Worried that your rooftop photovoltaic (PV) panels are cranking out less power than they should because they’re dusty? Don’t be. Paying someone to wash them is likely to cost more than what you’d gain in electrical output, engineers at the University of California, San Diego, said.According to an article in Laser Focus World, the efficiency of panels that hadn’t been cleaned or rained on for 145 days during a drought had dropped by 7.4%. But washing the panels midway through the summer would have boosted the value of electrical production by just $20.Power losses are greater for commercial installations, but even there probably not enough to justify the cost of a cleaning. For very large installations, however, washing made more sense economically.The conclusions were based on a study of 186 solar installations in California in 2010. Jan Kleissi, the report’s principal investigator, said pollution and dust levels in California are fairly typical of other parts of the country, if a little dustier. In areas that get more rain, panels would tend to stay cleaner, with even smaller energy losses, making for an even more compelling argument against paying for a cleaning.One exception is bird droppings. Rain won’t wash them off and they block virtually all light, the study said. So if your solar panels happen to be in an avian bombs-away zone, it probably makes sense to clean the panels once in a while.The study was originally published in Solar Energy.