The electrical grid of the future, one that relies on the sun and the wind, will also need ways to store that electricity for when we need it. And it's reviving interest in an old approach to storing power, a kind of battery that uses gravity and water. From Lakeside, Calif., Dan Charles reports.

DAN CHARLES, BYLINE: Northeast of San Diego, up a steep winding road and on the other side of a locked gate, there's a place that could store a huge amount of electricity.

CHARLES: Wow, this is so cool. I'm on top of a 300-foot-tall dam looking out at 2 1/2 square miles of water.

NEENA KUZMICH: And that's the San Vicente Reservoir.

CHARLES: Neena Kuzmich is deputy director of engineering at the San Diego Water Authority, which runs this place. The lake is gorgeous, surrounded by steep, barren mountainsides. But Kuzmich did not bring me here for the view. Remember what happened on September 6, she says.

KUZMICH: Everybody in the state of California, I believe, got a text message at 5:30 in the evening to turn off their appliances.

CHARLES: California's electrical grid was hitting its limit because the sun was going down, and so was all of the state's solar power. And yet earlier that same day, there was so much solar power available in California that the grid couldn't take it all. Enough electricity to power a small city was turned away, wasted because they couldn't store it for later when they really needed it.

KUZMICH: We have a problem if we're going to have these continuous heat waves. We need something - we need a facility that will store energy so we can't - we don't have to turn off our appliances.

CHARLES: Well, the San Diego Water Authority is hoping, maybe a decade from now, it could take all that extra solar power in the middle of the day and use it to pump water from this reservoir through big underground pipes up into a new, smaller reservoir a thousand feet higher.

KUZMICH: The upper reservoir would be up behind those mountains here that you see in front of you.

CHARLES: And then when the sun goes down and they really need the power, they'd open a valve and the force of 8 million tons of water falling back downhill would spin turbines and generate electricity.

KUZMICH: It's a water battery.

CHARLES: Big water batteries like this, called pumped hydro facilities, do exist already in dozens of places around the U.S. Many were built to store electricity that nuclear power plants generated during the night.

Malcolm Woolf, president and CEO of the National Hydropower Association, says nobody's built a big new one in more than a generation.

MALCOLM WOOLF: But in just the last several years, 92 projects have come into the developing pipeline.

CHARLES: Pumping water has some advantages over batteries, like the ones in electric cars. It can store a lot of power. That San Diego project could supply 100,000 homes for 8 hours or so. It doesn't require hard-to-find battery materials like cobalt and lithium. And the plants last for a century. The problem, at least according to some people, is it's hard to find places to build such things.

WOOLF: That is a myth that I am working hard to try to disabuse folks of.

CHARLES: Woolf says you don't even need a river for pumped storage - just two reservoirs, one high, one low, reusing the same water over and over in a closed loop.

Kelly Catlett, who's director of hydropower reform at American Rivers, an environmental advocacy group, says this technology is certainly worth considering.

KELLY CATLETT: There are good pumped storage projects and there are not-so-good pumped storage projects.

CHARLES: Her group won't support projects that build new dams on streams and rivers, disrupting sensitive aquatic ecosystems. But the plan for that reservoir in San Diego...

CATLETT: That looks like something we could potentially support.

CHARLES: She says this is a chance to do better now and make sure that power captured from falling water really is clean.

For NPR News, I'm Dan Charles in Lakeside, Calif.

(SOUNDBITE OF VENTURA'S "CITY OF THE SUN")

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