The San Diego 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.

“During off-peak periods – when power is inexpensive and renewable supplies from wind and solar facilities exceed demand – turbines will pump water to the upper reservoir where it will act as a battery of stored potential energy,” the San Diego County Water Authority’s website explains.

During times of high energy usage in the area, the system would discharge energy that was stored in the water from the upper reservoir to flow downhill through the turbines. The exchange would be closed-loop system, which means it won’t consume water while putting energy into the local grid.

Systems like this are called pumped storage hydropower, and the principle is already in operation at sites all over the country, according to NPR. Many were built in the 1970s and 1980s to store nuclear energy. The new project would take up to a decade to approve, plan, and construct.