Pacific Gas and Electric (PCG) in California announced last week it will buy 800 megawatts of solar-generated electricity from two companies, enough to light 239,000 homes. Within three years, PG&E will buy its solar energy from OptiSolar and SunPower, which plan to build the world's two largest solar farms in California as part of the deal.

It would nearly double the USA's entire solar-panel capacity. Driving the trend are solar's falling costs and state alternative-energy mandates.

Solar power has grown but still makes up well under 1% of U.S. power generation. More than 90% of solar panels have been installed on rooftops by maverick consumers and businesses. Utilities' embrace of solar energy will help push it to about 10% of power generation by 2025, predicts Ron Pernick, principal of research firm Clean Edge.

"Just a handful of utilities doing something big changes the scale of the entire market," says Julia Hamm of the Solar Electric Power Association.