Companies will build two solar power plants in California that together will put out more than 12 times as much electricity as the largest such plant today, the latest indication that solar energy is starting to achieve significant scale.

The plants will cover 12.5 square miles of central California with solar panels, and in the middle of a sunny day will generate about 800 megawatts of power, roughly equal to the size of a large coal-burning power plant or a small nuclear plant. A megawatt is enough power to run a large Wal-Mart store.

The 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 new plants, which will use photovoltaic technology to turn sunlight directly into electricity, to be competitive with other renewable energy sources, including wind turbines and solar thermal plants, which use the sun’s heat to boil water.

“These market-leading projects we have in California are something that can be extrapolated around the world,” Jennifer Zerwer, a spokeswoman for the utility, said. “It’s a milestone.”