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When it snowed in Flagstaff, Ariz., recently, thousands of people woke up and turned up their electric heating, and Arizona Public Service saw electricity demand reach a morning peak. To meet the demand, the company used the previous afternoon’s sunshine.

In a closely watched new solar project called Solana, the energy is gathered in a 3-square-mile patch of desert bulldozed flat near Gila Bend, about 50 miles southwest of Phoenix.

A sprawling network of parabolic mirrors focuses the sun’s energy on black-painted pipes, which carry the heat to huge tanks of molten salt. When the sun has set, the plant can draw heat back out of the molten salt to continue making steam and electricity.

The emerging technology is one way that the utility industry is trying to make electricity from the sun available even when it is not shining, overcoming one of the major shortcomings of solar power.

“We’re going to care more and more about that as time goes on,” said Brad Albert, the utility’s general manager of resource management, in a telephone interview.

The issue has also caught the attention of regulators. In California, the Public Utilities Commission approved a rule 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.

“There will be a trend towards storage as we see more variable renewables like photovoltaics and wind being added to the grid,” said Cara S. Libby, the project manager for solar research at the Electric Power Research Institute, a nonprofit utility organization in Palo Alto, Calif. The flexibility of such a system gets more important as a utility adds higher volumes of inflexible renewables, she said.

Thermal storage does not have to be storing heat. Calmac, based in Fair Lawn, N.J., installs systems that use off-peak electricity to make ice, and then uses the ice to run air-conditioners. Mark MacCracken, the chief executive, said he installed a system in Rockefeller Center in Manhattan that could reduce the center’s energy demand by one megawatt for six hours. Some California companies will meet the new storage requirement with ice systems, he said.

Solana is not the first renewable energy plant with storage; several have added banks of electric batteries. But battery storage is so expensive that these have been used mostly to smooth the output of the plant, not to store huge amounts overnight.

Batteries are expensive and have a limited lifetime. They are more economical in a car, where they help electricity substitute for something more expensive, like gasoline. But for utilities, they are nowhere near cheap enough to justify using them to avoid buying high-priced, late-afternoon electricity.

Storing energy as heat, instead of as electricity, is substantially cheaper. But it adds mechanical inefficiency, because the heat has to be transferred from oil to salt to water, losing a bit each time, and adding cost.

Arizona has set a goal of 15 percent renewable energy by 2025, said Steven Gotried, a spokesman for Arizona Public Service, and Solana will produce about 3 percent of what the utility sells.

In California, BrightSource Energy, the company behind Ivanpah, is planning to install storage on future projects, said Joseph F. Desmond, senior vice president of marketing.

“As you add more photovoltaic to the grid, it’s shifting the net peak to later in the day,” he said. And that improves the value of “projects that can deliver energy in those later hours.”