Nov. 9--Solar power installations are sprouting on California rooftops like leaves in the spring, but all that renewable energy poses new problems for the aging power grid.

Solar panels and wind turbines offer the promise of clean electricity by deriving their power from wind and the sun. But both suffer from the problem of intermittency: A gust of wind can cause a mill to spin faster, producing a power spike down the line, and a cloud passing over a solar cell can cause a sudden drop in electricity.

Meanwhile, utilities are rushing to upgrade the ability of the region's interconnected power grid, which is based on 100-year-old technology, to tap renewable resources without causing problems for users.

"We're going to see more change in the next 10 years than we've seen in the last 100," said Chris Baker, chief information officer for San Diego Gas & Electric Co. "This is happening fast."

California utilities don't have a lot of choice in the matter, either. The state has required them to get 20 percent of their power from renewable sources by 2010 and 33 percent by 2020. Today, much of that power is derived from large installations of solar panels or windmills, a central generation model that resembles the traditional electrical infrastructure of nuclear, coal and natural-gas plants.

But the state and federal government have both enacted subsidies for homeowners who want to install their own rooftop solar cells. Those subsidies, combined with a 9 to 13 percent drop in the cost of solar systems, have driven a sharp increase in demand for new solar systems.

Utilities each manage their own branches of a subsidy program called the California Solar Initiative. Southern California Edison's branch of the initiative said it has received more applications for solar cells in the first three quarters of 2009 than it did in all of 2008. In San Diego County, the program helped install 2.1 megawatts of rooftop solar capacity in October, the most ever for a month.

As the program meets new capacity goals, the government subsidies are reduced.

"The incentive is decreasing, yet the adoption rate is increasing," said Timothy Treadwell, an analyst for San Diego's initiative.

Rooftop installers say they're so busy, they're hiring new staff and new crews.

Daniel Jagudnik, a sales manager with Natural Energy in Escondido, said he has hired eight new salesmen and the company has brought on 10 new technicians since the first of the year.