Amid the enthusiasm, it's important to keep this latest twist in the solar saga in perspective. Solar energy of all kinds fulfills less than 0.1 percent of electrical demand in the United States, and affordable, commercially available panels have hovered near 15 percent efficiency for years. Despite the recent burst of corporate enthusiasm, the prices of solar modules are expected to continue inching down at just 5 percent a year, and grid parity—the point at which solar panels can compete subsidy-free with utilities—isn't expected until 2015 at the earliest.

It's too soon to say whether these costly corporate installations will go down in history as the first of a limited series of impulsive, feel-good publicity moves by tech start-up billionaires, or as the beginning of a longer-term movement that will help sustain the market for solar photovoltaics during the next decade and enable solar to finally become cost-competitive. One thing is certain: the movement will flourish only to the extent it is nurtured by a complex patchwork of economic and bureaucratic conditions. Of the 9509 new grid-tied solar installations in the United States in 2006, which totaled 101 MW, 70 percent of them were in California. And that's not just because it's sunny. As it turns out, California subsidizes solar in a particularly generous way.

Even without subsidies, solar panels may have found their logical home, at last, in the commercial world. The nice, flat roof design on most commercial buildings, unlike the pleasingly angular but less workable residential roof, is one obvious advantage. But some of the most compelling reasons are intangible.

“It's a key part of attracting and retaining employees,” says Doreen Reid, a senior associate at The Climate Group, a London-based nonprofit that helps companies reduce their carbon emissions. “Students coming out of college are more conscious of a company's environmental image.”

Robyn Beavers, Google's corporate environmental programs manager, confirms the transformative power of solar cells. “I've had so many people e-mail me and say, ‘This is why I love working at Google' and, ‘How can I install solar at home?' ” says Beavers, who presided over the installation project. Google's solar enterprise is part of a larger mission to promote the growth of solar energy, she says. Google founders Sergey Brin and Larry Page have invested heavily in Nanosolar, a start-up that specializes in thin-film solar cells. (Both declined to be interviewed for this article.)

For its rooftops, Google chose Sharp modules capable of generating 208 watts each. The polycrystalline silicon cells average 12.8 percent module conversion efficiency. Because solar panels produce dc current, each system requires inverters to change the current into usable ac, and Google used a set of utility-grade inverters with an average of 96 percent conversion efficiency made by SatCon Technology Corp. of Boston. It partnered with EI Solutions, a solar project developer with headquarters in San Rafael, Calif., to do the electrical design work.

Google won't say how much the whole project costs, other than to indicate that it expects to recoup its investment in five to seven years. Nonetheless, experts estimate that a solar installation costs between US $3 and $5 per watt in California, and between $6 and $10 per watt in the rest of the United States after factoring in local and federal rebates for the cost of the system. (According to the Northern California Solar Energy Association, the average cost of installing large systems in the Bay Area in 2006 was $8.58 per watt before rebates, on par with national 
figures.) Using data from California's Solar Initiative program and based on a $2.80-per-watt incentive rate, Google likely retrieved about $4.5 million from California on a project that in total probably cost more than $13 million. Federal tax breaks through the Energy Policy Act of 2005 also help to burnish the appeal of what is still, for many, a prohibitively expensive system.

For other companies, an important piece has been added to the picture for solar. Where research and development have so far failed to slash the price of solar, clever financing schemes have filled the breach. Google's solar project, for all its trendy impact, was financed the old-fashioned way—with cash. But customers without billions of dollars in liquid reserves tend to shy away from such a move.

Rather than requiring that customers buy all the equipment for an installation, which can run into the millions of dollars, solar service providers are persuading customers to sign agreements that in effect turn those providers into miniature utilities.

The office-supply company Staples was among the first to pursue such a scheme—in 2004 with SunEdison, a prominent Beltsville, Md., solar electricity service provider, for a 280-kW installation on two of its California warehouses. The solar installation covered about 10 percent of the facilities' electric loads.