"This is the largest customer-owned solar electric system at a corporate site," said Noah Kaye, director of public affairs at the Solar Energy Industries Association, an industry group based in Washington, D.C.

A Google executive said the company will rely on solar power to supply nearly a third of the electricity consumed by office workers at its roughly one-million-square-foot headquarters. This excludes power consumed by data centers that power many of Google's Web services worldwide, he said.

"We are going to be producing roughly 30 percent of the power that we use," David Radcliffe, vice president of real estate at Google, told Reuters in an interview. "This is for our corporate-office people center," he said.

Radcliffe declined to comment on the cost of the project or whether the solar generation equipment might pay for itself over time. "We wanted to dispel the myth that you can't be both Green and profitable," he said.

While the move marks a major demonstration of support for alternative energy, the project may only make only a small dent in the overall amount of energy consumed by Google. A utility industry rule of thumb is that data centers consume 10 times more electricity than buildings used to house office workers.

Most of the solar panels will sit on the rooftops of office buildings in the Googleplex -- the pet name for the site. Others will provide shaded parking as part of newly constructed solar-panel canopies over existing Google car parking lots.

Earlier this year, Google rival Microsoft got the jump on Google with a 2,288-panel solar system at its research site in Mountain View that is expected to produce 480 kilowatts at peak capacity, the first large-scale use of solar power at any Microsoft office worldwide.

Silicon Valley's Cypress Semiconductor, majority owner of solar cell maker SunPower Corp., has a 336-kilowatt system generating more than 10 percent of its corporate-office electricity needs, spokesman Matt Beevers said.

The consulting arm of Energy Innovations Inc., a company supported by venture backer Idealabs, has built 12 large-scale solar projects across California and is managing construction of the Google project.

Prior installations include the Richmond, California campus of Berlex, a unit of German drugmaker Schering AG, Sonoma State University and the Marin County General Services Building.

Sharp Electronics is supplying 9,212 solar panels for the Google project. Sharp Corp., parent of the solar panel maker, has a 5.2 megawatt solar generation system, the world's largest corporate solar system, at a Kameyama, Japan factory.

The world's largest dedicated solar-powered generation station is a 12-megawatt facility in Arnstein, Germany, near Frankfurt. The top US dedicated solar facility generates 4.6 megawatts and sits in the Arizona desert near Tucson, according to a solar generation database compiled by PVResources.com (http://www.pvresources.com/en/top50pv.php).