The largest utility in California, squeezed by rising demand for electricity and looming state deadlines to curb fossil fuels, has signed a deal to buy solar power from seven immense arrays of mirrors, towers and turbines to be installed in the Mojave Desert.

The contracts amount to the world’s largest single deal for new solar energy capacity, said officials from the utility, Southern California Edison, and BrightSource Energy, the company that would build and run the plants. When fully built, the solar arrays on a sunny day would supply 1,300 megawatts of electricity, somewhat more than a modern nuclear power plant.

That is enough electricity to power about 845,000 homes.

The companies acknowledged that several hurdles would have to be surmounted before the first surge of electricity flows from the desert  in theory around 2013  toward power-hungry cities more than 200 miles away.

First is approval by the state Public Utilities Commission. But more challenging, they said, is a series of permits for improving transmission lines. That process in the past has taken seven to 10 years per project, said Stuart R. Hemphill, vice president for renewable and alternative power for the utility.