Solar power keeps getting cheaper, but there’s a reason why utilities still turn to fossil fuels: it’s expensive to store solar energy for use at night or on a cloudy day, times when solar doesn’t work. But a massive new solar plant, sprawling over 1,670 acres near Las Vegas, was designed to solve that problem. It provides energy on demand, even when it’s dark.

Sitting in the Nevada desert, the new Crescent Dunes Solar Energy Project is covered with more than 10,000 mirrors, each the size of a small house, that track the sun throughout the day and focus it on a receiver filled with molten salt. The salt, heated to almost 1,000 degrees Fahrenheit, stores the energy as heat, so it’s always ready when it’s needed. When the grid needs power, the heat in the salt is released to turn water into steam, which drives generators to make energy. That can happen whether the sun is shining or not.

Using molten salt as both the heat transfer fluid and thermal energy storage medium provides efficient and cost-effective solar energy storage.

“Whether it’s in the daytime or the nighttime, it provides base-load stable power,” says Kevin Smith, CEO of SolarReserve, the company that built the new plant. “If you get a bit of cloud cover that goes across at three o’clock in the afternoon, we’re always drawing out of storage, so we continue to operate at 110 megawatts. We don’t miss a beat, and the utility doesn’t see any fluctuations in the power output over the day.”

Though people tend to use more power during the day when the sun is shining, peak periods for utilities often run into the night as well. That’s even more true in Las Vegas.

“In Nevada, with Las Vegas as their main load center, the utility’s peak goes upwards to kind of midnight,” says Smith. “So their peak is more of a noon to midnight kind of structure.” That means a normal solar plant—using PV panels, the kind of technology people have on their roofs—wouldn’t be able to meet demand without batteries or some other type of storage.

Crescent Dunes will deliver more than 500,000 megawatt-hours of electricity per year, and requires zero natural gas.

“The difficulty has been not only the cost but also the efficiency of batteries,” says Smith. “To get the size we’ve got, you’d need football fields of batteries . . . If you took all the utility-scale batteries and added them up around the world, this is bigger than those put together.” Right now, batteries are typically only used for very short backup—10 or 20 minutes at a time—rather than running all day.

As the first utility-scale solar plant of its kind in the world with built-in storage, Crescent Dunes cost around $1 billion to build. That’s cheaper than a solar PV plant with battery storage. It’s actually also cheaper than building a brand-new coal plant (with modern environmental protection) or a nuclear plant.