424B5

424B5 1 d129116d424b5.htm 424B5 424B5

Filed Pursuant to Rule 424(b)(5)
Registration No. 333-276062

PROSPECTUS SUPPLEMENT

(To Prospectus dated December 15, 2023)

Up to $1,000,000,000 Aggregate Offering Price of Common Stock

Alliant Energy Corporation

We have entered into a distribution agreement with (i) Barclays Capital Inc., BofA Securities, Inc., Goldman Sachs & Co. LLC, J.P. Morgan Securities LLC, KeyBanc Capital Markets Inc., Mizuho Securities USA LLC, MUFG Securities Americas Inc., TD Securities (USA) LLC, and Wells Fargo Securities, LLC, as sales agents (each of which, when acting in this capacity, we refer to as an agent and together as the agents) and forward sellers (as set out below) and (ii) the forward purchasers (as set out below), relating to the shares of common stock, par value $0.01 per share, offered by this prospectus supplement and the accompanying prospectus. In accordance with the terms of the distribution agreement, we may offer and sell common stock having an aggregate offering price of up to $1,000,000,000 from time to time through any agent, as our sales agent (acting in their capacity as our agents or as forward sellers), or directly to any agent acting as principal.

In addition to the issuance and sale of common stock by us through the agents, we also may enter into forward sale agreements between us and each of Barclays Bank PLC, Bank of America, N.A., Goldman Sachs & Co. LLC, JPMorgan Chase Bank, National Association, KeyBanc Capital Markets Inc., Mizuho Markets Americas LLC, MUFG Securities EMEA plc, The Toronto-Dominion Bank, and Wells Fargo Bank, National Association. We refer to each of these entities, when acting in such capacity, as a forward purchaser and, collectively, as the forward purchasers. In connection with each forward sale agreement, the relevant forward purchaser will, at our request, borrow from third parties and, through its relevant agent, sell a number of shares of our common stock equal to the number of shares of our common stock that underlie the forward sale agreement to hedge the forward sale agreement. We refer to each of Barclays Capital Inc., BofA Securities, Inc., Goldman Sachs & Co. LLC, J.P. Morgan Securities LLC, KeyBanc Capital Markets Inc., Mizuho Securities USA LLC, MUFG Securities Americas Inc., TD Securities (USA) LLC, and Wells Fargo Securities, LLC, when acting as the agent for a forward purchaser, as a forward seller and, collectively, as the forward sellers.

In no event will the aggregate sales price of shares of our common stock sold through the agents and through the forward sellers under this prospectus supplement exceed $1,000,000,000. The offering of our common stock pursuant to the distribution agreement will terminate upon the earliest of (1) the sale, under the distribution agreement, of shares of our common stock with an aggregate sales price of $1,000,000,000, (2) December 31, 2029 and (3) termination by us or the relevant agent at any time upon written notice, solely with respect to such agent.

Sales of the common stock, if any, made by the agents, as our sales agents, as contemplated by this prospectus supplement and the accompanying prospectus, may be made by means of ordinary brokers’ transactions on the Nasdaq Global Select Market at market prices, in privately negotiated transactions (including block transactions), or as otherwise agreed between us and the agents. We will pay each agent a commission of 1.00% of the gross sales price per share of common stock sold through such agent, as our sales agent, under the distribution agreement. In connection with each forward sale agreement, the relevant forward seller will receive, reflected in a reduced initial forward sale price payable by the relevant forward purchaser under its forward sale agreement, a commission of 1.00% of the volume weighted average of the sales prices of all borrowed shares of our common stock sold during the applicable period by it as a forward seller.

The agents are not required to sell any specific number or dollar amount of our common stock but have agreed to use their commercially reasonable efforts, consistent with their normal sales and trading practice, as our sales agents, and on the terms and subject to the conditions of the distribution agreement, to sell the common stock offered on terms agreed by the agents and us.

Under the terms of the distribution agreement, we also may sell common stock to the agents as principals at a price to be agreed upon at the time of sale. If we sell common stock to an agent as principal, we will enter into a separate terms agreement with such agent, and we will describe the public offering price, underwriting discount (which may exceed 1.00% of the public offering price) and other terms of the offering of those shares of common stock in a separate prospectus supplement or pricing supplement. See “Plan of Distribution (Conflicts of Interest)” for a description of compensation to be paid to the agents.

We will not initially receive any proceeds from the sale of borrowed shares of our common stock by a forward seller. We expect to receive proceeds from the sale of shares of our common stock upon future physical settlement of the relevant forward sale agreement with the relevant forward purchaser on dates specified by us on or prior to the maturity date of the relevant forward sale agreement. If we elect to cash settle or net share settle a forward sale agreement, we may not (in the case of cash settlement) or will not (in the case of net share settlement) receive any proceeds, and we may owe cash (in the case of cash settlement) or shares of our common stock (in the case of net share settlement) to the relevant forward purchaser. See “Plan of Distribution (Conflicts of Interest).”

Our common stock is listed on the Nasdaq Global Select Market under the symbol “LNT.” The last reported sale price of our common stock on the Nasdaq Global Select Market on March 18, 2026 was $71.76 per share.

Investing in our common stock involves risk. See “Risk Factors” on page S-4 of this prospectus supplement, page 4 of the accompanying prospectus and in the documents incorporated by reference into this prospectus supplement and the accompanying prospectus for a discussion of certain risks that prospective investors should consider before investing in our common stock.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is accurate or complete. Any representation to the contrary is a criminal offense.


Barclays		BofA Securities		Goldman Sachs & Co. LLC		J.P. Morgan


KeyBanc Capital Markets		Mizuho		MUFG


TD Securities		Wells Fargo Securities

The date of this prospectus supplement is March 19, 2026.

TABLE OF CONTENTS

Prospectus Supplement


	Page
About This Prospectus Supplement		ii
Forward-Looking Statements		iii
Summary		S-1
Risk Factors		S-4
Use of Proceeds		S-7
Plan of Distribution (Conflicts of Interest)		S-8
Where You Can Find More Information		S-13
Legal Matters		S-14
Experts		S-14

Prospectus


	Page
About This Prospectus		1
Forward-Looking Statements		2
Alliant Energy Corporation		3
Risk Factors		4
Use of Proceeds		5
Description of Common Stock		6
Description of Debt Securities		8
Description of Warrants		16
Description of Stock Purchase Contracts and Stock Purchase Units		17
Selling Shareowners		18
Plan of Distribution		19
Where You Can Find More Information		21
Legal Matters		21
Experts		22

-i-

ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock. The second part, the accompanying prospectus, gives more general information, some of which may not apply to this offering. You should read the entire prospectus supplement, as well as the accompanying prospectus and the documents incorporated by reference that are described under “Where You Can Find More Information” in this prospectus supplement. In the event that the description of the offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information contained in this prospectus supplement. Generally, when we refer to the “prospectus,” we are referring to this prospectus supplement and the accompanying prospectus combined.

Any statement made in this prospectus or in a document incorporated or deemed to be incorporated by reference into this prospectus will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus or any subsequently filed document that is also incorporated by reference into this prospectus modifies or supersedes that statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus.

You should rely only on the information contained or incorporated by reference in this prospectus or any “free writing prospectus” we may authorize to be delivered to you. “Incorporated by reference” means that we can disclose important information to you by referring you to another document filed separately with the Securities and Exchange Commission, or the SEC. We have not authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not making an offer to sell nor soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus and the documents incorporated by reference is accurate only as of the respective dates of those documents in which the information is contained. Our business, financial condition, results of operations and prospects may have changed since any such date.

Unless we otherwise indicate or unless the context requires otherwise, all references in this prospectus supplement to “Alliant Energy,” “we,” “our,” “us” or similar references mean Alliant Energy Corporation.

Our principal executive offices are located at 4902 North Biltmore Lane, Madison, Wisconsin 53718, and our telephone number is (608) 458-3311.

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FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the information incorporated by reference herein or therein contain forward-looking statements intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical fact, included in this prospectus supplement or the accompanying prospectus or incorporated by reference herein or therein, including statements regarding anticipated financial performance, business strategy and management’s plans and objectives for future operations, are forward-looking statements. These forward-looking statements can be identified as such because the statements include words such as “may,” “believe,” “expect,” “anticipate,” “plan,” “project,” “will,” “projections,” “estimate,” or other words of similar import. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those expressed in, or implied by, such statements. Readers are cautioned not to place undue reliance on these forward-looking statements. All forward-looking statements included in this prospectus supplement, the accompanying prospectus or in a document incorporated by reference herein or therein speak only as of the date of this prospectus supplement, the accompanying prospectus or the document incorporated by reference, as the case may be. Additional information concerning factors that could cause actual results to differ materially from those in the forward-looking statements is contained under “Risk Factors” on page S-4 of this prospectus supplement, page 4 of the accompanying prospectus and other documents that we file from time to time with the SEC that are incorporated by reference into this prospectus supplement and the accompanying prospectus, including, but not limited to, the risk factor disclosure beginning on page 16 of our Annual Report on Form 10-K for the fiscal year ended December 31, 2025. Some, but not all, of the risks and uncertainties that could materially affect actual results include:

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Interstate Power and Light Company’s, or IPL’s, and Wisconsin Power and Light Company’s, or WPL’s, ability to obtain adequate and timely rate relief to allow for, among other things, recovery of and/or the return on costs, including fuel costs, operating costs, transmission costs, capacity costs, costs of cancelled generation projects incurred prior to pursuing regulatory approval, as well as costs of generation projects incurred prior to regulatory approval or that exceed initial estimates, deferred expenditures, deferred tax assets, tax expense, interest expense, capital expenditures, marginal costs to service new customers, and remaining costs related to electric generating units, or EGUs, that have been or may be permanently closed and certain other retired assets, environmental remediation costs, and decreases in sales volumes, as well as earning their authorized rates of return, payments to their parent of expected levels of dividends, the impact of rate design on current and potential customers and demand for energy in their service territories, and the ability to obtain regulatory approval with acceptable conditions for individual customer rates for large load growth customers;

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the impact of IPL’s retail electric base rate moratorium;

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the ability to obtain regulatory approval for construction projects with acceptable conditions;

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the ability to complete construction of generation and energy storage projects by planned in-service dates and within the cost targets set by regulators due to cost increases of and access to materials, equipment and commodities, which could result from tariffs, including previously exempted tariffs related to solar project materials and equipment from certain countries, duties or other assessments, including antidumping or countervailing duties, inflation, labor issues or supply shortages, contractor performance, the ability to successfully resolve warranty issues or contract disputes and the ability to obtain adequate generator interconnection agreements to connect the new projects to Midcontinent Independent System Operator, Inc., or MISO, in a timely manner;

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weather effects on utility sales volumes and operations;

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the direct or indirect effects resulting from cybersecurity incidents or attacks on us, IPL, WPL, or our and their suppliers, contractors and partners, or responses to such incidents;

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the impact of customer- and third party-owned generation and other non-traditional service models, including alternative electric suppliers and potential policy changes, regulatory changes, or legislation

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that may enable large customers to source behind-the-meter generation directly from third parties or to own or otherwise procure on-site or behind-the-meter generation or participate in co-located resource arrangements, in IPL’s and WPL’s service territories on system reliability, operating expenses and customers’ demand for electricity;

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economic conditions in IPL’s and WPL’s service territories, including the potential impacts of business or facility closures and tariffs;

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the ability and cost to attract large load growth customers and to provide sufficient generation and the ability of ITC Midwest LLC and American Transmission Company LLC, or ATC, to provide sufficient transmission capacity for potential load growth timely, including significant new commercial or industrial customers, such as data centers;

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the ability of potential large load growth customers to timely construct new facilities, due to local or state regulatory actions, zoning, siting, or permitting actions, public or community opposition or other factors, as well as the resulting higher system load demand by expected levels and timeframes;

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the impact of large load growth customers altering, delaying or cancelling planned facilities, including any resulting impacts of overbuilt or under-utilized transmission capacity or generation assets;

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the impact of energy efficiency, franchise retention and customer disconnects on sales volumes and operating income;

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the impact that price changes may have on IPL’s and WPL’s customers’ demand for electric and gas services and their ability to pay their bills;

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changes in the price of delivered natural gas, transmission, purchased electric energy, purchased electric capacity and delivered coal, particularly during elevated market prices, and any resulting changes to counterparty credit risk, due to shifts in supply and demand caused by market conditions, regulations and MISO’s seasonal resource adequacy process;

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the ability to achieve the expected level of tax benefits for renewable generation and energy storage projects based on tax guidelines, timely beginning of construction and in-service dates, sourcing permissible amounts of construction and/or financing support from entities with ties to certain foreign countries, compliance with prevailing wage and apprenticeship requirements, project costs and the level of electricity output generated by qualifying generating facilities, and the ability to efficiently utilize the renewable generation and energy storage project tax benefits to achieve IPL’s authorized rate of return and for the benefit of IPL’s and WPL’s customers;

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federal and state regulatory or governmental actions, including the impact of legislation, Treasury regulations, executive orders, interpretations and guidance, and changes in public policy, including changes impacting renewable tax credits, including any repeal, modification, or reduced funding of the Inflation Reduction Act of 2022 and the impact of the One Big Beautiful Bill Act, and siting generation and energy storage projects;

