ESS Tech, Inc.

Unless the context otherwise requires, all references in this Annual Report on Form 10-K to “ESS,” “we,” “us,” “our,” or the “Company” refer to ESS Tech, Inc.

Business Combination

ESS Tech, Inc. (“Legacy ESS”) was founded in 2011 and became a publicly traded company through a business combination with a special purpose acquisition company named ACON S2 Acquisition Corp. (“STWO”) which changed its name to ESS Tech, Inc. upon closing (the “Business Combination”). As a result of the Business Combination, Legacy ESS survived and became a wholly owned subsidiary of ESS Tech, Inc. On March 31, 2024, Legacy ESS merged with ESS Tech, Inc. leaving ESS Tech, Inc. as the successor legal entity.

Business Overview

ESS is a long-duration energy storage company specializing in iron flow battery technology. We design and produce long-duration batteries predominantly using earth-abundant materials that we believe can be cycled over 20,000 times without capacity fade based on lab-scale results. Because our batteries are designed to operate using an electrolyte of primarily salt, iron and water, our products are environmentally sustainable and substantially recyclable or reusable. Our batteries provide flexibility to grid operators and energy assurance for commercial and industrial customers. Our technology addresses energy delivery, duration and cycle-life in a single battery platform that compares favorably to lithium-ion batteries, the most widely deployed alternative technology. We design and build this core technology, including battery stacks, electrolyte and electrolyte health management solutions into a core power train that is then sold in our current product focus: the gigawatt-hour scale, fully configurable Energy Base and core power trains that can be assembled at the customer site into complete storage solutions. Our original product offering included the Energy Warehouse and Energy Center, which we are not currently offering for sale. With each battery deployed, we further our mission to accelerate the transition to a zero-carbon energy future with increased grid reliability.

Our batteries are non-flammable and can operate in a wide range of temperatures from -15°C to 50°C with minimal auxiliary loads. This allows our energy storage products to be located in sites where lithium-ion batteries cannot be placed due to fire, chemical or explosion risks. In addition, our batteries are environmentally sustainable, predominantly utilizing readily sourced materials and recyclable or reusable components.

We believe that as we are able to increase our production, our per unit costs will decrease, allowing our battery technology to be priced more competitively. When comparing products on a lifetime levelized cost of storage (“LCOS”) basis, which is the total cost of the investment in an electricity storage technology divided by its cumulative delivered electricity, we expect our batteries to be less expensive than lithium-ion alternatives for storage durations greater than eight hours, which we believe is the economic maximum for lithium-ion technologies. Our cost advantage increases as the storage duration extends beyond eight hours because of the scalable nature of our technology.

Energy storage solutions of various sizes and durations must be installed throughout global electrical grids to enable decarbonization in line with global climate objectives. Our energy storage products are intended to supply long-duration power across this expanding spectrum of use cases. As described below in the subsection entitled “Our Technology and Products,” we believe our energy storage products will be capable of addressing customer needs across multiple use cases and markets. We are an early mover in long-duration energy storage and we believe our products will enable more rapid implementation of renewable energy while also improving grid stability. The safety, flexibility and durability of our energy storage products enable customers to use them in nearly any location globally. Examples of use cases range from localized energy storage at commercial and industrial sites to grid-scale use cases, such as peaker plant replacement and grid stabilization.

Our Technology and Products

Our long-duration iron flow batteries are the product of nearly 50 years of scientific advancement. In the 1970s, researchers first developed the concept of iron flow batteries. Despite realizing the batteries’ promising ability to store energy, these researchers found that the reaction between the positive and negative sides created hydroxide formations that clogged the electrodes and reduced the activity of the electrolytes. Hydroxide formation caused rapid degradation in early iron flow batteries after only a few cycles. Unable to prevent the hydroxide from forming, these scientists were forced to abandon their work.

