Hydrostor

Willow Rock per the California Energy Commission: 500 MW net, 4,000 MWh net, four compressor and turbine trains, thermal storage tanks, an excavated cavern and a 19-mile transmission line.

Sources for these notes

Type Editorial supply-chain classification based on the sourced portfolio: system integrator, cell manufacturer, non-lithium or mechanical storage developer, power equipment group, or software and controls supplier.

Long-duration technology 1

Chemistry Main storage chemistry of the flagship product. LFP is lithium iron phosphate; NMC is lithium nickel manganese cobalt oxide; LTO is lithium titanate. Mechanical covers gravity, compressed air and CO2 stores. Mixed means the vendor sells more than one chemistry at scale.

Mechanical 2

Flagship product The vendor's main grid-scale or commercial storage product line, as the vendor names it.

A-CAES 1

Energy per unit MWh Energy of one flagship unit (usually a 20-ft container or one enclosure), as the vendor publishes it. Rating basis (usable or nameplate, beginning of life) is in the spec notes.

Not published

Duration Discharge durations the vendor offers at rated power, in hours, as published (for example 2-4 h, or 100 h).

8 h 1

Deployed GWh Cumulative storage the vendor says it has deployed, delivered or shipped, in GWh, at the date of the cited source. Shipments and installed capacity differ; the spec notes say which one the vendor reports.

Not published

Deployed + contracted GWh Deployed plus contracted or awarded storage (backlog under contract) in GWh, as the vendor states it. Blank when the vendor publishes no contracted figure.

4 1

Cycle life cycles Rated cycle life of the flagship cell or system as the vendor publishes it. Test conditions (temperature, depth of discharge, end-of-life capacity) differ by vendor and are in the spec notes.

Not published

Warranty years Longest standard product or performance warranty term the vendor publishes for the flagship system, in years.

Not published

UL 9540 The vendor states a named system in this record is certified or listed to UL 9540. The spec notes identify any earlier product or model-specific scope.

Not published

UL 9540A A cited manufacturer statement or published report describes UL 9540A testing for a named system or cell in the record. Read the spec notes for the model and test level; this is not a blanket certification of all products.

Not published

Data center offer The vendor markets a storage product, reference or program for data centers: backup, UPS, grid-interactive UPS, load smoothing for AI training, or bridging power while a grid connection is built.

Not published

Parent Owner or parent group when the provider is a subsidiary or joint venture.

Not published

Products Named storage products and cells, with sizes where published.

Advanced compressed air energy storage (A-CAES) plants; Willow Rock, California, 500 MW / 4,000 MWh 1

Market status Dated ownership, deal or operating context: joint ventures, acquisitions and manufacturing agreements.

Not published

Services Published integration, controls or lifecycle services offered by the provider.

Not published

Reference project A named project and its reported status and capacity. Project capacity is not a product unit rating.

Not published

Headquarters Headquarters city and country only when an explicit cited company statement supports the location.

Not published

1. CleanTechnica: compressed gas storage (Willow Rock CEC certification) accessed 2026-11-11
2. Hydrostor home page accessed 2026-11-11