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the ability to utilize tax credits generated to date, and those that may be generated in the future, before they expire, as well as the ability to transfer tax credits that may be generated in the future at adequate pricing;

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the impacts of changes in the tax code, including tax rates, minimum tax rates, adjustments made to deferred tax assets and liabilities, changes in state income tax apportionment, and changes impacting the availability of and ability to transfer renewable tax credits, including preserving the qualification of any future tax credits;

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disruptions to ongoing operations and the supply of materials, services, equipment and commodities needed to continue to operate and maintain existing assets and to construct capital projects, which may result from geopolitical issues, tariffs, supplier manufacturing constraints, regulatory requirements, labor issues or transportation issues, and thus affect the ability to meet capacity requirements and result in increased capacity expense;

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inflation and higher interest rates;

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continued access to the capital markets on competitive terms and rates, and risks associated with potential increases in borrowing costs or reduced access to funding, and the actions of credit rating agencies;

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the future development of technologies related to electrification, and the ability to reliably store and manage electricity;

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employee workforce factors, including the ability to hire and retain employees with specialized skills, impacts from employee retirements, changes in key executives, ability to create desired corporate culture, collective bargaining agreements and negotiations, work stoppages or restructurings;

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disruptions in the supply and delivery of natural gas, purchased electricity and coal;

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changes to the creditworthiness of, or performance of obligations by, counterparties with which we, IPL and WPL have contractual arrangements, including large load growth customers, participants in the energy markets and fuel suppliers and transporters;

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the impact of penalties or third-party claims related to, or in connection with, a failure to maintain the security of personally identifiable information, including associated costs to notify affected persons and to mitigate their information security concerns;

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impacts that terrorist attacks may have on our, IPL’s and WPL’s operations and recovery of costs associated with restoration activities, or on the operations of our investments;

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changes to MISO’s interconnection or resource adequacy process establishing capacity planning reserve margin and capacity accreditation requirements that may impact how and when new and existing generating facilities, including IPL’s and WPL’s additional solar generation, may be accredited with energy capacity, and may require IPL and WPL to adjust their current resource plans, to add resources to meet the requirements of MISO’s process, or procure capacity in the market whereby such costs might not be recovered in rates;

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any legislative or regulatory changes that impose mandatory integrated resource planning requirements or materially modify existing planning processes, potentially affecting resource selection, cost recovery, and the ability to meet large load growth demand for energy;

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any material post-closing payments related to any past asset divestitures, including the transfer of renewable tax credits, which could result from, among other things, indemnification agreements, warranties, guarantees or litigation;

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issues associated with environmental remediation and environmental compliance, including compliance with all current environmental and emissions laws, regulations, siting requirements, and permits and future changes in environmental laws and regulations, including the Coal Combustion Residuals Rule, Cross-State Air Pollution Rule and federal, state or local regulations for emissions reductions, including greenhouse gases, or GHG, from new and existing fossil-fueled EGUs under the Clean Air Act, and litigation associated with environmental requirements;

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increased pressure from customers, investors and other stakeholders to more rapidly reduce GHG emissions;

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the timely development of technologies, innovations and advancements to provide cost effective alternatives to traditional energy sources;

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the ability to defend against environmental claims brought by state and federal agencies, such as the U.S. Environmental Protection Agency and state natural resources agencies, or third parties, such as the Sierra Club, and the impact on operating expenses of defending and resolving such claims;

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the direct or indirect effects resulting from breakdown or failure of equipment in the operation of electric and gas distribution systems, such as mechanical problems, disruptions in telecommunications,

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technological problems, and explosions or fires, and compliance with electric and gas transmission and distribution safety regulations, including regulations promulgated by the Pipeline and Hazardous Materials Safety Administration;

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issues related to the availability and operations of EGUs and energy storage facilities, including start-up risks, breakdown or failure of equipment, fires, availability of warranty coverage and successful resolution of warranty issues or contract disputes for equipment breakdowns or failures, performance below expected or contracted levels of output or efficiency, operator error, employee safety, transmission constraints, compliance with mandatory reliability standards and risks related to recovery of resulting incremental operating, capacity, fuel-related and capital costs through rates;

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impacts that excessive heat, excessive cold, storms, wildfires, or natural disasters may have on our, IPL’s and WPL’s operations and construction activities, and recovery of costs associated with restoration activities, or on the operations of our investments;

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our ability to sustain our dividend payout ratio goal;

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changes to costs of providing benefits and related funding requirements of pension and other postretirement benefits plans due to the market value of the assets that fund the plans, economic conditions, financial market performance, interest rates, timing and form of benefits payments, life expectancies and demographics;

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material changes in employee-related benefit and compensation costs, including settlement losses related to pension plans;

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risks associated with operation and ownership of non-utility holdings, including potential impairments;

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changes in technology that alter the channels through which customers buy or utilize our, IPL’s or WPL’s products and services;

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risks associated with third-party risk management practices, including vendor financial condition, operational performance, cybersecurity incidents, and compliance with contractual and regulatory requirements;

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risks associated with large-scale internal technology modernization initiatives, including enterprise asset management systems, operational technology/informational technology integration, cloud transformation, and digital modernization, and the potential for delays, cost overruns, or operational impacts;

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impacts on equity income from unconsolidated investments from changes in valuations of the assets held, as well as potential changes to ATC’s authorized return on equity;

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impacts of IPL’s future tax benefits from Iowa rate-making practices, including deductions for repairs expenditures and cost of removal obligations, allocation of mixed service costs and state depreciation, and recoverability of the associated regulatory assets from customers, when the differences reverse in future periods;

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current or future litigation, regulatory investigations, proceedings or inquiries;

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reputational damage from negative publicity, protests, fines, penalties and other negative consequences resulting in regulatory and/or legal actions;

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the direct or indirect effects resulting from pandemics;

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the effect of accounting standards issued periodically by standard-setting bodies;

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the ability to successfully complete tax audits and changes in tax accounting methods with no material impact on earnings and cash flows; and

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other factors listed in Management’s Discussion and Analysis of Financial Condition and Results of Operations and Risk Factors in the combined Annual Report on Form 10-K filed by IPL, WPL and us for the fiscal year ended December 31, 2025.

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We assume no obligation, and disclaim any duty, to update or revise publicly any forward-looking statements, whether as a result of new information, future events or otherwise, except as required by law.

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SUMMARY

This summary highlights information contained elsewhere in this prospectus supplement and the accompanying prospectus. This summary may not contain all of the information that may be important to you. You should read this entire prospectus supplement and the accompanying prospectus carefully before making a decision to invest in our common stock. References in this prospectus supplement to “common stock” refer to our common stock, $0.01 par value per share.

Alliant Energy Corporation

Alliant Energy Corporation is an energy-services provider that operates as a regulated investor-owned public utility holding company. Our primary focus is to provide regulated electric and natural gas service to customers in the Midwest through our two public utility subsidiaries, IPL and WPL. We also have non-utility operations. Our utility business is engaged principally in:

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the generation and distribution of electricity to retail customers in select markets in Iowa and Wisconsin;

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the distribution and transportation of natural gas to retail customers in select markets in Iowa and Wisconsin;

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the sale of electricity to wholesale customers in Illinois, Iowa and Wisconsin; and

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the generation and distribution of steam for two customers in Cedar Rapids, Iowa, which were each under contract through 2025 for taking minimum quantities of annual steam usage, with certain conditions. Subsequent to December 31, 2025, IPL exited the steam business.

We are a “holding company” under the Public Utility Holding Company Act of 2005 and are subject to regulation by the Federal Energy Regulatory Commission under that Act.

Our principal executive offices are located at 4902 North Biltmore Lane, Madison, Wisconsin 53718, and our telephone number is (608) 458-3311.

S-1

The Offering

Issuer

Alliant Energy Corporation

Common Stock Offered

Common stock, par value $0.01 per share, having an aggregate offering price of up to $1,000,000,000.

Use of Proceeds

We intend to use the net proceeds that we receive upon the issuance or sale of shares of common stock by us through the agents, after deducting the agents’ commissions and our offering expenses, for general corporate purposes, which may include repayment, refinancing, repurchase or redemption of debt, working capital, construction and acquisition expenditures and investments.

We will not initially receive any proceeds from the sale of borrowed shares of our common stock by a forward seller in connection with any forward sale agreement as a hedge of the forward sale agreement. We intend to use any cash proceeds that we receive upon physical settlement of any forward sale agreement, if physical settlement applies, or upon cash settlement of any forward sale agreement, if we elect cash settlement, for the purposes described above. Please read “Use of Proceeds.”

Risk Factors

You should read “Risk Factors” on page S-4 of this prospectus supplement, on page 4 of the accompanying prospectus and in the documents incorporated by reference herein for a discussion of certain risks that prospective investors should consider before investing in our common stock.

Transfer Agent and Registrar

EQ Shareowner Services

Nasdaq Global Select Market Symbol

LNT

Accounting Treatment of Forward Sales

In the event that we enter into any forward sale agreement, we expect that before the issuance of shares of our common stock, if any, upon physical or net share settlement of any forward sale agreement, the shares issuable upon settlement of that particular forward sale agreement will be reflected in our diluted earnings per share calculations using the treasury stock method. Under this method, the number of shares of our common stock used in calculating diluted earnings per share is deemed to be increased by the excess, if any, of the number of shares of our common stock that would be issued upon full physical settlement of that particular forward sale agreement over the number of shares of our common stock that could be purchased by us in the market (based on the average market price during the relevant period) using the proceeds receivable upon full physical settlement (based on the adjusted forward sale price at the end of the relevant reporting period).

Consequently, before physical or net share settlement of a particular forward sale agreement and subject to the occurrence of certain events, we anticipate there will be no dilutive effect on our earnings per share, except during periods when the average market price of our common stock is above the then-applicable forward sale price under that particular forward sale agreement. However, if we physically or net share settle a particular forward sale agreement, the delivery of

S-2

shares of our common stock would result in an increase in the number of shares outstanding and dilution to our earnings per share and return on equity.

Conflicts of Interest

Affiliates of each of the agents participate in our credit facilities. As a result, such affiliates may receive a portion of the net proceeds of this offering through the repayment of any borrowings under such facilities if we decide to pay down such indebtedness. In addition, the forward purchasers (or their respective affiliates) will receive the net proceeds of any sale of borrowed shares of our common stock pursuant to this prospectus supplement in connection with any forward sale agreement.

If certain agents or their affiliates were to receive part of the net proceeds from the sale of shares of our common stock by us through the agents or in connection with any forward sale agreement, such agents would be deemed to have a conflict of interest under Financial Industry Regulatory Authority, Inc. (FINRA) Rule 5121 to the extent such agents or affiliates receive at least 5% of the net proceeds of the offering. Any agent deemed to have a conflict of interest would be required to conduct the distribution of our common stock in accordance with FINRA Rule 5121. If the offering is conducted in accordance with FINRA Rule 5121, such agent would not be permitted to confirm a sale to an account over which it exercises discretionary authority without first receiving specific written approval from the account holder.

S-3

RISK FACTORS

Investing in our common stock involves risk. You should carefully consider the following risk factors together with all the other information contained in this prospectus supplement or incorporated by reference in this prospectus. You should also consider the risks and uncertainties discussed under the caption “Risk Factors” included in our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, which is incorporated by reference in this prospectus supplement, and which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future.

Sales of a substantial number of shares of our common stock, or the perception that such sales might occur, could adversely affect the trading price of our common stock.

As of December 31, 2025, we had 257,137,261 shares of our common stock outstanding. Also, we had, as of December 31, 2025, a total of approximately 11 million shares of common stock in the aggregate reserved for issuance pursuant to our 2020 Omnibus Incentive Plan, Shareowner Direct Plan and 401(k) Savings Plan. If this offering is completed, the number of shares of common stock that we have outstanding will increase. Sales of a substantial number of shares of our common stock, or the perception that such sales might occur, could adversely affect the trading price of our common stock. Further, issuance of shares pursuant to our 2020 Omnibus Incentive Plan, our Shareowner Direct Plan or our 401(k) Savings Plan, or the perception in the market that the holders of a large number of shares intend to sell shares, could reduce the market price of our common stock.

You may experience dilution.

Giving effect to the issuance of common stock in this offering, the receipt of the expected net proceeds and the use of those proceeds, this offering may have a dilutive effect on our expected earnings per weighted average common share. The actual amount of dilution cannot be determined at this time and will be based on a number of factors. Also, we are not restricted from issuing additional securities in the future, including shares of common stock, securities that are convertible into or exchangeable for, or that represent the right to receive, common stock or substantially similar securities. The price per share at which we sell additional shares of our common stock, or securities convertible or exchangeable into common stock, in future transactions may be higher or lower than the price per share paid by investors in this offering and investors purchasing shares or other securities in the future could have rights superior to existing shareowners. The issuance of these securities may cause further dilution to our shareowners.

The market price of our common stock may fluctuate significantly.

