The San Diego County Water Authority wants to keep the lights on, even when the Sun goes down. It plans to use San Vicente Reservoir to store solar power energy in so-called water batteries to maximize the city’s renewable energy potential, NPR reports.

Cities across California have an abundance of sunny days, which is perfect for providing renewable energy… as long as the Sun is up. The proposed project could store 4,000 megawatt-hours of energy per day, which could power 135,000 homes after the Sun goes down. To make this possible, the San Diego Water Authority would create a smaller upper reservoir just above the existing San Vicente Reservoir. These would be connected by a tunnel system and an underground powerhouse.