If solar energy is eventually going to matter — that is, generate a significant portion of the nation’s electricity — the industry must overcome a major stumbling block, experts say: finding a way to store it for use when the sun isn’t shining.

That challenge seems to be creating an opening for a different form of power, solar thermal, which makes electricity by using the sun’s heat to boil water. The water can be used to heat salt that stores the energy until later, when the sun dips and households power up their appliances and air-conditioning at peak demand hours in the summer.

Two California companies are planning to deploy the storage technology: SolarReserve, which is building a plant in the Nevada desert scheduled to start up next year, and BrightSource, which plans three plants in California that would begin operating in 2016 and 2017. Together, the four projects will be capable of powering tens of thousand of households throughout a summer evening.

Whether the technology will be widely adopted remains to be seen, but companies like Google, Chevron and Good Energies are investing in it, and the utilities NV Energy and Southern California Edison have signed long-term contracts to buy power from these radically different new power plants.