The market price of our common stock could be subject to significant fluctuations in response to factors such as the following, some of which are beyond our control:

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our financial condition, performance, liquidity and prospects;

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the market for similar securities;

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our issuance of common stock or other securities;

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changes in earnings estimates by analysts and our ability to meet analysts’ earnings estimates;

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announcements by third parties of significant claims or proceedings against us;

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government action or regulation;

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our dividend policy;

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changes in the ratings of our securities; and

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general economic, capital markets and real estate market conditions.

S-4

In addition, the stock markets in general, including the Nasdaq Global Select Market, are subject to significant price and trading fluctuations. These fluctuations have resulted in volatility in the market prices of securities that often has been unrelated or disproportionate to changes in operating performance and may affect adversely the market price of our common stock.

Settlement provisions contained in any forward sale agreement subject us to certain risks.

A forward purchaser will have the right to accelerate a forward sale agreement that it enters into with us and require us to physically settle such forward sale agreement (with respect to all or any portion of the transaction under such forward sale agreement that such forward purchaser determines is affected by such event) on a date specified by such forward purchaser if:

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in such forward purchaser’s commercially reasonable judgment, it or its affiliate is unable to hedge its exposure under such forward sale agreement because (x) insufficient shares of our common stock have been made available for borrowing by securities lenders or (y) the forward purchaser or its affiliate would incur a stock borrowing cost in excess of a specified threshold;

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we declare any dividend or distribution on shares of our common stock

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payable in cash in excess of specified amounts,

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that constitutes an extraordinary dividend under the forward sale agreement,

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payable in securities of another company as a result of a spinoff or similar transaction, or

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of any other type of securities (other than our common stock), rights, warrants or other assets for payment (cash or other consideration) at less than the prevailing market price;

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certain ownership thresholds applicable to such forward purchaser and its affiliates are exceeded;

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an event is announced that if consummated would result in a specified extraordinary event (including certain mergers or tender offers, as well as certain events involving our nationalization, a delisting of our common stock, or change in law); or

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certain other events of default or termination events occur, including, among others, any material misrepresentation made in connection with such forward sale agreement or our insolvency (each as more fully described in each forward sale agreement).

We expect that settlement of any forward sale agreement will generally occur no later than the date specified in such forward sale agreement. However, any forward sale agreement may be settled earlier than such specified date in whole or in part at our option. Except under the circumstances described above, we generally have the right to elect physical, cash or net share settlement under each forward sale agreement. Delivery of our common stock upon physical settlement of any forward sale agreement (or, if we elect net share settlement of any forward sale agreement, upon such settlement to the extent we are obligated to deliver our common stock) will result in dilution to our earnings per share and return on equity. If we elect to cash or net share settle all or a portion of the shares of our common stock underlying any forward sale agreement, we would expect the relevant forward purchaser or one of its affiliates to purchase shares of our common stock in secondary market transactions over an unwind period to:

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return shares of our common stock to securities lenders in order to unwind such forward purchaser’s hedge (after taking into consideration any shares of our common stock to be delivered by us to such forward purchaser, in the case of net share settlement); and

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•

if applicable, in the case of net share settlement, deliver shares of our common stock to us to the extent required in settlement of such forward sale agreement.

The forward sale price that we expect to receive upon physical settlement of any forward sale agreement will be subject to adjustment on a daily basis based on a floating interest rate factor equal to the overnight bank funding rate less a spread and will be subject to decrease on certain dates specified in the relevant forward sale agreement by the amount per share of quarterly dividends we currently expect to declare during the term of such forward sale agreement. If the overnight bank funding rate is less than the spread on any day, the interest rate factor will result in a daily reduction of the forward sale price. If the volume-weighted average price at which the relevant forward purchaser (or its affiliate) purchases shares during the applicable unwind period under a forward sale agreement is above the relevant forward sale price, in the case of cash settlement, we would pay the relevant forward purchaser under such forward sale agreement an amount in cash equal to the difference or, in the case of net share settlement, we would deliver to such forward purchaser a number of shares of our common stock having a value equal to the difference. Thus, we could be responsible for a potentially substantial cash payment in the case of cash settlement. If the volume-weighted average price at which the relevant forward purchaser (or its affiliate) purchases shares during the applicable unwind period under a forward sale agreement is below the relevant forward sale price, in the case of cash settlement, we would be paid the difference in cash by the relevant forward purchaser under such forward sale agreement or, in the case of net share settlement, we would receive from such forward purchaser a number of shares of our common stock having a value equal to the difference. Any such difference could be significant. See “Plan of Distribution (Conflicts of Interest)—Sales Through Forward Sellers.”

In addition, the purchase of our common stock by a forward purchaser or its affiliate to unwind the forward purchaser’s hedge position could cause the price of our common stock to increase over time (or prevent a decrease over time), thereby increasing the amount of cash (in the case of cash settlement), or the number of shares (in the case of net share settlement), that we would owe such forward purchaser upon settlement of the applicable forward sale agreement or decreasing the amount of cash (in the case of cash settlement), or the number of shares (in the case of net share settlement), that such forward purchaser would owe us upon settlement of the applicable forward sale agreement, as the case may be.

In certain bankruptcy or insolvency events, any forward sale agreements will automatically terminate, and we would not receive the expected proceeds from any forward sales of our common stock.

If we institute or consent to, or an appropriate regulatory or other authority institutes against us, a proceeding seeking a judgment in bankruptcy or insolvency or any other relief under any bankruptcy or insolvency law or other similar law affecting creditors’ rights or if we or such authority presents a petition for our winding up or liquidation or we consent to such a petition, any forward sale agreements that are then in effect will automatically terminate. If any such forward sale agreement so terminates, we would not be obligated to deliver to the relevant forward purchaser any shares of our common stock not previously delivered (or for which physical settlement has not been elected), and the relevant forward purchaser would be discharged from its obligation to pay the forward sale price per share in respect of any shares of our common stock not previously settled (or for which physical settlement has not been elected). Therefore, to the extent there are any shares of our common stock with respect to which we have not elected to physically settle under a forward sale agreement at the time of the institution of or consent to any such bankruptcy or insolvency proceedings or any such petition, we would not receive the forward sale price per share in respect of those shares of our common stock.

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USE OF PROCEEDS

We intend to use the net proceeds that we receive upon the issuance or sale of shares of common stock by us through the agents for general corporate purposes, which may include repayment, refinancing, repurchase or redemption of debt, working capital, construction and acquisition expenditures and investments. Net proceeds may be placed in short term instruments prior to use.

We will not initially receive any proceeds from the sale of borrowed shares of our common stock by the forward sellers, as agents for the forward purchasers, in connection with any forward sale agreement as a hedge of the forward sale agreement. In the event of full physical settlement of a forward sale agreement, which we expect to occur on or prior to the maturity date of such forward sale agreement, we expect to receive aggregate cash proceeds equal to the product of the initial forward price under the forward sale agreement and the number of shares of our common stock underlying the forward sale agreement, subject to the price adjustment and other provisions of the forward sale agreement. We currently intend to use any cash proceeds that we receive upon physical settlement of any forward sale agreement, if physical settlement applies, or upon cash settlement of any forward sale agreement, if we elect cash settlement, for the purposes provided in the immediately preceding paragraph.

If, however, we elect to cash settle or net share settle any forward sale agreement, we would expect to receive an amount of proceeds that is significantly lower than the product set forth in the second preceding sentence (in the case of any cash settlement) or will not receive any proceeds (in the case of any net share settlement), and we may owe cash (in the case of any cash settlement) or shares of our common stock (in the case of any net share settlement) to the relevant forward purchaser. See “Plan of Distribution (Conflicts of Interest).”

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PLAN OF DISTRIBUTION (CONFLICTS OF INTEREST)

We have entered into a distribution agreement with each of the agents, forward sellers and forward purchasers under which we may offer and sell common stock having an aggregate offering price of up to $1,000,000,000 from time to time through any agent, as our sales agent, or directly to any agent acting as principal. Further, the distribution agreement provides that, in addition to the offer and sale of shares of our common stock by us through the agents, we also may deliver from time to time instructions to any agent specifying that such agent, as a forward seller, use commercially reasonable efforts to sell shares of our common stock borrowed by the applicable forward purchaser in connection with one or more forward sale agreements, as described below. In no event will the aggregate number of shares of our common stock sold through the agents, each as an agent for us, as principal and as a forward seller, under the distribution agreement have an aggregate sales price in excess of $1,000,000,000. We will file the distribution agreement as an exhibit to a Current Report on Form 8-K, which will be incorporated by reference in this prospectus supplement.

Sales, if any, of shares of our common stock by any agent, acting as our sales agent, will be made in “at the market” offerings, as defined in Rule 415 under the Securities Act of 1933, as amended, or the Securities Act, including sales made directly on the Nasdaq Global Select Market, the existing trading market for our common stock, or sales made to or through a market maker or through an electronic communications network. In addition, shares of our common stock may be offered and sold by such other methods, including privately negotiated transactions (including block transactions), as we and any sales agent agree to in writing.

If common stock is sold by any agent, as our sales agent, in an at the market offering, the agents have agreed to confirm to us in writing the number of shares of common stock sold on the applicable trading day and the related gross sales price and net sales price of those shares of common stock on the immediately following trading day. We will report at least quarterly (1) the number of shares of common stock sold through the agents, as our sales agents, under the distribution agreement, (2) the number of borrowed shares of common stock sold by the forward sellers, as agents for the forward purchasers, in connection with any forward sale agreements as described below under “—Sales Through Forward Sellers” and (3) the net proceeds received by us and the compensation paid by us to the agents in connection with the transactions described in clauses (1) and (2).

In connection with the sale of our common stock on our behalf, the agents may be deemed to be “underwriters” within the meaning of the Securities Act, and the compensation paid to the agents may be deemed to be underwriting commissions or discounts. We have agreed to provide indemnification and contribution to the agents against certain liabilities, including liabilities under the Securities Act.

We have determined that our common stock is an “actively-traded security” excepted from the requirements of Rule 101 of Regulation M under the Securities Exchange Act of 1934, as amended, or the Exchange Act, by Rule 101(c)(1) of Regulation M. If any agent or we have reason to believe that the exemptive provisions set forth in Rule 101(c)(1) of Regulation M under the Exchange Act are not satisfied, that party has agreed to promptly notify the other parties and sales of common stock under the distribution agreement will be suspended until that or other exemptive provisions have been satisfied in the judgment of the agents and us.

The offering of common stock pursuant to the distribution agreement will terminate upon the earliest of (1) the sale of common stock having an aggregate sales price of $1,000,000,000, (2) the termination of the distribution agreement by us or the agents and (3) December 31, 2029. The distribution agreement may be terminated either by us or the agents at any time and in the sole discretion of us or the agents, as the case may be.

The expenses of the offering are estimated at approximately $1 million and are payable by us. We have agreed to reimburse the agents, the forward sellers and the forward purchasers for certain of their expenses.

Sales Through Agents

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our sales agents and on the terms and subject to the conditions of the distribution agreement, to sell the common stock offered on terms agreed upon by the agents and us. If we elect to offer common stock, we will instruct an agent as to the number of shares of common stock to be sold by it as our sales agent and the date or dates on which such common stock is to be sold. We may instruct such agent not to sell our common stock as our sales agent if the sales cannot be effected at or above a price designated by us. Such agent may decline to accept any such instructions that we may provide to it from time to time. We or any agent may suspend the offering of common stock by an agent, as our sales agent, upon notice to the other parties.

We will pay the agents a commission of 1.00% of the gross sales price per share of common stock sold through the agents, as our sales agents, under the distribution agreement. The remaining sales proceeds, after deducting any transaction fees, transfer taxes or similar taxes or fees imposed by any governmental, regulatory or self-regulatory organization in respect of the sale of the common stock, will constitute our net proceeds (before the expenses referred to in the next paragraph) from the sale of the common stock in the offering.

Under the terms of the distribution agreement, we may, if agreed to by any agent, also sell common stock to such agent, as principal, at a price per share to be agreed upon at the time of sale. If we sell common stock to an agent as principal, we will enter into a separate terms agreement with such agent, and we will describe the public offering price, underwriting discount (which may exceed 1.00% of the public offering price) and other terms of the offering of those shares of common stock in a separate prospectus supplement or pricing supplement. However, the agents have no obligation to agree to purchase common stock as principal or to enter into a terms agreement.

Settlement for sales of our common stock will occur on the second trading day (or on such other date as may be agreed upon by us and an agent) following the respective dates on which any such sales are made in return for payment of the net proceeds to us. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

Sales Through Forward Sellers

From time to time during the term of the distribution agreement, and subject to the terms and conditions set forth therein, we may enter into one or more forward sale agreements with a forward purchaser and deliver instructions to its agent, requesting that the agent execute sales of borrowed shares of our common stock as a forward seller in connection with the applicable forward sale agreement, and, subject to the terms and conditions of the distribution agreement, the relevant forward purchaser will attempt to borrow, and such forward seller will use its commercially reasonable efforts consistent with its normal trading and sales practices to sell, such shares of our common stock on such terms to hedge such forward purchaser’s exposure under such forward sale agreement. We or the relevant forward seller may immediately suspend the offering of shares of our common stock at any time upon proper notice to the other. We expect settlement between a forward purchaser and the relevant forward seller of sales of borrowed shares of our common stock, as well as the settlement between the relevant forward seller and buyers of such shares in the market, to occur on the first trading day following each date on which the sales are made unless another date shall be agreed to in writing by us and the relevant agent. The obligation of each forward seller under the distribution agreement to execute such sales of shares of our common stock is subject to a number of conditions, which each forward seller reserves the right to waive in its sole discretion.

