Cleaning solar panels is often not worth the cost if trying to improve their efficiency, scientists at the University of California, San Diego, say.

They found panels that hadn't been cleaned, or rained on, for 145 days during a summer drought in California lost only 7.4 percent of their efficiency, the university reported Wednesday.

For a typical residential solar system of 5 kilowatts, washing panels halfway through the summer would translate into a mere $20 gain in electricity production until the summer ends, the researchers said.

"You definitely wouldn't get your money back after hiring someone to wash your rooftop panels," mechanical and aerospace engineering Professor Jan Kleissl said.

That applies mostly to smaller systems, he acknowledged; for very large installations like solar farms, economies of scale could make it cost-effective to clean or wash panels.

In the small systems in the study, panels lost less than 0.05 percent of their overall efficiency per day, the researchers said.

"Dust on PV [photovoltaic] panels does make a difference but it's not a big enough factor in California to warrant cleaning," researcher Felipe Mejia of the university's Jacobs School of Engineering said.