Our founders, Craig Evans and Dr. Julia Song, began advancing this technology in 2011 and formed Legacy ESS. Building upon this promising concept, our team has significantly enhanced the technology, improved round-trip efficiency and developed an innovative and patented solution to the hydroxide build-up problem. Our proprietary solution to eliminate the

• 4 - hydroxide formation is known as the Proton Pump, which works by utilizing hydrogen generated by side reactions on the negative electrode. The Proton Pump converts the hydrogen back into protons in the positive electrolyte. This process eliminates the hydroxide and stabilizes the electrolytes’ pH level. The Proton Pump allows the electrolyte to be used for the 20,000 cycle-design without capacity fade.

Our iron flow batteries store energy by converting electrical energy into chemical energy. Each battery module is made up of one or more cells and each of these cells is made up of a negative electrode and a positive electrode, and these two electrodes are separated by a porous separator. On the positive side (positive electrode) of the battery during charge, ferrous iron (Fe+2) is oxidized into ferric iron (Fe+3) and on the negative side (negative electrode) of the battery, ferrous iron is reduced to iron metal. The porous separator is used to minimize the positive and the negative electrolyte from mixing, which helps improve the coulombic efficiency of the battery. The positive and negative electrolytes are stored separately in tanks outside of the battery and this electrolyte is constantly pumped in and out of the battery while in operation. To convert the chemical energy back to electrical energy, the reaction is reversed; on the positive side of the battery, ferric iron is reduced to ferrous iron and on the negative side, metallic iron is oxidized into ferrous iron. During these charge and discharge cycles, the pH of the positive and negative electrolyte can change dramatically. The Proton Pump ensures that the electrolyte pH remains stable and clear of any hydroxides.

Our iron flow batteries store energy by charging positive and negative electrolyte tanks that are separated by a membrane. In order to release energy, we generate a reaction between the tanks via the membrane. The power generated is a factor of the membrane size, while the duration of storage we can offer is a factor of the tank sizes and the addition of electrolytes to the tank, which can be accomplished at a relatively low cost. The duration of stored energy can be independently varied from the power. This allows for low marginal costs of energy, making our technology attractive for long-duration energy storage.

Using our iron flow battery technology, we have developed products to provide reliable, safe, long-duration energy storage solutions. Our third and current offering, the gigawatt-hour Energy Base storage product, is designed with a fully configurable layout that integrates with any site location and that allows the power (the rate of electricity flow) to be decoupled from the capacity (the total amount of energy held). This, combined with an expectation of 20,000 cycles and rapid response time, means that the performance of each Energy Base can be tailored to meet individual customer needs. As a result, users have the flexibility to use the battery for various use cases simultaneously on a project. We continue to perform warranty and maintenance activities for our first and second product generations, the Energy Warehouse and Energy Center, that remain under contract in the field.

The same core technology can enable energy storage solutions designed to meet specific customer needs, particularly when much larger (<5-100MW) requirements exist. In this arrangement, we offer core components configured as power trains, which can then be assembled on-site with various ‘balance of system’ components, including tanks, pumps, and plumbing, to create fully integrated storage solutions. These balance of system components are commercially available products used across a variety of industries and applications. ESS has developed and will continue to expand its documentation, reference design and recommendations to allow customers and partners to build and operate integrated storage solutions based on our core technology.

For all of our energy storage products, the intellectual property and points of differentiation are contained within the core technology components, the Proton Pump, power module, Battery Management System and electrolyte. These components are protected by trade secrets, patents (both granted and in process) and years of research. The balance of system components required to build complete solutions are intentionally designed to be easily produced using broadly available pumps, tanks and other equipment and easily fabricated enclosures, allowing energy storage products to be assembled and deployed almost anywhere at an efficient cost.

Customers

Our current and potential customers include utilities, Independent Power Providers (“IPPs”) and Commercial and Industrial (“C&I”) end users. We intend to serve customers in both ‘behind-the-meter’ and ‘front-of-the-meter’ markets. In behind-the-meter applications, customers will use our energy storage products to reduce energy costs, integrate with renewable energy solutions to achieve corporate sustainability goals, and to enhance their energy resiliency. Behind-the-meter customers may include data centers, microgrids, critical infrastructure assets and commercial and industrial customers.