In connection with each forward sale agreement, the relevant forward seller will receive, reflected in a reduced initial forward sale price payable by the relevant forward purchaser under its forward sale agreement, a commission of 1.00% of the volume weighted average of the sales prices of all borrowed shares of our common stock sold during the applicable period by it as a forward seller. We refer to this commission rate as the forward selling commission.

The initial forward sale price per share under each forward sale agreement will equal the product of (1) an amount equal to one minus the applicable forward selling commission and (2) the volume weighted average price

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per share at which the borrowed shares of our common stock were sold pursuant to the distribution agreement by the relevant forward seller to hedge the relevant forward purchaser’s exposure under such forward sale agreement. Thereafter, the initial forward sale price will be subject to price adjustment as described below. If we elect to physically settle any forward sale agreement by delivering shares of our common stock, we will receive an amount of cash from the relevant forward purchaser equal to the product of the initial forward sale price per share under such forward sale agreement and the number of shares of our common stock underlying such forward sale agreement, subject to the price adjustment and other provisions of such forward sale agreement. Each forward sale agreement will provide that the initial forward sale price, as well as the sales prices used to calculate the initial forward sale price, will be subject to adjustment based on a floating interest rate factor equal to the overnight bank funding rate less a spread. In addition, the initial forward sale price will be subject to decrease on certain dates specified in the relevant forward sale agreement by the amount per share of quarterly dividends we currently expect to declare during the term of such forward sale agreement. If the overnight bank funding rate is less than the spread on any day, the interest rate factor will result in a daily reduction of the forward sale price.

Before any issuance of shares of our common stock upon settlement of any forward sale agreement, we expect that the shares issuable upon settlement of such forward sale agreement will be reflected in our diluted earnings per share calculations using the treasury stock method. Under this method, the number of shares of our common stock used in calculating diluted earnings per share is deemed to be increased by the excess, if any, of the number of shares that would be issued upon full physical settlement of such forward sale agreement over the number of shares that could be purchased by us in the market (based on the average market price of our common stock during the applicable reporting period) using the proceeds receivable upon full physical settlement (based on the adjusted forward sale price at the end of the reporting period). Consequently, we anticipate there will be no dilutive effect on our earnings per share except during periods when the average market price of shares of our common stock is above the applicable adjusted forward sale price subject to increase or decrease as described in the immediately preceding paragraph. However, if we decide to physically settle or net share settle any forward sale agreement, delivery of our shares to the relevant forward purchaser on the physical settlement or net share settlement of the forward sale agreement would result in dilution to our earnings per share and return on equity.

•

if applicable, in the case of net share settlement, deliver shares of our common stock to us to the extent required in settlement of such forward sale agreement.

If the volume-weighted average price at which the relevant forward purchaser (or its affiliate) purchases shares during the applicable unwind period under a forward sale agreement is above the relevant forward sale price, in the case of cash settlement, we would pay the relevant forward purchaser under such forward sale agreement an amount in cash equal to the difference or, in the case of net share settlement, we would deliver to such forward purchaser a number of shares of our common stock having a value equal to the difference. Thus, we could be responsible for a potentially substantial cash payment in the case of cash settlement. If the volume-weighted average price at which the relevant forward purchaser (or its affiliate) purchases shares during the applicable unwind period under a forward sale agreement is below the relevant forward sale price, in the case of cash settlement, we would be paid the difference in cash by the relevant forward purchaser under such forward sale agreement or, in the case of net share settlement, we would receive from such forward purchaser a number of shares of our common stock having a value equal to the difference.

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A forward purchaser will have the right to accelerate the forward sale agreement that it enters into with us and require us to physically settle such forward sale agreement (with respect to all or any portion of the transaction under such forward sale agreement that such forward purchaser determines is affected by such event) on a date specified by such forward purchaser if:

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we declare any dividend or distribution on shares of our common stock

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payable in cash in excess of specified amounts,

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that constitutes an extraordinary dividend under the forward sale agreement,

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payable in securities of another company as a result of a spinoff or similar transaction, or

•

of any other type of securities (other than our common stock), rights, warrants or other assets for payment (cash or other consideration) at less than the prevailing market price;

•

certain ownership thresholds applicable to such forward purchaser and its affiliates are exceeded;

•

A forward purchaser’s decision to exercise its right to accelerate any forward sale agreement and to require us to settle any such forward sale agreement will be made irrespective of our interests, including our need for capital. In such cases, we could be required to issue and deliver shares of our common stock under the terms of the physical settlement provisions of the applicable forward sale agreement irrespective of our capital needs, which would result in dilution to our earnings per share and return on equity. In addition, upon certain events of bankruptcy, insolvency or reorganization relating to us, the forward sale agreement will terminate without further liability of either party. Following any such termination, we would not issue any shares and we would not receive any proceeds pursuant to the forward sale agreement.

Selling restrictions

No action has been taken in any jurisdiction (except in the United States) that would permit a public offering of the common stock, or the possession, circulation or distribution of this prospectus supplement, the accompanying prospectus, any free writing prospectus or any other offering material relating to us or our common stock where action for that purpose is required. Accordingly, our common stock may not be offered or sold, directly or indirectly, and neither this prospectus supplement, the accompanying prospectus, any free writing prospectus nor any other offering material or advertisements in connection with our common stock may be distributed or published in or from any country or jurisdiction, except in compliance with any applicable rules and regulations of any such country or jurisdiction.

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The agents may arrange to sell the common stock offered by this prospectus supplement and the accompanying prospectus in certain jurisdictions outside the United States, either directly or through affiliates, where they are permitted to do so.

Other relationships

The agents and their respective affiliates are full service financial institutions engaged in various activities, which may include sales and trading, commercial and investment banking, advisory, investment management, investment research, principal investment, hedging, market making, brokerage and other financial and non-financial activities and services. The agents and their respective affiliates have provided, and may in the future provide, a variety of these services to us and to persons and entities with relationships with us, for which they received or will receive customary fees and expenses.

In the ordinary course of their various business activities, the agents and their respective affiliates, officers, directors and employees may purchase, sell or hold a broad array of investments and actively trade securities, derivatives, loans, commodities, currencies, credit default swaps and other financial instruments for their own account and for the accounts of their customers, and such investment and trading activities may involve or relate to our assets, securities and/or instruments (directly, as collateral securing other obligations or otherwise) and/or persons and entities with relationships with us. The agents and their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such assets, securities or instruments and may at any time hold, or recommend to clients that they should acquire, long and/or short positions in such assets, securities and instruments.

Conflicts of Interest

Among other relationships, an affiliate of each of the agents is a lender under our credit facility. As a result, such affiliates may receive a portion of the net proceeds of this offering through the repayment of any borrowings under such facilities if we decide to pay down such indebtedness. The forward purchasers will receive the net proceeds of any sale of borrowed shares of our common stock pursuant to this prospectus supplement in connection with any forward sale agreement. Because certain agents or their affiliates are expected to receive part of the net proceeds from the sale of shares of our common stock in connection with any forward sale agreement, such agents would be deemed to have a conflict of interest under FINRA Rule 5121 to the extent such agents or affiliates receive at least 5% of the net proceeds of the offering. Any agent deemed to have a conflict of interest would be required to conduct the distribution of our common stock in accordance with FINRA Rule 5121. If the offering is conducted in accordance with FINRA Rule 5121, such agent would not be permitted to confirm a sale to an account over which it exercises discretionary authority without first receiving specific written approval from the account holder.

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WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC (File No. 001-09894). We also filed a registration statement on Form S-3, including exhibits, under the Securities Act, with respect to the securities offered by this prospectus supplement and the accompanying prospectus. This prospectus supplement does not contain all of the information included in the registration statement or the exhibits. You can find our public filings with the SEC on the internet at a web site maintained by the SEC located at www.sec.gov.

We are “incorporating by reference” specified documents that we file with the SEC, which means:

•

incorporated documents are considered part of this prospectus supplement;

•

we are disclosing important information to you by referring you to those documents; and

•

information we file with the SEC will automatically update and supersede information contained in this prospectus supplement.

We incorporate by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, after the date of this prospectus supplement and before the end of the offering of the securities pursuant to this prospectus supplement:

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our Annual Report on Form 10-K for the fiscal year ended December 31, 2025;

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our Current Reports on Form 8-K filed on January 8, 2026, and March 4, 2026; and

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the description of our common stock filed as Exhibit 4.32 to our Annual Report on Form 10-K for the fiscal year ended December 31, 2023, and any amendment or report updating that description.

Notwithstanding the foregoing, information furnished under Items 2.02 and 7.01 of any Current Report on Form 8-K, including the related exhibits under Item 9.01, is not incorporated by reference in this prospectus.

You may obtain copies of documents incorporated by reference in this prospectus, at no cost, by request directed to us at the following address or telephone number:

Alliant Energy Corporation

4902 North Biltmore Lane

Madison, Wisconsin 53718

Attention: Omar Chaudhary

Corporate Secretary

(608) 458-3311

You can also find these filings on our website at www.alliantenergy.com. However, we are not incorporating the information on our website other than these filings into this prospectus.

Any statement contained herein or in a document incorporated or deemed to be incorporated by reference herein will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained herein, in any other subsequently filed document which also is or is deemed to be incorporated by reference herein or in any prospectus supplement, modifies or supersedes such statement. Any such statement so modified or superseded will not be deemed, except as so modified and superseded, to constitute a part of this prospectus.

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LEGAL MATTERS

The validity of the common stock offered in this prospectus supplement and certain legal matters will be passed upon for us by Perkins Coie LLP and for the agents, the forward sellers and the forward purchasers by Gibson, Dunn & Crutcher LLP.

EXPERTS

The consolidated financial statements, and the related consolidated financial statement schedule, incorporated by reference in this prospectus from Alliant Energy Corporation’s Annual Report on Form 10-K for the fiscal year ended December 31, 2025, and the effectiveness of Alliant Energy Corporation and subsidiaries’ internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports. Such financial statements and financial statement schedule are incorporated by reference in reliance upon the reports of such firm given their authority as experts in accounting and auditing.

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PROSPECTUS

Alliant Energy Corporation

Common Stock

Debt Securities

Warrants

Stock Purchase Contracts

and

Stock Purchase Units

We may offer and sell from time to time securities in one or more offerings in amounts, at prices and on terms determined at the time of the offering. This prospectus provides you with a general description of the securities we may offer.

We may offer and sell the following securities:

•

common stock;

•

debt securities, which may be convertible into our common stock;

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warrants to purchase debt securities, common stock or other securities; and

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stock purchase contracts and stock purchase units.

Each time securities are sold using this prospectus, we will provide one or more prospectus supplements containing specific information about the offering and the terms of the securities being sold, including the offering price. Each prospectus supplement may also add, update or change information contained in this prospectus. You should carefully read this prospectus and the prospectus supplement relating to the specific issue of securities, together with the documents we incorporate by reference, before you invest.

We may offer and sell these securities to or through underwriters, dealers or agents, or directly to investors, on a continuous or a delayed basis. The applicable prospectus supplement will provide the specific terms of the plan of distribution.

In addition, selling shareowners to be named in a prospectus supplement may offer and sell from time to time shares of our common stock in such amounts as set forth in a prospectus supplement. Unless otherwise set forth in a prospectus supplement, we will not receive any proceeds from the sale of shares of our common stock by any selling shareowners.

Our common stock is listed on the Nasdaq Global Select Market, or Nasdaq, under the symbol “LNT.”

Investing in our securities involves risk. See “Risk Factors” on page 4 of this prospectus and in any prospectus supplement or such other document we refer you to in any prospectus supplement for a discussion of certain risks that prospective investors should consider before investing in our securities.

This prospectus is dated December 15, 2023.

TABLE OF CONTENTS


	Page
About This Prospectus		1
Forward-Looking Statements		2
Alliant Energy Corporation		3
Risk Factors		4
Use of Proceeds		5
Description of Common Stock		6
Description of Debt Securities		8
Description of Warrants		16
Description of Stock Purchase Contracts and Stock Purchase Units		17
Selling Shareowners		18
Plan of Distribution		19
Where You Can Find More Information		21
Legal Matters		21
Experts		22

ABOUT THIS PROSPECTUS

References in this prospectus to “we,” “us,” “our” and the “Company” refer to Alliant Energy Corporation, unless the context otherwise requires. References to “common stock” refer to our common stock, $0.01 par value per share.

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, or the SEC, utilizing a “shelf” registration process. Under this shelf registration process, we may, from time to time, sell the securities or combinations of the securities described in this prospectus, and one or more of our shareowners may sell our common stock, in one or more offerings. This prospectus provides you with a general description of those securities. Each time we, or, under certain circumstances, our shareowners, offer securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplement may also add, update or change information contained in this prospectus and may include other special considerations applicable to such offering of securities. If there is any inconsistency between the information in this prospectus and any prospectus supplement, you should rely on the information in the prospectus supplement. You should read this prospectus and any prospectus supplement together with the additional information described under the heading “Where You Can Find More Information.”

