SRP and ESS unveil 50 MWh long-duration energy storage pilot to power a cleaner grid

Salt River Project (SRP) and ESS Inc. (NYSE: GWH) have announced a 50-megawatt-hour (MWh) long-duration energy storage (LDES) pilot project, signaling a major step forward for grid-scale decarbonization in the U.S. Southwest. Expected to be among the largest of its kind using iron-flow battery technology, the project will deliver valuable operational data on extended-duration energy storage and its role in enhancing reliability across Arizona’s expanding clean-energy network.

The collaboration underscores SRP’s long-term resource plan, which targets an 82 percent reduction in carbon intensity by 2035 and net-zero emissions by 2050. Partnering with ESS, a leading developer of non-lithium flow batteries, enables SRP to diversify its storage technologies beyond conventional lithium-ion systems and evaluate multi-hour solutions capable of supporting renewable integration at scale.

Why SRP’s decision to expand long-duration pilots could redefine Arizona’s renewable reliability strategy

Arizona’s energy system faces a dual challenge: rapid population growth and increasing dependence on solar generation. As daily demand peaks occur after sunset, short-duration lithium-ion batteries have proved insufficient for long-term balancing. SRP’s 50 MWh project aims to fill that critical gap by testing iron-flow batteries designed for continuous operation across eight to twelve hours without performance degradation.

Iron-flow chemistry operates on a simple yet durable principle. Energy is stored in liquid electrolytes containing iron, circulated through cell stacks where oxidation and reduction occur. Because the active material is non-flammable and abundant, the technology avoids supply-chain and safety constraints associated with lithium, cobalt, or nickel. It also offers near-unlimited cycling capability, with a projected 20,000-plus cycle life and stable round-trip efficiencies approaching 70 to 80 percent.

SRP’s experience with smaller-scale projects, such as its 5 MW, 10-hour CMBlu Energy pilot, laid the groundwork for this next phase. The new deployment reflects growing confidence in flow-battery durability, scalability, and adaptability to Arizona’s desert conditions, where thermal management is critical.

How ESS Inc. could strengthen investor confidence through high-visibility pilot performance

For ESS Inc., the SRP partnership represents both technological validation and a potential turning point in market perception. The company’s Energy Warehouse and Energy Center systems have already been installed across several municipal and industrial sites worldwide, but the SRP collaboration marks one of its most visible utility-scale integrations in the United States.

ESS’s technology differentiates itself by leveraging iron, salt, and water as its core materials—making it fully recyclable and free of toxic components. For utilities under pressure to decarbonize supply chains, this environmentally benign chemistry could provide a long-term procurement advantage.

From a financial perspective, ESS Inc. stock has experienced wide trading fluctuations through 2025, reflecting broader investor caution toward unproven storage chemistries. Yet analysts note that performance milestones from large-scale demonstrations like SRP’s could reshape valuation narratives. If the pilot achieves high availability and demonstrates cost parity with lithium systems over a 20-year lifecycle, it could strengthen ESS’s position in subsequent procurement rounds and future Department of Energy (DOE) funding programs.

Market sentiment toward long-duration storage remains mixed but optimistic. Institutional investors continue to view energy storage as a key enabler of renewable integration, with a compound annual growth rate projected near 25 percent through 2030. However, only a handful of developers—ESS among them—have achieved grid-connected demonstrations exceeding 10 hours of duration. That exclusivity gives the company early-mover visibility in a market segment estimated by BloombergNEF to exceed $50 billion globally by 2040.

How long-duration storage fits into the next phase of U.S. grid modernization

Beyond Arizona, the SRP-ESS collaboration aligns with a national trend toward deeper grid flexibility and resilience. The DOE’s “Pathways to Commercial Liftoff: Long Duration Energy Storage” report estimates that 225 GW of long-duration capacity will be needed by mid-century to enable a net-zero grid. This scale is not merely aspirational—it is essential for mitigating the seasonal intermittency of renewables.

SRP’s participation in the DOE’s Long Duration Energy Storage Council and its broader procurement roadmap demonstrate a recognition that grid reliability cannot depend solely on fossil peakers. As natural gas plants retire and renewable penetration climbs, multi-hour storage will become critical for both operational stability and economic dispatch.

Other U.S. utilities are pursuing similar strategies. Pacific Gas and Electric in California is testing multi-day compressed-air and thermal systems; Xcel Energy in Colorado has partnered on iron-air storage through Form Energy; and Duke Energy in Florida is evaluating modular flow-battery units for distributed applications. Collectively, these pilots represent the largest wave of non-lithium demonstrations in U.S. history, all seeking cost-effective long-term energy shifting.

Arizona’s policy framework also provides fertile ground for LDES adoption. The state’s Integrated Resource Planning (IRP) process now explicitly encourages technology diversity, while regulators are evaluating tariff structures that reward long-duration flexibility. SRP’s decision to advance this 50 MWh pilot places it at the forefront of that transition, potentially shaping future regulatory baselines for clean-energy procurement.

What success could mean for ESS Inc., SRP, and the broader clean-energy economy

If the ESS pilot meets performance expectations, it could signal the commercial maturity of long-duration storage and accelerate its inclusion in utility resource plans nationwide. Beyond reliability, LDES technologies enable renewable over-generation to be stored and dispatched during peak hours, reducing curtailment and improving project economics for solar developers.

For SRP, the benefits extend to system resilience and emissions compliance. A successful trial would help the utility model future hybrid systems combining solar, flow batteries, and demand response—effectively creating a dispatchable renewable backbone. That approach mirrors initiatives in Europe and Australia, where similar iron-flow pilots have proven durable across diverse climates.

For ESS Inc., the project’s outcome could significantly influence future procurement decisions. Positive data on cost per cycle, round-trip efficiency, and heat tolerance could strengthen its competitive standing in DOE loan programs and international markets seeking sustainable alternatives to lithium. Analysts expect that even moderate technical success would attract strategic partnerships or licensing interest from major OEMs and power producers.

The ripple effects would extend to manufacturing, workforce development, and regional supply chains. Flow-battery systems can be assembled using domestically sourced materials, aligning with U.S. Inflation Reduction Act incentives for energy-storage localization. As ESS expands its production capabilities, Arizona’s position as a clean-energy innovation hub could grow, supporting local employment and research partnerships.

From an investor standpoint, ESS’s role in this pilot adds credibility to its commercialization roadmap. Market watchers note that long-duration storage represents one of the few remaining “white spaces” in grid decarbonization—an area where successful technology can achieve both environmental and financial durability. The SRP project may thus become a bellwether for how quickly non-lithium chemistries transition from pilot scale to full-fleet deployment.

How the SRP-ESS collaboration could shape the global long-duration storage landscape

Globally, utilities are reaching similar inflection points. The United Kingdom’s National Grid has begun evaluating multi-day storage options to support offshore wind integration, while Australia’s New South Wales government has launched tenders for 8-hour and longer-duration projects. These markets watch U.S. pilots closely for cost and reliability benchmarks.

If the SRP-ESS project succeeds, it could position the U.S. as a leader in sustainable LDES manufacturing, export, and standards development. Moreover, it could validate the broader thesis that renewable reliability depends as much on duration as on capacity. As grids move toward higher shares of variable generation, the ability to store and deliver energy over extended periods may determine not just energy independence but also economic competitiveness.

The 50 MWh ESS pilot stands as more than a technological experiment. It embodies the next step in redefining what “baseload” means in a renewable era—where iron, water, and innovation may one day replace gas turbines as the backbone of reliability. Whether this demonstration delivers full commercial success or simply foundational learnings, it marks a decisive turning point in how utilities approach the energy transition.