Front-of-the-meter customers, in contrast, are primarily utilities experiencing high rates of renewable energy penetration and requiring energy storage to help balance the grid, and IPPs who can leverage energy storage to improve the economics of renewable energy projects. These customers use our energy storage products to store energy on a utility scale that they can then utilize or sell to their customers when needed and ensure overall grid reliability at a time of increasing demand for

• 5 - energy and grid reliability. Over time, we expect our customer base to expand to include additional types of energy suppliers and an even broader variety of use cases.

We believe our solutions are well-suited to address these diverse applications and use cases due to their flexibility, longevity and safety, among other factors.

Suppliers

Our batteries are made predominantly of earth-abundant, environmentally sustainable materials. These materials are considerably less expensive than rare earth metals that make up other batteries and as a result make up a low percentage of the total costs for our batteries. Because these materials are widely available, there are multiple suppliers for each input. In addition, we use limited high-cost materials such as platinum in our manufacturing. While supply chain disruptions have in the past impacted the ability of some of our suppliers to deliver certain components of our batteries to us in a timely manner as described in the section entitled “Part I—Item 1A. Risk Factors—Risks Related to Our Technology, Products and Manufacturing—We depend on third-party suppliers for the development and supply of key raw materials and components for our energy storage products. We also depend on vendors for the shipping of our energy storage products. Quality issues or delays in our supply or delivery chain and shipments could harm our ability to manufacture, supply and commercialize our energy storage products,” the mechanical elements and control systems of our batteries are comprised of commercially available equipment that can be supplied by multiple manufacturers.

Partnerships

We have various partnerships, including:

•OneBeacon Insurance: Through OneBeacon, we offer project surety capacity and corporate bonding options to our customers.

•Export-Import Bank of the United States: The Export-Import Bank of the United States (“EXIM”) is the official export credit agency of the United States. EXIM equips U.S. businesses with the financing tools necessary to compete for global sales when private sector lenders are unable or unwilling to provide financing. Our energy storage products are qualified by EXIM, which can provide financing for qualified overseas buyers.

Research & Development

Since January 1, 2019, we have invested approximately $184.7 million in improving our technology and bringing our energy storage products to market. Our research and development efforts are conducted in Oregon and supported by our approximately 21 research and development employees.

We aim to be a leader in the long-duration energy storage market, and in order to do so, it is essential to continue our ongoing research and development activities. We have a research and development roadmap for additional breakthroughs to extend our technology advantages further. Expanding our technological capabilities to alternate chemistries and technologies is also a long-term goal of our research and development team.

Intellectual Property

Intellectual property is an integral differentiator for our business, and we rely upon a combination of patents, copyrights and trade secrets to protect our proprietary technology. We believe that we have enforceable intellectual property protection over all critical design elements as well as the key enabling technologies for iron flow batteries. We have developed a significant patent portfolio. We have over 315 patents that are granted, applied for or in the pipeline for application, and an undisclosed number of trade secrets. Without taking into account any possible patent term adjustments or extensions, the earliest our current, issued patents will begin to expire in 2028. We continually review our efforts to assess the existence and patentability of new proprietary technology. We intend to leverage our iron flow expertise to drive innovation and are pursuing additional technological advancements.

Competition

The energy storage industry is highly competitive. The declining cost of renewable energy, the decrease in battery costs, improving battery technologies, and public financial support in the form of grants and tax incentives are shifting customer demands causing the industry to evolve and expand. The main competitive factors in the energy storage market include, but are not limited to, safety and reliability, duration, performance and uptime, operational flexibility, asset life length and cyclability, ease of integration, operability in extreme temperatures, environmental sustainability, historical track record, and field-proven technology.

With the rising demand for clean electric power solutions with lower greenhouse gas emissions, there has been a transition to renewable energy sources and increasing penetration of distributed energy infrastructure. The proliferation of intermittent generating resources has created new challenges to electric grid stability, and thus an opportunity for an

