The issue has also caught the attention of regulators. In California, the Public Utilities Commission approved a rule on Thursday that will require the state’s three big investor-owned utilities and other electric industry players to install storage by 2024.

“The impetus to require storage is definitely inspired by the success of solar,” said Robert Gibson, vice president of the Solar Electric Power Association, a nonprofit educational group. “Hopefully the California initiative is going to kick-start this and bring down costs,” he said. Battery makers have predicted progress, he said, adding that cost-effective storage “has always been a few years out.”

Like other utilities, Arizona Public Service faces its biggest challenge in the early morning, before the sun is high enough to hit conventional solar panels, the kind installed on rooftops to turn sunlight into electric current. Arizona and, increasingly, California see the same problem in the evening, when the sun is too low for the panels to work, just as thousands of people are returning home and workplaces are still humming. Solar panels can help utilities meet afternoon peaks, but not morning or evening ones; by 6 p.m., panels are producing only about half their maximum, even if they are installed on tracking devices that tilt the panels to follow the sun across the sky.

Solana is a $2-billion project built with a $1.45 billion loan guarantee from the Department of Energy. Close behind is the Ivanpah project in California. It uses a field of mirrors the size of garage doors, mounted on thousands of pillars, to focus the sun’s light on a tower with a tank painted black. Engineers say that design could incorporate storage efficiently, because the tank reaches very high temperatures. That plant will enter commercial operation by the end of October.

Solana can gather heat roughly 1.75 times as fast as its steam turbines can use it, so on a sunny day the plant is turning out power steadily even if clouds obscure the sun. Its capacity is about six hours. Its production, up to 280 megawatts, can be throttled back at hours when photovoltaic cells are churning out current, or at night when demand is low.