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. Though the California installations will generate...