You should rely only on the information contained or incorporated by reference in this prospectus and in any prospectus supplement. “Incorporated by reference” means that we can disclose important information to you by referring you to another document filed separately with the SEC. We have not authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it.

This prospectus and any prospectus supplement or information incorporated by reference herein or therein contains summaries of certain agreements that we have filed as exhibits to various SEC filings, as well as certain agreements that we will enter into in connection with the offering of securities covered by any prospectus supplement. The descriptions of these agreements contained in this prospectus and any prospectus supplement or information incorporated by reference herein or therein do not purport to be complete and are subject to, or qualified in their entirety by reference to, the definitive agreements. Copies of the definitive agreements will be made available without charge to you by making a written or oral request to us. See “Where You Can Find More Information.”

We are not making offers to sell nor soliciting offers to buy, nor will we make an offer to sell nor solicit an offer to buy, securities in any jurisdiction where the offer or sale is not permitted.

You should assume that the information appearing in this prospectus and any supplement to this prospectus, or the information we file or previously filed with the SEC that we incorporate by reference in this prospectus or any prospectus supplement, is accurate only as of the respective dates of those documents in which the information is contained. Our business, financial condition, results of operations and prospects may have changed since those dates.

FORWARD-LOOKING STATEMENTS

This prospectus, any prospectus supplement and the information incorporated by reference herein or therein contain forward-looking statements intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical fact, included in this prospectus or any prospectus supplement or incorporated by reference herein or therein, including statements regarding anticipated financial performance, business strategy and management’s plans and objectives for future operations, are forward-looking statements. These forward-looking statements can be identified as such because the statements generally include words such as “may,” “believe,” “expect,” “anticipate,” “plan,” “project,” “will,” “projections,” “estimate,” or other words of similar import. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those expressed in, or implied by, these statements. Readers are cautioned not to place undue reliance on these forward-looking statements. All forward-looking statements included in this prospectus, any prospectus supplement or in a document incorporated by reference herein or therein speak only as of the date of this prospectus, the applicable prospectus supplement or the document incorporated by reference, as the case may be. Additional information concerning factors that could cause actual results to differ materially from those in the forward-looking statements is contained under “Risk Factors” on page 4 of this prospectus and other documents that we file from time to time with the SEC that are incorporated by reference into this prospectus and any prospectus supplement. Numerous important factors described in this prospectus, any prospectus supplement and/or any document incorporated by reference in this prospectus and/or any prospectus supplement could affect these statements and could cause actual results to differ materially from our expectations. We assume no obligation, and disclaim any duty, to update or revise publicly any forward-looking statements, whether as a result of new information, future events or otherwise, except as required by law.

ALLIANT ENERGY CORPORATION

We are an energy-services provider that operates as a regulated investor-owned public utility holding company, and our purpose-driven strategy is to serve our customers and build stronger communities. Our primary focus is to provide regulated electric and natural gas service to customers in the Midwest through our two public utility subsidiaries, Interstate Power and Light Company, or IPL, and Wisconsin Power and Light Company, or WPL. We also have non-utility operations. Our utility business is engaged principally in:

•

the generation and distribution of electricity to retail customers in select markets in Iowa and Wisconsin;

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the distribution and transportation of natural gas to retail customers in select markets in Iowa and Wisconsin;

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the sale of electricity to wholesale customers in Minnesota, Illinois, Iowa and Wisconsin; and

•

the generation and distribution of steam for two customers in Cedar Rapids, Iowa and various other energy-related products and services.

We are a “holding company” under the Public Utility Holding Company Act of 2005 and are subject to regulation by the Federal Energy Regulatory Commission, or the FERC, under that Act.

Our principal executive offices are located at 4902 North Biltmore Lane, Madison, Wisconsin 53718, and our telephone number is (608) 458-3311.

RISK FACTORS

Investing in our securities involves risk. You should carefully consider the specific risks discussed or incorporated by reference in the applicable prospectus supplement, together with all the other information contained in the prospectus supplement or incorporated by reference in this prospectus. You should also consider the risks and uncertainties discussed under the caption “Risk Factors” included in our Annual Report on Form 10-K for the year ended December 31, 2022 and our subsequent Quarterly Reports on Form 10-Q, which are incorporated by reference in this prospectus, and which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future.

USE OF PROCEEDS

Unless we inform you otherwise in the applicable prospectus supplement, we intend to use the net proceeds from the sale of the securities for general corporate purposes, which may include repayment or refinancing of debt, acquisitions, working capital, capital expenditures, investments and repurchases and redemptions of securities. Net proceeds may be temporarily invested prior to use.

DESCRIPTION OF COMMON STOCK

The following description of our common stock summarizes material terms and provisions that apply to our common stock. The summary may not contain all of the information that is important to you and is subject to and qualified in its entirety by reference to our Restated Articles of Incorporation, as amended (our “Charter”), and our Amended and Restated Bylaws (our “Bylaws”), each of which is filed as an exhibit to the registration statement of which this prospectus is a part. See “Where You Can Find More Information.”

General

Our Charter provides that we have authority to issue 480,000,000 shares of common stock, $0.01 par value per share. We do not have the authority under our Charter to issue shares of preferred stock.

Common Stock

Holders of our common stock are entitled to such dividends as our board of directors may declare from time to time in accordance with applicable law. Our ability to pay dividends is dependent upon a number of factors, including the ability of our subsidiaries to pay dividends. Our utility subsidiaries each have restrictions on the payment of dividends on their common stock based on the regulatory restrictions applicable to them, and IPL has restrictions on the payment of dividends on its common stock based on the terms of its outstanding preferred stock.

Holders of our common stock are entitled to one vote per share of common stock held by them on all matters properly submitted to a vote of shareowners, subject to Section 180.1150 of the Wisconsin Business Corporation Law. See “—Anti-Takeover Effects of Various Provisions of Wisconsin Law, Our Charter and Our Bylaws.” Holders of our common stock will be entitled to vote for the election of members to our board of directors and on all other matters. Unless otherwise provided in our Charter, each director to be elected shall be elected by a plurality of the votes cast by the common stock shares entitled to vote in the election of directors at an annual or special meeting of shareowners at which a quorum is present. Shareowners have no cumulative voting rights, which means that the holders of shares entitled to exercise more than 50% of the voting power are able to elect all of the directors to be elected. Our board of directors is divided into three classes, with staggered terms of three years each. Action on all other matters besides election of directors shall be approved if the votes cast within the voting group favoring the action exceed the votes cast opposing the action, unless our Charter or the Wisconsin Business Corporation Law requires a greater number of affirmative votes. All shares of common stock are entitled to participate equally in distributions in liquidation. Holders of common stock have no preemptive rights to subscribe for or purchase our shares. There are no conversion rights, sinking fund or redemption provisions applicable to our common stock.

Anti-Takeover Effects of Various Provisions of Wisconsin Law, Our Charter and Our Bylaws

Provisions of our Charter, our Bylaws and Wisconsin law may have an anti-takeover effect and may delay, defer or prevent a tender offer or takeover attempt that a shareowner might consider in his, her or its best interest, including an attempt that might result in a shareowner’s receipt of a premium over the market price for his, her or its shares. These provisions are also designed, in part, to encourage persons seeking to acquire control of the Company to first negotiate with our board of directors, which could result in an improvement of their terms.

Wisconsin Law

Section 196.795(3) of the Wisconsin Statutes provides that no person may take, hold or acquire, directly or indirectly, more than 10% of the outstanding voting securities of a public utility holding company, such as us, without the approval of the Public Service Commission of Wisconsin, or the PSCW.

Section 180.1150 of the Wisconsin Business Corporation Law provides that the voting power of shares of public Wisconsin corporations, such as us, held by any person or persons acting as a group in excess of 20% of the corporation’s voting power is limited to 10% of the full voting power of those shares, unless full voting power of those shares has been restored pursuant to a vote of shareowners. Sections 180.1140 to 180.1144 of the Wisconsin Business Corporation Law contain some limitations and special voting provisions applicable to specified business combinations involving Wisconsin corporations, such as us, and a significant shareowner, unless the board of directors of the corporation approves either the business combination or the purchase of shares that causes the shareowner to become a significant shareowner before such purchase occurs.

Similarly, Sections 180.1130 to 180.1133 of the Wisconsin Business Corporation Law contain special voting provisions applicable to some business combinations, unless specified minimum price and procedural requirements are met. Following commencement of a takeover offer, Section 180.1134 of the Wisconsin Business Corporation Law imposes special voting requirements on share repurchases effected at a premium to the market and on asset sales by the corporation, unless, as it relates to the potential sale of assets, the corporation has at least three independent directors and a majority of the independent directors vote not to have the provision apply to the corporation.

Shareowner Meetings; Requirements for Advance Notice of Shareowner Nominations and Proposals

Our Bylaws establish a procedure that shareowners seeking to call a special meeting of shareowners must satisfy. This procedure involves notice to us, the receipt by us of written demands for a special meeting from holders of our common stock representing at least 10% of the votes entitled to be cast on any issue proposed to be considered, a review of the validity of such demands by an independent inspector appointed by us and the fixing of the record and meeting dates by our board of directors. In addition, shareowners demanding such a special meeting must deliver to us a written agreement to pay the costs incurred by us in holding a special meeting, including the costs of preparing and mailing the proxy materials for our solicitation of proxies for use at such meeting, in the event such shareowners are unsuccessful in their proxy solicitation.

Our Bylaws also provide our board of directors with discretion in postponing shareowner meetings, including, within certain limits, special meetings of shareowners. Additionally, our chairman or board of directors (acting by resolution) may adjourn a shareowner meeting at any time prior to the transaction of business at such meeting, within certain limits.

Our Bylaws also contain strict time deadlines and procedures applicable to shareowners seeking to nominate a person for election as a director or to otherwise bring business before a meeting. These provisions may have the effect of precluding the conduct of certain business at a meeting if the proper procedures are not followed. To bring business before an annual meeting, a shareowner must give written notice that complies with our Bylaws to our Corporate Secretary not earlier than the close of business on the 120th day and not later than the close of business on the 90th day prior to the first anniversary of the preceding year’s annual meeting.

DESCRIPTION OF DEBT SECURITIES

The following description of the debt securities, or notes, sets forth certain material terms and provisions of the notes to which any prospectus supplement may relate. The following description does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the form of senior note indenture, as it may be supplemented and amended from time to time, which has been filed as an exhibit to the registration statement of which this prospectus is a part. The terms of the notes will include those stated in the indenture and those made a part of the indenture by reference to the Trust Indenture Act of 1939, as amended, which we refer to as the 1939 Act.

The particular terms of the notes offered by any prospectus supplement and the extent, if any, to which the provisions described in this prospectus may apply to the offered notes will be described in the prospectus supplement relating to the offered notes. Accordingly, for a description of the terms of a particular issue of notes, reference must be made to both the prospectus supplement relating thereto and to the following description.

The notes will be issued as unsecured debt securities under the indenture and will rank equally with all of our other unsecured and unsubordinated debt. The notes will be effectively subordinated to all of our secured debt. The indenture does not limit the aggregate principal amount of notes that may be issued under the indenture and provides that notes may be issued from time to time in one or more series pursuant to the terms of one or more supplemental indentures, board resolutions or officer’s certificates pursuant to a supplemental indenture or a board resolution. The indenture gives us the ability to reopen a previous issue of notes and issue additional notes of such series, unless otherwise provided.

