The brief

Analyst view

• Willow Rock stores 4 GWh over eight hours at 500 MW, the largest standalone grid storage project in California.
• Construction begins mid-2026; facility comes online 2030; capital cost ~$1.5B.
• The system eliminates fossil fuel combustion during discharge by capturing and storing compression heat in thermal tanks.

Technical approach

As reported

Storage

Underground air caverns

Air is compressed into water-filled rock caverns 2,000 feet deep, where the geology is stable for decades of operation.

Heat recovery

Thermal storage

Compression heats the air; this heat is captured in thermal storage tanks and used to reheat the air during discharge, eliminating the need for natural gas.

Discharge

Turbine generation

Compressed air expands through turbines to generate electricity on demand, delivering 500 MW continuously for 8 hours.