The intermittent nature of solar power has long been its most obvious weakness. What happens when the sun stops shining? People still need power.

One California-based solar company, BrightSource Energy, says it has developed a way to solve the problem. On Monday the company announced that it will be including its SolarPLUS thermal energy storage technology to several of its concentrated solar power (CSP) solar plants. The storage units will allow power from the sun to be stored for the evening when the sun is no longer shining but demand is up.

BrightSource Energy currently has three signed power purchase agreements with Southern California Edison for about 4 million MWh of electricity from seven CSP plants. The new contract would be for the same amount of power but from six solar plants, leaving one of the original seven unbuilt. Two of the CSP plants are scheduled to come online in 2015 and three are scheduled to come online in 2016 and 2017.

“With these agreements, we’re demonstrating that power tower technology is not only advancing the solar thermal industry, but that utility-scale solar generation can be both cost effective and reliable,” said BrightSource Energy President and CEO John Woolard in a statement.

Politicians and engineers welcomed the announcement, noting that it would not only improve grid reliability, but could also reduce variability and integration costs of solar power and help solar power achieve more stable prices.

“Energy storage improves the overall efficiency of our electric system which will lower costs for consumers,” said California Assembly Member Nancy Skinner (D-Berkeley). Skinner recently authored a bill, Assembly Bill 2514, which encourages utilities to adopt energy storage technologies.

In a CSP plant, heliostats (like the ones pictures above) are controlled by software that shifts them with the sun to capture the sun’s rays throughout the day. The heat from the mirrors is focused on a boiler at the top of a tower. The steam from the boiler spins a turbine. When storage technology is introduced, the steam heats molten salts which store the heat until its needed. When needed, the molten salts are used to heat the water and produce the steam.

Photo Caption: Some of the 24,000 mirrors called ‘heliostats’ at the eSolar Sierra SunTower power plant in Lancaster, California in the Mojave Desert approximately 70 miles (110 km) north of Los Angeles May 12, 2011. eSolar’s concentrated solar power (CSP) system uses the movable heliostats to reflect solar heat to a thermal receiver mounted atop two towers. Electrical power is produced when the focused heat boils water within the thermal receiver and produces steam, which is then piped to a nearby reconditioned 1947 GE turbine generator to produce electricity. Sierra SunTower is the only commercial CSP tower facility in North America.