ESS Tech makes long-duration energy storage systems using proprietary iron flow battery technology. The core product is the Energy Base, a modular, grid-scale battery system that stores energy by cycling iron-salt-water electrolytes through a cell stack. ESS's key technical innovation, the "Proton Pump," prevents electrolyte degradation that plagued earlier iron flow batteries, enabling 10+ hours of discharge and 20,000+ cycles with no capacity fade. The Energy Base's architecture decouples power from energy capacity, meaning duration can be extended cheaply by adding more electrolyte and larger tanks rather than more battery packs. ESS sells complete systems directly to utilities and independent power producers through competitive RFP processes, targeting grid-scale front-of-meter deployments. ESS manufactures its core technology — battery stacks, electrolyte, and the Proton Pump — at its Wilsonville, Oregon facility, with over 98% U.S.-sourced components. This domestic manufacturing makes ESS eligible for Section 45X Advanced Manufacturing Production Tax Credits, which ESS monetizes with third parties for cash. ESS is pre-scale and carries a going concern disclosure, with operations funded through equity issuances and debt financing while the company ramps its first Energy Base deliveries. ESS's first commercial-scale project is a 5 MW / 50 MWh pilot with Salt River Project, with a roadmap to extend duration to 16 hours and reduce installed cost to compete with lithium-ion on a fully installed $/kWh basis by 2027–2028.