Inclusion Expands Institutional Visibility as ESS Targets High-Growth AI and Energy Markets

ESS Tech, Inc. (NYSE:GWH) ("ESS" or the "company"), a leading provider of non-lithium energy storage solutions, has been added as a member of the Russell Microcap® Index, which was effective upon U.S. market openings on June 29, as part of the first 2026 Russell indexes reconstitution. Inclusion in the Russell Microcap® Index further validates ESS’ strategic focus on addressing rapidly expanding demand for safe, non-lithium energy storage solutions.

"We are pleased to be added to the Russell Microcap® Index at a pivotal moment for ESS," said Drew Buckley, CEO of ESS. "We are accelerating development of our U.S. made, sodium-ion battery platform to meet growing demand from AI infrastructure, data centers, and other high-growth markets, with early-stage opportunities approaching $1 billion, while continuing to advance our iron flow technology for long-duration applications. We believe inclusion in the Russell Index will expand our visibility with institutional investors as we focus on delivering long-term value for our customers and shareholders."

Russell indexes are widely used by investment managers and institutional investors for index funds and as benchmarks for active investment strategies. According to data as of the end of June 2025, about $12.2 trillion in assets are benchmarked against the Russell U.S. Indexes, which belong to FTSE Russell, the global index provider.

The June reconstitution of the Russell US indexes captures up to the 4,000 largest US stocks as of April 30, ranking them by total market capitalization. Membership in the Russell 3000^® Index is re-evaluated twice per year and includes an automatic addition to the large-cap Russell 1000^® Index or small-cap Russell 2000^® Index as well as the appropriate growth and value style indexes. FTSE Russell determines membership for its Russell indexes primarily by objective, market-capitalization rankings and style attributes.

For more information on the Russell Microcap® Index and the Russell indexes reconstitution, go to the "Russell Reconstitution" section on the FTSE Russell website.

About ESS Tech, Inc.

ESS (NYSE:GWH) is the leading provider of non-lithium energy storage solutions. ESS was established in 2011 with a mission to accelerate decarbonization safely and sustainably through longer lasting energy storage. Using easy-to-source materials, ESS solutions enable energy security, reliability and resilience. We build flexible storage solutions that allow our customers to meet increasing energy demand without power disruptions and maximize the value potential of excess energy. For more information visit www.essinc.com.

Forward Looking Statements

This communication contains forward-looking statements (including within the meaning of Section 21E of the Securities Exchange Act of 1934, as amended, and Section 27A of the Securities Act of 1933, as amended) concerning the company and other matters that involve substantial risks and uncertainties. These statements may discuss the management team’s goals, beliefs, hopes, intentions and expectations as to future plans, trends, events, results of operations and financial condition, or otherwise, based on current beliefs of the management of the company, as well as assumptions made by, and information currently available to, the company’s management. These forward-looking statements can be identified by the use of forward-looking terminology, including the words "anticipate," "believe," "continue," "could," "estimate," "expect," "intends," "may," "might," "plan," "possible," "potential," "predict," "project," "should," "will," "would," or, in each case, their negative or other variations or comparable terminology may identify forward-looking statements, but the absence of these words does not mean that a statement is not forward-looking. These forward-looking statements, which are subject to risks, uncertainties and assumptions about us, may include our anticipated growth strategies and anticipated trends in our business. Examples of forward-looking statements include, among others, ESS’ addition to the Russell Microcap® Index as part of the 2026 Russell indexes reconstitution and any resulting expansion of our visibility among institutional investors or broadening of our shareholder base, ESS’ plans for its business, ESS’ potential opportunities that approach $1 billion in sodium-ion opportunities, the demand for non-lithium battery solutions, the timeline for ESS’ development of sodium-ion BESS, statements by ESS’ CEO, and ESS’ ability to grow and our ability to meet the growing demand for energy storage. These forward-looking statements are based on ESS’ current expectations and beliefs concerning future developments and their potential effects on ESS. Many factors could cause actual future events to differ materially from the forward-looking statements in this communication. There can be no assurance that the future developments affecting ESS will be those that we have anticipated. These forward-looking statements involve a number of risks, uncertainties (some of which are beyond ESS control) or other assumptions that may cause actual results or performance to be materially different from those expressed or implied by these forward-looking statements, which include, but are not limited to, the demand for our sodium-ion BESS not developing as anticipated; our strategy to allocate resources toward sodium-ion BESS not achieving the anticipated benefits and adversely affecting the development of our iron flow battery technology; our ability to realize and capitalize on sodium-ion opportunities; delays in the development of our sodium-ion BESS; our ability to expand our portfolio; our ability to execute and meet timelines related to Project New Horizon; our products being in the early stage of commercialization and aspects of our technology not having been fully field tested; our ability to secure binding orders; failure to deliver the benefits offered by our technology; inability to achieve market acceptance of our products; our ability to sell effectively to large customers; failure to accurately estimate future supply and demand for our products and services; failure to manage our growth effectively; failure to meet the obligations under our sales contracts and service agreements; expansions into new markets, product lines or services; our ability to raise capital in the near future; and other risks and uncertainties described more fully in Exhibit 99.2 of the Current Report on Form 8-K filed by the company on June 23, 2026 and the company’s other filings with the U.S. Securities and Exchange Commission. Except as required by law, ESS is not undertaking any obligation to update or revise any forward-looking statements whether as a result of new information, future events or otherwise.

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Company:

investors@essinc.com

Investor Relations:

Chris Tyson

Executive Vice President

MZ Group - MZ North America

(949) 491-8235

GWH@mzgroup.us

www.mzgroup.us

Media:

Brad Dore

VP, Marketing

ESS Tech, Inc.

(916) 207-7355

Brad.Dore@essinc.com