• 6 - increased role for long-duration energy storage solutions. In recent years, there have been, and in the future there may continue to be, more unpredictable weather events including extreme temperatures, hurricanes and wildfires. Our technology can operate efficiently and effectively in these extreme weather conditions. Our key competitors include different energy storage technologies such as lithium-ion batteries, lithium metal batteries, lithium iron phosphate batteries, sodium-ion batteries including those developed by Contemporary Amperex Technology Co., Limited, vanadium or zinc bromine batteries, sodium sulfur batteries, compressed air, hydrogen, fuel cell and pumped-storage hydropower. Key competitors in the traditional lithium-ion space include Contemporary Amperex Technology Co. Limited, LG Chem, Ltd., Samsung Electronics Co., Ltd., Sungrow Power Supply Co., Ltd., and Tesla, Inc. Key competitors in the non-lithium-ion space include EnerVenue, Inc., Invinity Energy Systems, CellCube, CMBlu Energy AG, Energy Dome, Energy Vault, Enerox GmbH, Eos Energy Enterprises, Inc., Form Energy, Highview Power PTY Ltd., Hydrostor, Lockheed Martin (GridStar Flow), and Malta Inc.. Many of our competitors have substantially greater financial, marketing, personnel and other resources than we do. Although we may be small compared to some of our competitors, we believe we are well-positioned to compete with them in the market supported by our innovative iron flow battery technology, strategic partnerships and premier leadership team with a proven track record of success.

New technologies may enter the market that may have additional or superior advantages to our offerings. We believe there remains significant government support available for energy storage technologies, and a variety of newer and emerging companies have announced plans to develop energy storage products using a variety of technologies, including compressed air, thermal energy, and solid-state batteries among others. Although many of these companies are not in commercial production today, they may in the future offer solutions that become competitive with our offerings. We intend to continuously improve our product offerings and maintain robust research and development efforts in order to stay ahead of existing and emerging competitors.

Government Regulations and Compliance

We operate in the heavily regulated energy sector. Our operations are subject to a variety of federal, state and local regulations governing manufacturing, project development, transportation, environmental, grid interconnection, and market participation matters. As such, there are a variety of federal, state and local regulations and agencies that impact our operations. As a participant in the renewable energy sector specifically, there are federal and state regulations, tax incentives and support mechanisms that impact our business. In addition, renewable energy and energy storage is a priority for many national, state and local governments.

On the U.S. federal level, tax credits remain in place that incentivize both the manufacturing and deployment of renewable energy and battery storage. The Inflation Reduction Act of 2022 established advanced manufacturing production credits and investment tax credits for energy storage. The One Big Beautiful Bill Act (H.R. 1) (the “OBBB”), enacted in July 2025, preserved these tax credits but introduced complexity by requiring compliance with certain “foreign entity of concern” (FEOC) rules. Beginning in 2026, energy storage technology will need to comply with these rules, which generally restrict entities linked to adversarial nations, particularly China, from accessing the benefits of U.S. tax credits including by placing limitations on the eligibility of equipment procured from such entities for such credits. As a manufacturer of energy storage systems, we may be eligible for advanced manufacturing production credits to produce eligible battery components. For project deployments, battery energy storage systems are eligible for investment tax credits. These credits are available for standalone storage systems regardless of whether they are charged by renewable energy sources, allowing project developers to monetize and sell the tax credits they receive from the creation of their projects. Additionally, battery storage projects are eligible for accelerated depreciation via the federal government’s Modified Accelerated Cost Recovery System. These policies provide tax and financing advantages for battery storage projects, lower the capital requirements for renewable energy projects to be developed and open a new source of funding for these projects, although the complexity introduced by the FEOC rules, in particular as the industry is still awaiting guidance, may counteract these benefits and limit our and our customers’ ability to take advantage of such incentives.

State incentives have also driven growth in the deployment of renewable energy and energy storage. States with high Renewable Portfolio Standards (“RPS”), such as California, have seen greater deployment of renewables than states with similar renewable resources that lack such requirements. Other states, such as Texas, have seen significant renewable deployment driven by favorable market conditions. In both types of markets, the deployment of intermittent renewables is straining the electric grid and thus driving demand for energy storage.

We have made sales in Australia, where government incentive programs support renewable energy and energy storage deployment. The Australian federal government and several state governments have established renewable energy targets and energy storage procurement goals that may drive future demand for our products.

