General Motors liked the idea of using the sun to power its buildings. But until recently, one immutable economic fact held G.M. back: The upfront costs were simply too high to justify the ultimate payoff.

G.M. is not alone. Even solar energy’s biggest fans concede that the high investment costs have kept companies from pursuing what is arguably the cleanest, most renewable and least politically sensitive energy source around.

But now, G.M. and a small but growing number of other companies and municipalities are getting solar energy from systems installed by others. Even though the installations are right on their own roofs, they buy the electricity much as they would from a utility’s grid. And because the companies that paid for the systems will get a steady income, they can provide power from the sun at competitive electricity rates.

Since June, the roof of G.M.’s parts warehouse in Cucamonga, Calif., has been host to a photovoltaic array with the ability to generate as much as 1.5 million kilowatt hours of electricity a year. The installation, which G.M. expects will provide half of the building’s electricity, cost G.M. nothing.

A solar developer called Developing Energy Efficient Roof Systems — commonly called Deers — bought the equipment with money it raised from private financiers. Deers and its investors own the cells; G.M. signed a long-term contract to purchase the solar-generated electricity from them, at a discount to the prevailing rate for electricity in the region.

These days, that rate is 9 cents to 10 cents a kilowatt hour; G.M. expects that the solar system will reduce its overall electricity costs by 10 percent a year.

The installation, which G.M. expects will provide half of the building’s electricity, cost G.M. nothing.