Generally, don't hire someone to wash your dirty solar panels, wait for the rain to do it for you. That's the conclusion of a study recently conducted by a team of engineers at the University of California, San Diego.
Researchers 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. Overall, 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, which would not cover the cost to clean.
For larger commercial rooftop systems, the financial losses are bigger but still rarely enough to warrant the cost of washing the panels. On average, panels lost a little less than 0.05 percent of their overall efficiency per day.
Data came from the output at 186 residential and commercial sites.
They compared output after more than 0.1 inches of rain fell on the panels with output during the 145-day summer drought California experienced that year. The panels would have been cleaned by rain but would have remained dirty during the drought, researchers reasoned.
"Dust on PV panels does make a difference but it's not a big enough factor in California to warrant cleaning as it is removed when rain comes"
Researchers also found that solar panels mounted at an angle of less than five degrees caused bigger losses in efficiency. That's because dirt slips off panels that are installed at a steeper angle.
Panels heavily soiled with bird droppings for example should be cleaned. That's because the droppings essentially block all sunlight and will not be washed away when it rains. Engineers also found that at a few sites, photovoltaic panels were dirty enough to warrant cleaning due to very specific and localized circumstances. For example, being directly next to and downwind of a highway, factory or significant dust from agriculture may generate enough dirt to warrant cleaning.