Report from Burbank Water and Power Documents Successful Installation and Operation of ESS Battery system in Real-World Utility Setting

Demonstration Supports Value of Iron Flow Technology for Long-Duration Energy Storage Applications

ESS Tech, Inc. (NYSE:GWH) ("ESS" or the "Company"), a leading manufacturer of long-duration iron flow energy storage systems ("LDES") for commercial and utility-scale applications, highlighted the findings of a final report issued in connection with Burbank Water and Power for the American Public Power Association (APPA) to evaluate the application of ESS' Iron Flow Battery technology in a real-world utility environment.

The report, prepared under the APPA's Demonstration of Energy & Efficiency Developments program, documented the installation, operation, and evaluation of an ESS Energy Warehouse system. According to the report, the system was successfully installed, energized, and operated through the 21-month project evaluation period co-located with a solar resource, with Burbank Water and Power concluding that ESS' Iron Flow Battery technology works and there is a place for this battery in a utility's overall energy storage strategy.

"This project provides another example of how ESS iron flow technology can perform in an operating utility environment," said Drew Buckley, Chief Executive Officer of ESS. "Third-party demonstration projects like this are important because they show how the technology performs in real-world utility applications. The project helps validate the underlying strengths of our core technology, which carry through to our next generation Energy Base product: long-duration performance, a safe and non-flammable chemistry based on iron, salt and water, and the value of a solution manufactured in America as utilities look for durable energy storage alternatives to lithium-ion."

The Burbank Water and Power final report noted several attributes of the system and underlying chemistry, including the use of iron, salt, and water, non-flammable and sustainable, easily sourced components, domestic manufacturing, and a projected long operating life. The report also stated that utility personnel were successfully trained on operation and maintenance of the system and that the project supported broader education and engagement around iron flow battery technology.

"We collaborated with APPA to understand the potential for long duration energy storage to support our future energy and zero carbon energy generation requirements," said Mandip Samra, General Manager of Burbank Water and Power. "The project with ESS and APPA demonstrated that safe, long duration battery alternatives are viable, and can benefit our grid. We appreciate APPA's leadership."

The findings align with ESS' focus on delivering long-duration energy storage solutions designed to support grid reliability, renewable integration, and energy resilience for utilities, independent power producers, and commercial and industrial customers. That focus is reflected in the Company's broader commercial momentum, including Project New Horizon with Salt River Project in Arizona, which includes Google participation, ESS' contract supporting a large-capacity energy storage deployment with the United States Air Force Research Laboratory, and ongoing collaboration with key partners to advance the commercialization of iron flow battery systems.

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.

Cautionary Language on 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 projections of our future financial performance, our anticipated growth strategies and anticipated trends in our business. Examples of forward-looking statements include, among others, statements pertaining to market opportunities for ESS' products, pace of commercial activity, the timing for manufacturing and delivery for Project New Horizon, the timing of delivery commencing for the Company's projects, ESS product development and manufacturing, and relationships with customers. 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, barriers we face in our attempts to produce our energy storage products; our products being in the early stage of commercialization and aspects of our technology not having been fully field tested; our inability to develop our business and effectively commercialize our energy storage products; our dependence on third-party suppliers; delays in our manufacturing operations, our ability to control our costs and achieve our cost reduction strategy; our dependence on complex machinery; our ability to increase our production capacity; required maintenance being performed incorrectly or maintenance requirements exceeding our current expectations; our history of losses; failure to deliver the benefits offered by our technology; inability to achieve market acceptance of our products; our warranty obligations; our relationships with related parties; regulatory challenges; our ability to protect our intellectual property; and our ability to raise capital in the near future; 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-K filed on March 5, 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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ESS Highlights Third-Party Utility Validating Iron Flow Battery Technology