California Energy Commission approves final permit for Hydrostor's 500 MW Willow Rock A-CAES facility

Hydrostor/Willow Rock A-CAES

The California Energy Commission issued a final permit on December 19, 2025 for Hydrostor's 500 MW / 4 GWh Willow Rock advanced compressed air storage facility in Kern County. The company plans to break ground mid-2026, with the facility coming online in 2030. The project will be the largest standalone energy storage facility in California.

• 500 MW power output, 4,000 MWh (8 hours continuous discharge) storage capacity
• Permits issued: California Energy Commission (Dec 2025), Kern County Board (Dec 2025), conditional letter of support (Apr 2025)
• Estimated cost ~$1.5B; facility designed for 50-year operation
• Uses advanced A-CAES technology: compresses air into 2,000-foot underground cavern, captures compression heat in thermal tanks, reheats air during discharge to eliminate fossil fuel use