SustainX is developing and demonstrating a modular, market-ready energy storage system that uses compressed air as the storage medium. SustainX uses a hydraulic drivetrain to convert electrical energy into potential energy stored as compressed air. The technology does not need to be paired with a secondary heat source, such as a gas combustion turbine, and no fuel is involved. SustainX has successfully demon- strated a 1 kW prototype that has proven the two core technologies of this project, isothermal gas cycling and staged hydraulic conversion. SustainX will first develop a 50 kW energy storage system (the ESS-50). The lessons learned from this initial scale-up will be used to produce the ESS-1000—a 1 MW power output, 4 MWh storage system that will be demonstrated by partner AES Energy Storage. SustainX technology relies on off-the-shelf, reliable, long-lived components.