Provisions of a Particular Series

The prospectus supplement applicable to each series of notes will specify, among other things:

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the title of such notes;

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any limit on the aggregate principal amount of such notes which may be authenticated and delivered under the indenture;

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the person to whom interest on a note of the series shall be payable if other than the person in whose name that note is registered at the close of business on the regular record date for such interest;

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the date or dates on which the principal of such notes is payable or the method or means by which those dates will be determined, and our right, if any, to extend those dates and the duration of any such extension;

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the rate or rates at which such notes shall bear interest, if any, or any method by which such rate or rates will be determined, the date or dates from which such interest will accrue, the interest payment dates on which such interest shall be payable, the regular record date for the interest payable on any interest payment date, and our right, if any, to extend the interest payment periods and the duration of any such extension;

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the place or places where the principal of, premium, if any, and interest, if any, on such notes shall be payable;

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the methods by which registration of transfer of such notes and exchanges of such notes may be effected, and by which notices and demands to or upon us in respect of such notes and the indenture may be made, given, furnished, filed or served;

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the note registrar and paying agent or agents for such notes and, if such is the case, a statement that the principal of such notes will be payable without presentment or surrender of the notes;

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if the time for giving redemption notices for such notes is different from that provided in the indenture, such different time, and the period or periods within which, or date or dates on which, the price or prices at which and the terms and conditions upon which such notes may be redeemed, in whole or in part, at our option, as well as any restrictions on such redemptions;

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our obligation(s), if any, to redeem, purchase or repay such notes pursuant to any sinking fund or analogous provision or at the option of a holder and the terms and conditions upon which such notes will be so redeemed, purchased or repaid, as well as any applicable exceptions to the indenture provisions relating to notices of redemption in the case of mandatory redemption or redemption or repayment at the option of a holder;

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the denominations in which such notes shall be issuable;

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the currency or currencies, including composite currencies, in which the principal, premium, if any, and interest, if any, on such notes will be payable if other than U.S. dollars and the method for determining the equivalent amount in U.S. dollars;

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if the principal, premium, if any, or interest, if any, on such notes are to be payable, at our election or at the election of a holder, in a coin or currency other than that in which such notes are stated to be payable: (i) the coin or currency in which any such payment will be payable and (ii) the period(s) within which, and the terms upon which, such election may be made;

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if the principal, premium, if any, or interest, if any, on such notes are to be payable, or are to be payable at our election or at the election of a holder, in securities or other property, the type and amount of such securities or other property, or the method by which such amount shall be determined, and the period(s) within which, and terms upon which, such election may be made;

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if the amount payable in respect of principal of or any premium or interest on any notes may be determined with reference to an index or formula, the manner in which such amount will be determined;

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any deletions from, modifications of or additions to the events of default or covenants as provided in the indenture pertaining to such notes, and any change in the rights of the trustee or holders pursuant to a supplemental indenture;

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any additions to the definitions set forth in the indenture with respect to such notes;

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whether such notes shall be issued in whole or in part in the form of a global security or securities; the terms and conditions, if any, upon which such global security or securities may be exchanged for certificated notes of such series and of like tenor of any authorized denomination and the circumstances under which such exchange may occur, if other than as provided in the indenture; the depositary for such global security or securities if other than The Depository Trust Company; and the form of any legend(s) to be borne by any such global security or securities in addition to or in lieu of the legend provided in the indenture;

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any limitations on the rights of the holders of such notes to transfer or exchange such notes or to obtain registration of transfer of such notes; and if a service charge will be made for the registration of transfer or exchange of such notes, the amount or terms of the service charge;

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any restriction or condition on the transferability of such notes;

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if other than the entire principal amount, the portion of the principal amount of such notes payable upon declaration of acceleration of maturity;

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the terms, if any, pursuant to which such notes may be converted into or exchanged for our or another person’s shares of capital stock or other securities;

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if such notes are denominated in a currency other than U.S. dollars or in a composite currency, the obligations or instruments, if any, which may be deposited in connection with a satisfaction and discharge, and any additional or alternative provisions for the reinstatement of our indebtedness in respect of such notes after satisfaction and discharge as provided in the indenture;

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any exception to the indenture provisions relating to legal holidays or variation in the definition of “business day” with respect to such notes;

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any non-applicability of the limitation on liens contained in the indenture to the notes of such series or any exceptions or modifications to the limitation on liens with respect to such notes; and

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any other terms of such notes.

The indenture does not contain provisions that afford holders of notes protection in the event of a highly leveraged transaction involving us.

Some of the notes may be issued as original issue discount notes. Such notes are sold at a discount below their stated principal amount. If we issue these securities, the prospectus supplement will describe any special tax, accounting or other information which we think is important. We encourage you to consult with your own tax and financial advisors on these important matters.

Registration and Transfer

We will not be required to (i) issue, register the transfer of or exchange notes of any series during a period of 15 days immediately preceding the date notice is given identifying the notes of such series called for redemption, or (ii) issue, register the transfer of or exchange any notes so selected for redemption, in whole or in part, except the unredeemed portion of any note being redeemed in part.

Payment and Paying Agent

Unless otherwise indicated in an applicable prospectus supplement, payment of principal of any notes will be made only against surrender to the paying agent of such notes. Principal of and interest on notes will be payable, subject to any applicable laws and regulations, at the office of such paying agent or paying agents as we may designate from time to time. Payment of interest on notes on any interest payment date will be made to the person in whose name the notes (or predecessor security) are registered at the close of business on the record date for such interest payment.

Unless otherwise indicated in an applicable prospectus supplement, the trustee will act as paying agent with respect to the notes. We may at any time designate additional paying agents or rescind the designation of any paying agents or approve a change in the office through which any paying agent acts.

All moneys paid by us to a paying agent for the payment of the principal of (and premium, if any) or interest on the notes of any series which remain unclaimed at the end of two years after such principal or interest shall have become due and payable will be repaid to us, upon request and subject to applicable abandoned property laws, and the holder of such notes will from that time forward look only to us for payment of such principal, premium and interest.

Limitation on Liens

Unless otherwise specified in a prospectus supplement for notes of a particular series, the following covenant will apply to the notes of that series.

So long as any notes remain outstanding, we will not secure any indebtedness with a lien on any shares of the capital stock of any of our significant subsidiaries, which shares of capital stock we directly or indirectly own from the date of the indenture or thereafter, unless we equally and ratably secure all notes. However, this restriction does not apply to or prevent:

(1)

(i) liens upon capital stock later acquired, directly or indirectly, by us to secure (A) the purchase price of such capital stock or (B) indebtedness incurred solely for the purpose of financing the acquisition of any such capital stock, (ii) liens existing on any such capital stock at the time of acquisition, and (iii) extensions, renewals or replacements of any of the foregoing, provided that in connection with clause (iii), the principal amount of indebtedness so secured shall be for the same or a lesser principal amount of the indebtedness secured by the lien and no such lien shall extend to or cover any capital stock other than the capital stock being acquired or to more than the same proportion of all shares of capital stock as was covered by the lien that was extended, renewed or replaced; or

(2)

attachment, judgment or other similar liens arising in connection with court proceedings, provided that the execution or other enforcement of such liens is effectively stayed and (i) the claims secured by the lien are being actively contested in good faith by appropriate proceedings or (ii) payment of the claims is covered in full (subject to customary deductible amounts) by insurance maintained with responsible insurance companies.

Liens on any shares of the capital stock of any of our significant subsidiaries to secure any indebtedness, other than liens described in (1) and (2) above, are referred to in this prospectus as “restricted liens.” The foregoing limitations do not apply to the extent that we create any restricted liens to secure indebtedness that, together with all of our other indebtedness secured by restricted liens, does not at the time exceed 10% of our consolidated net tangible assets, as determined by us as of a month end not more than 90 days prior to the closing or consummation of the proposed transaction.

For this purpose, “indebtedness” applied to any person means (i) any liability of such person (A) for borrowed money, or under any reimbursement obligation relating to a letter of credit (other than trade letters of credit) issued to support indebtedness or obligations of such person or others of the kinds referred to in this definition, or (B) evidenced by a bond, note, debenture or similar instrument, or (C) for payment obligations arising under any conditional sale or other title retention

arrangement (including a purchase money obligation) given in connection with the acquisition of any businesses, properties or assets of any kind, or (D) for the payment of money relating to a capitalized lease obligation; (ii) any liability of others described in the preceding clause (i) that such person has guaranteed or that is otherwise its legal liability; and (iii) any amendment, supplement, modification, deferral, renewal, extension or refunding of any liability of the types referred to in clauses (i) and (ii) above.

For this purpose, “consolidated net tangible assets” means the total amount of our assets determined on a consolidated basis in accordance with generally accepted accounting principles, or GAAP, less (i) the sum of our consolidated current liabilities determined in accordance with GAAP and (ii) the amount of our consolidated assets classified as intangible assets determined in accordance with GAAP, including but not limited to, such items as goodwill, trademarks, trade names, patents, and unamortized debt discount and expense and regulatory assets carried as an asset on our consolidated balance sheet.

For this purpose, “significant subsidiary” means any direct, majority owned subsidiary of us that is a “significant subsidiary” as defined in Regulation S-X promulgated by the SEC.

Regulations prohibit or restrict the encumbrance or pledge of public utility assets for the benefit of an associated company. Any pledge of WPL’s common stock to secure the notes could require approval of the FERC or the PSCW. Any pledge of IPL’s common stock to secure the notes could require approval of the FERC. Even with a valid pledge of IPL’s or WPL’s common stock, foreclosure under the indenture may be subject to applicable regulatory requirements, including approval by each of the FERC or the PSCW, if foreclosure or the sale of the pledged IPL or WPL common stock may constitute a transfer of control of IPL or WPL. Applicable law gives each of the FERC and the PSCW broad discretion to define “control” for these purposes, and any such determination would depend upon the facts and circumstances existing at the time. Accordingly, the ability to foreclose on and dispose of IPL or WPL common stock may be restricted or delayed by applicable regulatory requirements.

Consolidation, Merger and Sale

We may not consolidate with or merge into any other corporation or convey, transfer or lease our properties and assets substantially as an entirety to any person, unless:

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the corporation formed by such consolidation or into which we are merged or the person that acquires by conveyance or transfer, or that leases, our properties and assets substantially as an entirety, is a corporation organized and existing under the laws of the United States, any State in the United States or the District of Columbia and such other corporation or person expressly assumes, by supplemental indenture executed and delivered to the trustee, the payment of the principal of (and premium, if any) and interest on all the notes and the performance of every covenant of the indenture on our part to be performed or observed;

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immediately after giving effect to such transactions, no event of default, and no event that, after notice or lapse of time or both, would become an event of default, shall have happened and be continuing; and

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we have delivered to the trustee an officer’s certificate and an opinion of counsel, each stating that such transaction complies with the provisions of the indenture governing consolidation, merger, conveyance, transfer or lease and that all conditions precedent to the transaction have been complied with.

Notwithstanding the foregoing, we may merge or consolidate with or transfer all or substantially all of our assets to an affiliate that has no significant assets or liabilities and was formed solely for the purpose of changing our jurisdiction of organization or our form of organization; provided that the amount of our indebtedness is not increased; and provided, further that the successor assumes all of our obligations under the indenture.

Modification

The indenture contains provisions permitting us and the trustee, with the consent of the holders of not less than a majority in aggregate principal amount of the outstanding notes of each series that is affected, to modify the indenture or the rights of the holders of the notes of such series; provided that no such modification may, without the consent of the holder of each outstanding note that is affected:

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change the stated maturity of the principal of, or any installment of principal of or interest on, any note, or reduce the principal amount of any note or the rate of interest on any note or any premium payable upon the redemption of

any note, or change the method of calculating the rate of interest of any note, or impair the right to institute suit for the enforcement of any such payment on or after the stated maturity of any note (or, in the case of redemption, on or after the redemption date); or

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reduce the percentage of principal amount of the outstanding notes of any series, the consent of whose holders is required for any such supplemental indenture, or the consent of whose holders is required for any waiver (of compliance with certain provisions of the indenture or certain defaults under the indenture and their consequences) provided for in the indenture; or

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modify any of the provisions of the indenture relating to supplemental indentures requiring the consent of holders, waiver of past defaults, or waiver of certain covenants, except to increase any percentage vote required for such an amendment or supplemental indenture or to provide that certain other provisions of the indenture cannot be modified or waived without the consent of each affected holder of an outstanding note.

In addition, we and the trustee may execute, without the consent of any holders of notes, any supplemental indenture for certain other customary purposes, including the creation of any new series of notes.

Events of Default

The indenture provides that any one or more of the following described events with respect to the notes of any series, which has occurred and is continuing, constitutes an “event of default” with respect to the notes of such series:

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failure for 30 days to pay interest on the notes of such series, when due on an interest payment date other than at maturity or upon earlier redemption; or

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failure to pay principal or premium, if any, or interest on the notes of such series when due at maturity or upon earlier redemption; or

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failure for 30 business days to deposit any sinking fund payment when due by the terms of a note of such series; or

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failure to observe or perform any other covenant or warranty of ours in the indenture (other than a covenant or warranty which has expressly been included in the indenture solely for the benefit of one or more series of notes other than such series) for 90 days after written notice to us from the trustee or the holders of at least 33% in principal amount of the outstanding notes of such series; or

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certain events of bankruptcy, insolvency or reorganization of us; or

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any other event of default specified by us pursuant to the indenture with respect to notes of that series.

The holders of not less than a majority in aggregate outstanding principal amount of the notes of any series have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee with respect to the notes of such series. If an event of default occurs and is continuing with respect to the notes of any series, then the trustee or the holders of not less than 33% in aggregate outstanding principal amount of the notes of such series may declare the principal amount of the notes due and payable immediately by notice in writing to us (and to the trustee if given by the holders), and upon any such declaration such principal amount shall become immediately due and payable. At any time after such a declaration of acceleration with respect to the notes of any series has been made and before a judgment or decree for payment of the money due has been obtained as provided in the indenture, the holders of not less than a majority in aggregate outstanding principal amount of the notes of such series may rescind and annul such declaration and its consequences if the default has been cured or waived and we have paid or deposited with the trustee a sum sufficient to pay all matured installments of interest and principal (and premium, if any) due otherwise than by acceleration and all sums paid or advanced by the trustee, including reasonable compensation and expenses of the trustee.

The holders of not less than a majority in aggregate outstanding principal amount of the notes of any series, on behalf of the holders of all the notes of such series, may waive any past default with respect to such series, except (i) a default in the payment of principal (or premium, if any) or interest or (ii) a default in respect of a covenant or provision which as provided under the indenture cannot be modified or amended without the consent of the holder of each outstanding note of such series affected.

