Southern California Edison has finished California's largest commercial rooftop solar panel installation to date – the first step in its $875 million plan to put 250 megawatts of solar panels on two square miles of rooftops.

The utility said Monday that its installation on a 600,000-square-foot roof of a Fontana, Calif. distribution warehouse owned by ProLogis will generate enough electricity to power 1,300 homes in Riverside and San Bernardino counties.

Phoenix, Ariz.-based First Solar (NDSQ: FLSR), which supplied the 33,700 thin-film panels for the Fontana project, said it would generate 2 megawatts of electricity (see First Solar Scores SCE Panel Bid).

Applied Materials and Google have rooftop solar installations at their respective California corporate campuses that generate 1.9 megawatts and 1.6 megawatts respectively. Both of those installations are made from crystalline solar cells. There is also a 14 megawatt ground-mounted solar installation at Nellis Air Force Base. Japan and Germany also sport several large privately-owned solar installations.

First Solar, which now holds a lead in the thin-film solar panel market with its cadmium-telluride panels, will also supply Southern California Edison's next solar rooftop project, a 1-megawatt installation on an industrial building owned by Multi-Employer Property trust in Chino, Calif.

Southern California Edison made waves in March when it unveiled its massive-scale commercial rooftop plans, meant to supply enough electricity to power 165,000 homes. The utility hasn't announced any other sites for its solar rooftops projects beyond the first two.

Those plans are just one part of a big push by the utility and others in California to sign up renewable energy projects to meet a state mandate to produce 20 percent of its electricity from renewable sources by 2010 (see California to Get More Solar-Thermal and PG&E to Buy 800MW From OptiSolar, SunPower). The state may not hit its goal, according to many, but it is already contemplating upping its goals to get 33 percent of its power from renewable sources not including large-scale hydroelectric by 2020.

In March, Southern California Edison said it wanted to have its first rooftop solar system up and running by August, which would put Monday's completion four months behind schedule.

But the utility had to wait until September for the California Public Utilities Commission to authorize the first three installations. Southern California Edison said its entire $875 million project is awaiting regulatory approval due in March.

While Southern California Edison may be behind schedule in its solar rooftops project, it hasn't had to scale it back yet. That's what happened in North Carolina in October, when utility Duke Energy cut in half an ambitious two-year solar installation plan after facing criticism over its projected costs (see Duke Chops $100M Distributed Solar Project in Half).

Duke Energy originally planned to spend $100 million to build 16 megawatts of rooftop and ground-based solar power projects (see Duke Eyes Residential Rooftops). Its scaled-back plan will seek $50 million to install about 8 megawatts of solar power projects instead.

Duke Energy is also facing a state mandate to increase its share of renewables. North Carolina requires its utilities to get 12.5 percent of its power from renewable sources by 2021 and mandates that they include solar power in that mix starting in 2010.

About half of the states in the country have similar mandates to promote renewable energy, which has pushed utilities to sign contracts with renewable power developers – mostly solar and wind power.

The Californian mandate for 20 percent renewable power by 2010 applies only to investor-owned utilities, not to public utilities like the Los Angeles Department of Water and Power.

But Los Angeles Mayor Antonia Villaraigosa said last week said he'd like the city to get 1.3 gigawatts of power from solar sources by 2020 (see L.A. Trots Out Ambitious Solar Plan).