All of these governmental programs supporting demand for our products depend on continued legislative support and favorable regulatory implementation. Changes in policy guidance or budget priorities could reduce or eliminate these benefits, including through repeal of modification of tax credits, weakening of state renewable energy mandates, or

• 7 - reduction in grid modernization funding. Any such changes would adversely affect demand for our products and our financial position. Additionally, new or modified regulations could impose compliance costs or affect our operations, which could materially harm our business.

In addition to benefiting from governmental laws and regulations supporting energy storage, we are subject to federal, state and local requirements related to the environment, transportation, health, safety and employment. Our manufacturing operations are subject to environmental regulations, and our products are subject to regulations addressing safety and reliability. We are also subject to state utility commission requirements, grid interconnection timelines, as well as other permitting processes for project development. Our Oregon manufacturing facility is subject to the requirements of the Occupational Safety and Health Act, local wage regulations and rigorous health and safety regulations in the State of Oregon.

Human Capital Management

We pride ourselves on our clean, innovative technology and our employees are dedicated to our strategic mission to deliver reliable, resilient and safe renewable energy storage solutions to communities worldwide. As of December 31, 2025, we employed 62 full-time employees, based primarily at our headquarters in Wilsonville, Oregon.

In order to achieve our mission to create a reliable, resilient, and safe renewable energy future, we are committed to investing in our employees and building a respectful and diverse work environment. We provide equal employment opportunities to all persons regardless of race, age, color, gender, sexual orientation, national origin, physical or mental disability, religion, or any other characteristic protected by federal, state, or local law. Our employees are key to our success as a company, and we are committed to attracting, developing and retaining the best talent. Many of our employees are highly skilled in technical areas related to energy storage, and our philosophy is to both develop talent from within as well as strategically hire qualified individuals with the skills, experience and industry knowledge necessary to contribute to the growth and success of ESS. We also supplement our workforce with consultants or independent contractors as necessary. In order to retain top talent, we have designed our compensation program to provide employees with competitive compensation and benefits consistent with positions, skill levels, experience, knowledge, and geographic location. We are also continuously working to improve our recruiting, retention and development processes as our operations grow.

Executive management assists our board of directors (the “Board”) in its oversight of human capital management including with respect to corporate culture, diversity and inclusion, recruiting, retention, attrition, talent management, career development and progression, succession and employee relations.

The success of our business is connected to the well-being of our team members. Accordingly, we are committed to the health, safety and wellness of our team members worldwide. To date, we have not experienced any work stoppages and we consider our relationship with our employees to be good. None of our employees are represented by a labor union or subject to a collective bargaining agreement.

Available Information

Our investor relations website is located at https://investors.essinc.com/, our Company X account is located at https://x.com/ESS_info, and our corporate LinkedIn account is located at https://www.linkedin.com/company/energy-storage-systems/. The information on, or that can be accessed through, our website and the aforementioned X account and LinkedIn account is not incorporated by reference into this Annual Report and should not be considered to be part of this Annual Report on Form 10-K unless expressly noted. Further, our references to website URLs are intended to be inactive textual references only. We may use our investor relations website and the aforementioned X account and LinkedIn account to post important information for investors, including news releases, analyst presentations, and supplemental financial information, and as a means of disclosing material non-public information and for complying with our disclosure obligations under Regulation FD. Accordingly, investors should monitor our investor relations website and the aforementioned X account and LinkedIn account, in addition to following press releases, filings with the Securities and Exchange Commission (the “SEC”) and public conference calls and webcasts.

We will file or furnish periodic reports and amendments thereto, including our Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q, Current Reports on Form 8-K (and amendments to those reports), proxy and information statements and other information filed or furnished pursuant to Sections 13(a) and 15(d) of the Exchange Act with the SEC. The SEC maintains a website that contains reports, proxy and information statement, and other information regarding issuers that file electronically, which may be accessed through the SEC at http://www.sec.gov. Our reports, amendments thereto, proxy statements and other information are also made available, free of charge, on our investor relations website as soon as reasonably practicable after we electronically file or furnish such information with the SEC. All statements made in any of our securities filings, including all forward-looking statements or information, are made as of the date of the document in

• 8 - which the statement is included, and we do not assume or undertake any obligation to update any of those statements or documents unless we are required to do so by law.