Satisfaction and Discharge

We will be discharged from our obligations on the notes of any series, or any portion of the principal amount of the notes of any series, if we irrevocably deposit with the trustee or any paying agent (other than us) sufficient cash or eligible

obligations (or a combination of both) to pay the principal, or portion of principal, premium, if any, and interest, if any, due and to become due, and deliver to the trustee or such paying agent:

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if such deposit shall have been made prior to the maturity of the notes of such series, a company order stating that the money and eligible obligations shall be held in trust and deposited in accordance with the indenture; and

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if eligible obligations have been deposited, an opinion of counsel that the obligations so deposited constitute eligible obligations and do not contain provisions permitting the redemption or other prepayment at the option of the issuer thereof and an opinion of an independent public accountant to the effect that the eligible obligations will be sufficient in accordance with the indenture; and

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if such deposit shall have been made prior to the maturity of the notes of the series, (i) an officer’s certificate stating our intention that, upon delivery of the officer’s certificate, our indebtedness in respect of those notes, or the portions thereof, will have been satisfied and discharged as contemplated in the indenture, and (ii) if such deposit shall have been made prior to the maturity of the notes of the series, an opinion of counsel to the effect that, as a result of a change in law or a ruling of the U.S. Internal Revenue Service issued after the date of issuance of such notes, the holders of the notes of the series, or portions thereof, will not recognize income, gain or loss for U.S. federal income tax purposes as a result of the satisfaction and discharge of our indebtedness and will be subject to U.S. federal income tax on the same amounts, at the same times and in the same manner as if we had not so satisfied and discharged our indebtedness.

For this purpose, “eligible obligations” for U.S. dollar-denominated notes, means securities that are direct obligations of, or obligations the full and timely payment of which are unconditionally guaranteed by, the United States, entitled to the benefit of the full faith and credit thereof, or depository receipts issued by a bank as custodian with respect to these obligations or any specific interest, principal or other payments due in respect thereof held by the custodian in accordance with the indenture for the account of the holder of a depository receipt.

In the event that all of the conditions set forth above have been satisfied for any series of notes, or portions thereof, except that, for any reason, we have not delivered the officer’s certificate and opinion of counsel described in the second and third bulleted items above, the holders of those notes will no longer be entitled to the benefits of certain of our covenants under the indenture, including the covenant described above in “– Limitation on Liens.” Our indebtedness under those notes, however, will not be deemed to have been satisfied and discharged prior to maturity, and the holders of those notes may continue to look to us for payment of the indebtedness represented by those notes.

The indenture will be deemed satisfied and discharged when no notes remain outstanding and we have paid all other sums payable by us under the indenture and delivered an officer’s certificate and opinion of counsel to the trustee stating that the conditions to satisfaction and discharge contained in the indenture have been met. All moneys we pay to the trustee or any paying agent on notes (or hold in trust for payment thereof) which remain unclaimed at the end of two years after payments have become due will be paid to us (or, discharged if so held in trust) upon request, subject to certain notice provisions and applicable abandoned property laws. Thereafter, the holder of those notes may look only to us for payment and not the trustee or any paying agent.

Information Concerning the Trustee

The trustee, prior to an event of default with respect to notes of any series, undertakes to perform, with respect to notes of such series, only such duties as are specifically set forth in the indenture and, in case an event of default with respect to notes of any series has occurred and is continuing, shall exercise, with respect to notes of such series, such of the rights and powers vested in it by the indenture, and use the same degree of care and skill as a prudent individual would exercise in the conduct of his or her own affairs. Subject to such provision, the trustee is under no obligation to exercise any of the powers vested in it by the indenture at the request of any holder of notes of any series, unless offered reasonable security and indemnity by such holder against the costs, expenses and liabilities which might be incurred by the trustee. The trustee is not required to expend or risk its own funds or otherwise incur any financial liability in the performance of its duties if the trustee reasonably believes that repayment or adequate indemnity is not reasonably assured to it. The trustee may decline to follow any such direction from a holder if, among other reasons, the trustee determines in good faith that the actions or proceedings as directed may not lawfully be taken, would involve the trustee in personal liability or would be unduly prejudicial to the holders of the notes of such series not joining in such direction.

Pursuant to the 1939 Act, if a default occurs with respect to notes of any series, the trustee may be required to resign as trustee under the indenture if it has a conflicting interest (as defined in the 1939 Act), unless the default is cured, duly waived or otherwise eliminated within 90 days.

We and certain of our subsidiaries may maintain deposit accounts and banking relationships with the trustee. The trustee and certain of its affiliates may also serve as trustee under other indentures pursuant to which our securities and certain of our subsidiaries are outstanding.

Governing Law

The indenture and the notes will be governed by, and construed in accordance with, the internal laws of the State of New York.

Global Securities

We will issue the notes in whole or in part in the form of one or more global certificates, which we refer to as global securities. We will deposit the global securities with or on behalf of The Depository Trust Company, which we refer to as DTC, and registered in the name of Cede & Co., as nominee of DTC.

We have provided the following descriptions of the operations and procedures of DTC solely as a matter of convenience. These operations and procedures are solely within the control of DTC and are subject to change by them from time to time. Neither we, any underwriter nor the trustee take any responsibility for these operations or procedures, and you are urged to contact DTC or its participants directly to discuss these matters.

DTC has advised us that:

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DTC is a limited-purpose trust company organized under the New York Banking Law, a “banking organization” within the meaning of the New York Banking Law, a member of the Federal Reserve System, a “clearing corporation” within the meaning of the New York Uniform Commercial Code and a “clearing agency” registered under Section 17A of the Securities Exchange Act of 1934, as amended, or the Exchange Act;

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DTC holds securities that its direct participants deposit with DTC and facilitates the settlement among direct participants of securities transactions, such as transfers and pledges, in deposited securities through electronic computerized book-entry changes in direct participants’ accounts, thereby eliminating the need for physical movement of securities certificates;

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direct participants include securities brokers and dealers, trust companies, clearing corporations and other organizations;

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DTC is a wholly-owned subsidiary of The Depository Trust & Clearing Corporation, which is owned by the users of its regulated subsidiaries;

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access to the DTC system is also available to indirect participants such as securities brokers and dealers, banks and trust companies that clear through or maintain a custodial relationship with a direct participant, either directly or indirectly; and

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the rules applicable to DTC and its direct and indirect participants are on file with the SEC.

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We expect that under procedures established by DTC:

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upon deposit of the global securities with DTC or its custodian, DTC will credit on its internal system the accounts of direct participants designated by the underwriters with portions of the principal amounts of the global securities; and

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ownership of the notes will be shown on, and the transfer of ownership of the notes will be effected only through, records maintained by DTC or its nominee, with respect to interests of direct participants, and the records of direct and indirect participants, with respect to interests of persons other than participants.

The laws of some jurisdictions require that purchasers of securities take physical delivery of those securities in the form of a certificate. For that reason, it may not be possible to transfer interests in a global security to those persons. In addition,

because DTC can act only on behalf of its participants, who in turn act on behalf of persons who hold interests through participants, the ability of a person having an interest in a global security to pledge or transfer that interest to persons or entities that do not participate in DTC’s system, or otherwise to take actions in respect of that interest, may be affected by the lack of a physical definitive security in respect of that interest.

So long as DTC or its nominee is the registered owner of a global security, DTC or that nominee will be considered the sole owner or holder of the notes represented by that global security for all purposes under the applicable indenture and under the notes. Except as described below, owners of beneficial interests in a global security will not be entitled to have notes represented by that global security registered in their names, will not receive or be entitled to receive the notes in the form of a physical certificate and will not be considered the owners or holders of the notes under the applicable indenture or under the notes, and may not be entitled to give the trustee directions, instructions or approvals. For that reason, each holder owning a beneficial interest in a global security must rely on DTC’s procedures and, if that holder is not a direct or indirect participant in DTC, on the procedures of the DTC participant through which that holder owns its interest, to exercise any rights of a holder of notes under the applicable indenture or the global security.

Neither we nor the trustee will have any responsibility or liability for any aspect of DTC’s records relating to the notes or relating to payments made by DTC on account of the notes, or any responsibility to maintain, supervise or review any of DTC’s records relating to the notes.

We will make payments (including principal, premium, if any, and interest) on the notes represented by the global securities to DTC or its nominee, as the registered owner of the notes, by wire transfer of immediately available funds to the accounts specified by the registered owner of the notes. We expect that when DTC or its nominee receives any payment on the notes represented by a global security, DTC will credit participants’ accounts with payments in amounts proportionate to their beneficial interests in the global security as shown in DTC’s records. We also expect that payments by DTC’s participants to owners of beneficial interests in the global security held through those participants will be governed by standing instructions and customary practice as is now the case with securities held for the accounts of customers registered in the names of nominees for such customers. DTC’s participants will be responsible for those payments.

Initial settlement for the notes will be made in immediately available funds. Transfers between participants in DTC will be effected in accordance with DTC’s procedures. The notes represented by the global securities are expected to be eligible to trade in DTC’s Same-Day Funds Settlement System, and any permitted secondary market trading activity in such notes will, therefore, be required by DTC to be settled in immediately available funds.

Certificated Notes

We will issue certificated notes to each person that DTC identifies as the beneficial owner of notes represented by the global securities upon surrender by DTC of the global securities only if:

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DTC notifies us that it is no longer willing or able to act as a depository for the global securities, and we have not appointed a successor depository within 90 days of that notice;

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an event of default with respect to the notes has occurred and is continuing; or

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we decide not to have the notes represented by a global security.

Neither we nor the trustee will be liable for any delay by DTC, its nominee or any direct or indirect participant in identifying the beneficial owners of the related notes. We and the trustee may conclusively rely on, and will be protected in relying on, instructions from DTC or its nominee, including instructions about the registration and delivery, and the respective principal amounts, of the notes to be issued.

We will make all payments of principal, interest and premium, if any, with respect to certificated notes by wire transfer of immediately available funds to the accounts specified by the holders of the certificated notes or, if no such account is specified, by mailing a check to each such holder’s registered address. We expect that secondary trading in any certificated notes will also be settled in immediately available funds.

DESCRIPTION OF WARRANTS

The following summary of certain provisions of the warrants does not purport to be complete and is subject to, and is qualified in its entirety by reference to, all provisions of the applicable warrant agreement.

We may issue warrants for the purchase of debt securities, common stock or other securities. Warrants may be issued independently or together with debt securities, common stock or other securities offered by any prospectus supplement and may be attached to or separate from any such offered securities. Each series of warrants will be issued under a separate warrant agreement to be entered into between us and a bank or trust company, as warrant agent, all as will be set forth in the prospectus supplement relating to the particular issue of warrants. The warrant agent will act solely as our agent in connection with the warrants and will not assume any obligation or relationship of agency or trust for or with any holders of warrants or beneficial owners of warrants.

Reference is made to the prospectus supplement relating to the particular issue of warrants offered pursuant to such prospectus supplement for the terms of and information relating to such warrants, including, where applicable:

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the designation, aggregate principal amount, currencies, denominations and terms of the series of debt securities purchasable upon exercise of warrants to purchase debt securities and the price at which such debt securities may be purchased upon such exercise;

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the number of shares of common stock purchasable upon the exercise of warrants to purchase common stock and the price at which such number of shares of common stock may be purchased upon such exercise;

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the designation and number of units of other securities purchasable upon the exercise of warrants to purchase other securities and the price at which such number of units of such other securities may be purchased upon such exercise;

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the date on which the right to exercise such warrants will commence and the date on which such right will expire;

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U.S. federal income tax consequences applicable to such warrants;

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the number of warrants outstanding as of the most recent practicable date; and

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any other terms of such warrants.

Warrants will be issued in registered form only. The exercise price for warrants will be subject to adjustment in accordance with provisions described in the applicable prospectus supplement.

Each warrant will entitle the holder thereof to purchase such principal amount of debt securities or such number of shares of common stock or other securities at such exercise price as will in each case be set forth in, or calculable from, the prospectus supplement relating to the warrants, which exercise price may be subject to adjustment upon the occurrence of certain events as set forth in such prospectus supplement. After the close of business on the expiration date, or such later date to which such expiration date may be extended by us, unexercised warrants will become void. The place or places where, and the manner in which, warrants may be exercised will be specified in the prospectus supplement.

Prior to the exercise of any warrants to purchase debt securities, common stock or other securities, holders of such warrants will not have any of the rights of holders of debt securities, common stock or other securities, as the case may be, purchasable upon such exercise, including the right to receive payments of principal of, premium, if any, or interest, if any, on the debt securities purchasable upon such exercise or to enforce covenants in the applicable indenture, or to receive payments of dividends, if any, on the common stock purchasable upon such exercise, or to exercise any applicable right to vote.

DESCRIPTION OF STOCK PURCHASE CONTRACTS AND STOCK PURCHASE UNITS

The following summary of certain provisions of the stock purchase contracts and stock purchase units does not purport to be complete and is subject to, and is qualified in its entirety by reference to, all provisions of the applicable stock purchase contracts or stock purchase units.

