Researchers in California discovered that solar panels that had not been rained on or cleaned for 145 days lost only 7.4% of their efficiency. For a typical consumer with solar panels on her house, having the panels professionally washed would translate only into a $20 gain in electricity production over the course of a year – hardly worth it when compared to the cost of professional washing.
Commercial rooftop systems also do not have enough of a gain to justify the cost of washing the solar panels. On average, panels lost a little less than 0.05 percent of their overall efficiency per day.
The researchers noted that they focused primarily on smaller residential and commercial solar systems, so time may tell whether or not washing large installations is cost effective.
During the research, engineers did discover that panels mounted at an angle of less than five degrees caused losses in efficiency since dirt slips off panels that are installed at a steeper angle.
There are special circumstances that do make having solar panels professionally cleaned a great idea. Dust storms, bird droppings, massive air pollution, construction residue, pollen, sea salt… But washing away normal daily dirt accumulation does not seem to be worth the cost and effort.