As California utilities scramble to buy more renewable energy, Pacific Gas and Electric Co. and a Palo Alto startup will announce plans today to build a solar power plant big enough to light more than 132,000 homes. Ausra Inc. will design and build the plant, which will be located on the Carrizo Plain of eastern San Luis Obispo County and could begin operating as soon as 2010. San Francisco's PG&E has agreed to buy the plant's power for 20 years. Like the rest of California's big utilities, PG&E faces a state-imposed deadline to derive 20 percent of its power from certain renewable sources by the end of 2010. So the company is turning to solar thermal power plants, which can generate large amounts of energy on a reliable basis. In July, the company agreed to buy power from a solar plant planned for the Southern California desert, which will generate 553 megawatts, enough for more than 414,000 homes. PG&E plans to buy 1,000 megawatts of solar thermal energy within the next five years. Solar works best when it's really hot, and that's when we need a lot of power," said Peter Darbee, the utility's chief executive officer. "So solar is something we're exploring more." Solar thermal plants do not use the solar cells that more Californians are bolting to their rooftops. Instead, they use the sun's energy to heat liquids that turn turbines and generate power. Ausra's technology uses flat mirrors that focus sunlight on tubes carrying water, which then turns to steam. The plants can produce far more electricity than silicon solar cells provide and at a far lower price. Ralph Cavanagh, with the Natural Resources Defense Council, said he's pleased to see the recent attention on solar thermal plants. "They're a very good idea for California, and they're also a really good idea for the world," said Cavanagh, director of the environmental group's energy program. "This is one of the scalable solutions that can make a big difference."