We may issue stock purchase contracts, including contracts obligating holders to purchase from us, and obligating us to sell to the holders, a specified number of shares of common stock or other securities at a future date or dates. The price per security and the number of securities may be fixed at the time the stock purchase contracts are issued or may be determined by reference to a specific formula set forth in the stock purchase contracts. The stock purchase contracts may be issued separately or as part of stock purchase units consisting of a stock purchase contract and debt securities, warrants, other securities or debt obligations of third parties, including U.S. treasury securities, securing the holders’ obligations to purchase the securities under the stock purchase contracts. The stock purchase contracts may require holders to secure their obligations under the stock purchase contracts in a specified manner. The stock purchase contracts also may require us to make periodic payments to the holders of the stock purchase units or vice versa, and those payments may be unsecured or refunded on some basis.

The stock purchase contracts, and, if applicable, collateral or depositary arrangements relating to the stock purchase contracts or stock purchase units, will be filed with the SEC in connection with the offering of stock purchase contracts or stock purchase units. The prospectus supplement relating to a particular issue of stock purchase contracts or stock purchase units will describe the terms of those stock purchase contracts or stock purchase units, including the following:

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if applicable, a discussion of material U.S. federal income tax considerations; and

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any other information we think is important about the stock purchase contracts or the stock purchase units.

If we issue stock purchase units where debt obligations of third parties are used as security for your obligations to purchase or sell shares of common stock or other securities, we will include in the prospectus supplement relating to the offering information about the issuer of the debt obligations. Specifically, if the issuer has a class of securities registered under the Exchange Act and is either eligible to register its securities on Form S-3 under the Securities Act of 1933, as amended, or the Securities Act, or meets the listing criteria to be listed on a national securities exchange, we will include a brief description of the business of the issuer, the market price of its securities and how you can obtain more information about the issuer. If the issuer does not meet the criteria described in the previous sentence, we will include substantially all of the information that would be required if the issuer were making a public offering of the debt obligations.

SELLING SHAREOWNERS

We may register shares of common stock covered by this prospectus for re-offers and resales by any selling shareowners to be named in a prospectus supplement. We may register these shares to permit selling shareowners to resell their shares when they deem appropriate. A selling shareowner may resell all, a portion or none of such shareowner’s shares at any time and from time to time. Selling shareowners may also sell, transfer or otherwise dispose of some or all of their shares of our common stock in transactions exempt from the registration requirements of the Securities Act. We do not know when or in what amounts the selling shareowners may offer shares for sale under this prospectus and any prospectus supplement. We will not receive any proceeds from any sale of shares by a selling shareowner under this prospectus and any prospectus supplement. We may pay all expenses incurred with respect to the registration of the shares of common stock owned by the selling shareowners, other than underwriting fees, discounts or commissions which will be borne by the selling shareowners. We will provide you with a prospectus supplement naming the selling shareowners, the amount of shares to be registered and sold and any other terms of the shares of common stock being sold by each selling shareowner.

PLAN OF DISTRIBUTION

We may sell our securities, and any selling shareowner may sell shares of our common stock, in any one or more of the following ways from time to time: (1) through agents; (2) to or through underwriters; (3) through brokers or dealers; (4) in “at the market offerings” within the meaning of Rule 415(a)(4) under the Securities Act, to or through a market maker or into an existing trading market, on an exchange or otherwise; (5) directly by us or any selling shareowners to purchasers, including through a specific bidding, auction or other process; or (6) through a combination of any of these methods of sale. The applicable prospectus supplement will contain the terms of the transaction, name or names of any underwriters, dealers, agents and the respective amounts of securities underwritten or purchased by them, the initial public offering price of the securities, and the applicable agent’s commission, dealer’s purchase price or underwriter’s discount. Any selling shareowners, dealers and agents participating in the distribution of the securities may be deemed to be underwriters, and compensation received by them on resale of the securities may be deemed to be underwriting discounts. Additionally, because selling shareowners may be deemed to be “underwriters” within the meaning of Section 2(11) of the Securities Act, selling shareowners may be subject to the prospectus delivery requirements of the Securities Act.

Any initial offering price, dealer purchase price, discount or commission may be changed from time to time.

The securities may be distributed from time to time in one or more transactions, at negotiated prices, at a fixed price or fixed prices (that may be subject to change), at market prices prevailing at the time of sale, at various prices determined at the time of sale or at prices related to prevailing market prices.

Offers to purchase securities may be solicited directly by us or any selling shareowner or by agents designated by us from time to time. Unless otherwise indicated in the prospectus supplement, any such agent will use its commercially reasonable efforts to solicit purchases for the period of its appointment or to sell securities on a continuing basis. Agents may receive compensation in the form of commissions, discounts or concessions from us. Agents may also receive compensation from the purchasers of the securities for whom they sell as principals. Each particular agent will receive compensation in amounts negotiated in connection with the sale, which might be in excess of customary commissions. Any such agent may be deemed to be an underwriter, as that term is defined in the Securities Act, of the securities so offered and sold. Accordingly, any commission, discount or concession received by them and any profit on the resale of the securities purchased by them may be deemed to be underwriting discounts or commissions under the Securities Act. We have not entered into any agreements, understandings or arrangements with any underwriters or broker-dealers regarding the sale of their securities. As of the date of this prospectus, there are no special selling arrangements between any broker-dealer or other person and us. No period of time has been fixed within which the securities will be offered and sold.

If underwriters are utilized in the sale of any securities in respect of which this prospectus is being delivered, such securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including negotiated transactions, at fixed public offering prices or at varying prices determined by the underwriters at the time of sale. Securities may be offered to the public either through underwriting syndicates represented by managing underwriters or directly by one or more underwriters. If any underwriter or underwriters are utilized in the sale of securities, unless otherwise indicated in the applicable prospectus supplement, the obligations of the underwriters are subject to certain conditions precedent, and the underwriters will be obligated to purchase all such securities if they purchase any of them.

If a dealer is utilized in the sale of the securities in respect of which this prospectus is delivered, we will sell such securities, and any selling shareowner will sell shares of our common stock to the dealer, as principal. The dealer may then resell such securities to the public at varying prices to be determined by such dealer at the time of resale. Transactions through brokers or dealers may include block trades in which brokers or dealers will attempt to sell shares as agent but may position and resell as principal to facilitate the transaction or in cross trades, in which the same broker or dealer acts as agent on both sides of the trade. Any such dealer may be deemed to be an underwriter, as such term is defined in the Securities Act, of the securities so offered and sold. In addition, any selling shareowner may sell shares of our common stock in ordinary brokerage transactions or in transactions in which a broker solicits purchases.

Offers to purchase securities may be solicited directly by us or any selling shareowner, and the sale thereof may be made by us or any selling shareowner, directly to institutional investors or others who may be deemed to be underwriters within the meaning of the Securities Act with respect to any resale thereof.

Agents, underwriters and dealers may be entitled under relevant agreements with us or any selling shareowner to indemnification by us against certain liabilities, including liabilities under the Securities Act, or to contribution with respect to payments which such agents, underwriters and dealers may be required to make in respect thereof. The terms and conditions of any indemnification or contribution will be described in the applicable prospectus supplement.

We may pay all expenses incurred with respect to the registration of the shares of common stock owned by any selling shareowners, other than underwriting fees, discounts or commissions, which will be borne by the selling shareowners. We or any selling shareowner may also sell shares of our common stock through various arrangements involving mandatorily or optionally exchangeable securities, and this prospectus may be delivered in connection with those sales.

We or any selling shareowner may enter into derivative, sale or forward sale transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If the applicable prospectus supplement indicates, in connection with those transactions, the third parties may sell securities covered by this prospectus and the applicable prospectus supplement, including in short sale transactions and by issuing securities not covered by this prospectus but convertible into, exchangeable for or representing beneficial interests in securities covered by this prospectus, or the return of which is derived in whole or in part from the value of such securities. The third parties may use securities received under derivative, sale or forward sale transactions or securities pledged by us or any selling shareowner or borrowed from us, any selling shareowner or others to settle those sales or to close out any related open borrowings of stock, and may use securities received from us or any selling shareowner in settlement of those transactions to close out any related open borrowings of stock. The third party in such sale transactions will be an underwriter and will be identified in the applicable prospectus supplement.

Additionally, certain selling shareowners may engage in hedging transactions with broker-dealers in connection with distributions of shares or otherwise. In those transactions, broker-dealers may engage in short sales of shares in the course of hedging the positions they assume with such selling shareowner. Certain selling shareowners also may sell shares short and redeliver shares to close out such short positions. Certain selling shareowners may also enter into option or other transactions with broker-dealers which require the delivery of shares to the broker-dealer. The broker-dealer may then resell or otherwise transfer such shares pursuant to this prospectus. Certain selling shareowners also may loan or pledge shares, and the borrower or pledgee may sell or otherwise transfer the shares so loaned or pledged pursuant to this prospectus. Such borrower or pledgee also may transfer those shares to investors in our securities or the selling shareowner’s securities or in connection with the offering of other securities not covered by this prospectus.

Underwriters, broker-dealers or agents may receive compensation in the form of commissions, discounts or concessions from us or any selling shareowner. Underwriters, broker-dealers or agents may also receive compensation from the purchasers of shares for whom they act as agents or to whom they sell as principals, or both. Compensation as to a particular underwriter, broker-dealer or agent will be in amounts to be negotiated in connection with transactions involving shares and might be in excess of customary commissions. In effecting sales, broker-dealers engaged by us or any selling shareowner may arrange for other broker-dealers to participate in the resales.

Any securities offered other than common stock will be a new issue and, other than the common stock, which is listed on the Nasdaq, will have no established trading market. We may elect to list any series of securities on an exchange, and in the case of the common stock, on any additional exchange, but, unless otherwise specified in the applicable prospectus supplement and/or other offering material, we shall not be obligated to do so. No assurance can be given as to the liquidity of the trading market for any of the securities.

Agents, underwriters and dealers may engage in transactions with, or perform services for, us or our subsidiaries or any selling shareowner in the ordinary course of business.

Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time. An underwriter may carry out these transactions on the Nasdaq, in the over-the-counter market or otherwise.

The place and time of delivery for securities will be set forth in the accompanying prospectus supplement.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC (File No. 1-9894). We also filed a registration statement on Form S-3, including exhibits, under the Securities Act, with respect to the securities offered by this prospectus. This prospectus is a part of the registration statement, but does not contain all of the information included in the registration statement or the exhibits.

We are “incorporating by reference” specified documents that we file with the SEC, which means:

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incorporated documents are considered part of this prospectus;

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we are disclosing important information to you by referring you to those documents; and

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information we file with the SEC will automatically update and supersede information contained in this prospectus.

We incorporate by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, after the date of this prospectus and before the end of the offering of the securities pursuant to this prospectus:

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our Annual Report on Form 10-K for the year ended December 31, 2022;

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our Quarterly Reports on Form 10-Q for the quarters ended March 31, 2023, June 30, 2023 and September 30, 2023;

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our Current Reports on Form 8-K filed January 4, 2023, January 13, 2023, March 3, 2023, March 13, 2023, March 16, 2023, March 30, 2023, May 25, 2023, July 14, 2023, September 21, 2023, October 26, 2023, and November 13, 2023; and

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the description of our common stock in our Registration Statement on Form 8-A, dated December 28, 2018, as updated by the description of our common stock contained in Exhibit 4.29 to our Annual Report on Form 10-K for the fiscal year ended December 31, 2022, including any amendment or report filed for the purpose of updating such description.

You may obtain copies of documents incorporated by reference in this prospectus, at no cost, by request directed to us at the following address or telephone number:

Alliant Energy Corporation

4902 North Biltmore Lane

Madison, Wisconsin 53718

Attention: Omar N. Chaudhary, Corporate Secretary

(608) 458-3311

Our filings are available to the public over the Internet on the SEC’s website at h ttp://w ww.sec.gov, as well as on our website at w ww.alliantenergy.com. However, we are not incorporating the information on our website other than these filings into this prospectus.

LEGAL MATTERS

Unless otherwise specified in a prospectus supplement accompanying this prospectus, the validity of the securities offered in this prospectus and certain legal matters will be passed upon for us by Perkins Coie LLP. If the securities are being distributed in an underwritten offering, certain legal matters will be passed upon for the underwriters by counsel identified in the related prospectus supplement.

EXPERTS

The consolidated financial statements, and the related financial statement schedules, of Alliant Energy Corporation and subsidiaries incorporated in this prospectus by reference from Alliant Energy Corporation’s Annual Report on Form 10-K and the effectiveness of Alliant Energy Corporation and subsidiaries’ internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports. Such financial statements and financial statement schedules are incorporated by reference in reliance upon the reports of such firm, given their authority as experts in accounting and auditing.

$1,000,000,000

Alliant Energy Corporation

Common Stock

PROSPECTUS SUPPLEMENT

March 19, 2026

Barclays

BofA Securities

Goldman Sachs & Co. LLC

J.P. Morgan

KeyBanc Capital Markets

Mizuho

MUFG

TD Securities

Wells Fargo Securities

SEC Form 424B5 filed by Alliant Energy Corporation