Two California companies said Thursday that they would each build solar power plants that were 10 times bigger than the largest now in service, creating the first true utility-scale use of a technology now mostly confined to rooftop supplements to conventional power supplies.

The solar power will be sold to Pacific Gas & Electric, which is under a state mandate to get 20 percent of its electricity from renewable sources by 2010. The utility said that it expected the plants, both using photovoltaic technology, which turns electricity directly into sunlight, would be competitive with other renewable sources, including wind and solar thermal, which uses the sun's heat to boil water. To date, the only large plants have been solar thermal.

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