Demonstrates 12 to 17 Hour Duration with Significant RTE Increase

Extends Duration for less than $90/kWh

ESS Tech, Inc. (“ESS” or the “Company”) (NYSE: GWH), a leading manufacturer of iron flow long-duration energy storage (LDES) systems for commercial- and utility-scale applications, achieved significant technical milestones in June far ahead of schedule. These milestones include dramatic improvements from a new material substitution within the core ESS stack technology. The results demonstrate 12.2 hour duration at rated power and 17.8 hour duration at reduced power. The results also demonstrate higher round trip efficiency (RTE), cycling durability, and iron plating robustness, representing an 18-month product roadmap acceleration. This improvement not only increases the value of the iron flow battery faster than expected, but also reduces material costs, accelerating the Company’s cost down objectives. For customers, the RTE increase translates to more energy and lower operating costs with a 6.7% charge time improvement and 6.3% additional input energy per day at standard conditions. Moreover, the increased energy capacity aligns with the market on maximizing clean energy captured and consumed, moving ESS closer to delivering green baseload power, everyday.

These advancements directly support the Company’s strategic pivot to execute on a rigorous and accelerated timeline utilizing agile build principles. “We are in a position where we have to do things better, faster, cheaper. I am pleased to report that our shift in strategy is yielding operational efficiencies with significantly reduced cash requirements and technical results that align with the voice of our customers. We continue to make progress on key customer contracts for our U.S. manufactured, scalable Energy Base^TM solution, which further supports ongoing discussions with potential capital providers and available financing options to execute on our business plan,” said Kelly Goodman, Interim CEO of ESS.

About ESS Tech, Inc.

At ESS (NYSE: GWH), our mission is to accelerate global decarbonization by providing safe, sustainable, long-duration energy storage that powers people, communities and businesses with clean, renewable energy anytime and anywhere it’s needed. As more renewable energy is added to the grid, long- duration energy storage is essential to providing the reliability and resiliency we need when the sun is not shining, and the wind is not blowing.

Our technology uses earth-abundant iron, salt and water to deliver environmentally safe solutions capable of providing 10+ hours of flexible energy capacity for commercial and utility-scale energy storage applications. Established in 2011, ESS enables project developers, independent power producers, utilities and other large energy users to deploy reliable, sustainable long-duration energy storage solutions. For more information visit www.essinc.com.

This release contains certain forward-looking statements, including statements regarding the Company 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” and “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 product performance and technical features, contracts and relationships with third parties, and potential capital raising measures. These forward-looking statements are based on the Company’s current expectations and beliefs concerning future developments and their potential effects on the Company and involve a number of risks, uncertainties (some of which are beyond the Company’s 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, technical advancements, partnerships and customer relationships not resulting in expectant benefits, and the Company’s inability to raise additional capital 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 May 15, 2025, and the Company’s other filings with the U.S. Securities and Exchange Commission. Except as required by law, the Company is not undertaking any obligation to update or revise any forward-looking statements whether as a result of new information, future events or otherwise.

Source: ESS Inc.

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