Agreement Supports Advancement and Real-World Evaluation of ESS's Non-Lithium Long Duration Energy Storage Energy Base Technology

Collaboration Expected to Support Utility and Large-Load Sustainability Objectives, and Clean Energy Goals of Project Participants

ESS Tech, Inc. (NYSE:GWH) (ESS, or the Company), a leading manufacturer of long-duration energy storage (LDES) systems for commercial and utility-scale today applications, today announced a collaboration framework with Salt River Project (SRP) and Google for Project New Horizon to advance long-duration energy storage innovation at SRP's Copper Crossing Energy and Research Center in Florence, Arizona.

Project New Horizon is a five-megawatt (MW), 50 megawatt hour (MWh) system that will deploy ESS' iron flow Energy Base technology and represents a significant commercial validation milestone for the Company's next-generation, utility-scale platform. The pilot is expected to support SRP's evaluation of emerging non-lithium LDES technologies and, if successfully executed, could help unlock follow-on commercial opportunities with utilities and large energy users seeking scalable, LDES solutions.

ESS's Energy Base is designed for utility-scale deployments and differs from prior containerized solutions by enabling flexible configuration across diverse use cases and is expected to deliver cost advantages at larger scale.

The Project New Horizon pilot was awarded through SRP's competitive solicitation process for LDES technologies. Design is underway, with manufacturing expected to begin in 2026 and delivery targeted for December 2027.

Under the agreement, SRP intends to partner with Electric Power Research Institute (EPRI) to support project operational testing under an initial, multi-year research scope, with potential for future work. All tests required per the Energy Storage Agreement (ESA) will be performed by SRP and ESS for the duration of the agreement term, with EPRI providing independent oversight.

Google is a participant in the Copper Crossing project, funding a portion of the project's payments through a cost sharing arrangement with SRP. The collaboration is intended to evaluate real-world operational performance of non-lithium LDES and generate learnings relevant to grid reliability, renewable integration, and broader LDES deployment pathways. SRP and Google have both stated goals to expand clean energy and help make reliable, affordable electricity available around the clock.

"This phase of Project New Horizon represents an important commercial validation of Energy Base and a clear signal that utilities are actively pursuing scalable, non-lithium long-duration storage as part of long-term resource planning," said Drew Buckley, Chief Executive Officer of ESS. "The collaboration with SRP, along with Google's participation and cost sharing, reinforces the relevance of long-duration storage for grid reliability and for large energy users focused on carbon-free energy. Successful execution can serve as a blueprint for broader deployments and meaningful follow-on opportunities."

"Project New Horizon is part of SRP's effort to evaluate the impact of long-duration energy storage technologies that could ultimately support SRP's mission of providing reliable, affordable and sustainable power," said Chico Hunter, Manager, Innovation and Development at SRP. "We look forward to working with ESS, Google, and EPRI to assess performance in real-world conditions and advance our understanding of how long-duration storage can serve future system needs."

"Long-duration energy storage is needed to improve grid resilience and unlock around-the-clock clean energy," said Lucia Tian, Head of Advanced Energy Technologies, at Google. "This collaboration with SRP is intended to accelerate learning and innovation from real-world deployments of a portfolio of emerging long-duration energy storage technologies, including this first pilot with ESS. We're excited to work with a utility partner who shares our commitment to supporting a range of technologies and diverse supply chains that can bring reliable, affordable, and clean energy to Arizona and across the United States."

About ESS Tech, Inc.

ESS (NYSE:GWH) is the leading manufacturer of long-duration iron flow 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 iron, salt, and water, ESS iron flow technology enables 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 certain forward-looking statements, including statements regarding ESS and its management team's expectations, hopes, beliefs, intentions or strategies regarding the future. The words "anticipate," "believe," "continue," "could," "estimate," "expect," "intends," "may," "might," "plan," "possible," "potential," "predict," "project," "should," "will," "would" and similar expressions may identify forward-looking statements, but the absence of these words does not mean that a statement is not forward-looking. Examples of forward-looking statements include, among others, statements regarding the Company's productization, manufacturing and delivery of the Energy Base and relationships with customers, including Salt River Project. 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, general economic and market conditions as well as geopolitical developments and other risks and uncertainties described more fully in the section titled "Risk Factors" in the Company's Quarterly Report on Form 10-Q filed on November 13, 2025, 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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ESS Announces Agreement to Join Salt River Project and Google Long Duration Energy Storage Collaboration