BrightSource Energy Inc on Tuesday said it was now able to store the energy its solar thermal power plants produce, allowing them to function during after the sun sets.

The company, which filed with U.S. securities regulators for an initial public offering earlier this year, said it is combining its power tower solar thermal technology with molten salt storage.

Oakland, California-based BrightSource is calling the new offering, which it is aiming at utilities, SolarPLUS. It said the technology should help reduce the cost of solar power because the plant is able to produce more energy than it would without the storage component.

"A standard (photovoltaic solar) plant might do 1,900 hours per year, and with roughly six hours of storage we'll do two-and-a-half times that," BrightSource Chief Executive John Woolard said in an interview. "If I can deliver two-and-a-half times the amount of power from the same capacity asset, I've got a huge cost advantage on a per kilowatt hour basis."

BrightSource's storage offering can be adjusted depending on the needs of the customer, Woolard said. For instance, some utilities want six hours of storage that can generate power late into the evening, while others may only need two to three hours of storage. Currently, he said utilities must rely on fossil fuel plants to guarantee reliable power during those hours.

"(Utilities) take an intermittent asset, PV or wind, and then they have to back it up with separate natural gas and fossil plants to produce reliable power at low cost to ratepayers," Woolard said. "We're able to do all of that in one offering."

Energy storage is considered critical to widespread adoption of renewable energy sources such as wind and solar power because the sun does not always shine and the wind does not always blow.

Storage is also needed to balance supply on transmission networks due to the inherent volatility of renewable power.

Molten salt storage technology is widely used in Spain, according to BrightSource. Another California company, SolarReserve, licenses molten salt storage technology from United Technologies Corp (UTX.N).

BrightSource said its "solar salts" are made of sodium nitrate and potassium nitrate.

Woolard would not comment on when the company's shares might begin trading.

California Renewable energy Energy Solar energy