Green energy advocates and utility companies for years have struggled to clear one of the last great hurdles facing solar and wind power: how to provide electricity even when the sun's not shining and the wind's not blowing.

Storage is the Holy Grail, a way to stash extra energy for later but batteries remain too expensive and inefficient to provide large scale power.

One company, though, now says it has a solution – or, at least, an alternative.

On Monday, the California firm Solar Reserve unveiled the world's first solar plant able to generate power 24/7 without any kind of coal- or gas-fired backup – and not just for one or tens or hundreds of homes, but on a "utility scale:" for tens of thousands of households.

"This is really the only large-scale storage project in the U.S. that doesn’t need natural gas backup," CEO Kevin Smith says. "It's significant. We can put it in the middle of nowhere – in Africa or the California desert."

The plant, built in Tonopah, Nevada – population 2,478 – already works: Since the fall, the lights in Las Vegas and Reno have blinked with power generated three hours away at the site, known as Crescent Dunes. These days, it's churning out 110 megawatts from noon to midnight – roughly the 12-hour window of peak demand in Vegas, and enough electricity for 75,000 homes.

The process it uses is different than typical photovoltaic solar panels. It's known as thermal solar: 10,347 curved mirrors sit in a circle 1.73 miles across, following and focusing the sun's rays onto black tubes coiled around the top of a tower. Inside, more than 3 million gallons of liquid salt – kept at 500 degrees in a so-called "cold tank" to stay molten – flow through the tubes, baking until they reach 1,050 degrees. The fluid then plunges into a pair of 40-foot-tall insulated tanks, which hold the heat until it's needed to turn water into ​​​​steam. The steam spins a turbine to create electricity.

For the next 25 years, the company has contracted with NV Energy, owned by Warren Buffett's Berkshire Hathaway, to pump electrons into the grid. By day, the 640-foot tower casts an otherworldly glow that prompted one airline passenger to write on the blog UFOsightingsdaily.com ​that it has the "appearance of a landed UFO on the ground."​ Once stored inside the insulated tanks, the heat won't dissipate for weeks, if needed, and there's enough salt to provide 10 hours of steady power.

"We can store energy in it, extract the energy, recharge it almost like a battery being charged and discharged," Smith says. "Because that storage is flexible, we can design the facility to meet demand whenever the utility wants."

At $997 million – financed in part with $737 million from the Department of Energy – the project isn't likely to overtake natural gas in the U.S., where the market is awash in cheap natural gas from the fracking boom of the last decade. A new natural gas plant can provide power for as low as $52 per megawatt-hour, or MWh, according to figures compiled by the consulting firm Lazard. Crescent Dunes, by contrast, is supplying the NV Energy at $135 per megawatt-hour – a contract awarded in 2009 when natural gas was more expensive.

"It works in niche applications," Chris Vlahoplus, of the consulting firm ScottMadden, says of solar thermal, namely the "southwest U.S. desert environment."

Politics also pose a challenge: Solar Reserve unveiled Crescent Dunes amid a groundswell of conservative opposition to renewables and government-funded efforts to combat climate change. Even as NV Energy trumpeted its partnership with Crescent Dunes, it also supported a move by state regulators to increase the costs on traditional rooftop solar, which has eaten away at utilities' revenue.

More broadly, 27 states have sued to stop the Clean Power Plan, the first federal law limiting heat-trapping carbon emissions from existing power plants and the centerpiece of the Obama administration's efforts to slow climate change. In an unprecedented move, the Supreme Court ruled last month to halt the plan's implementation while the legal challenge moves forward.