Hey Mr. Green,

I generate about 140 to 160 percent of my annual electricity consumption while putting excess power back on the grid. But I have never bothered using software to track my solar panel performance. How much better do solar panels perform if they're washed each year? 

—Michael in Santa Clara, California

University of San Diego researchers looked into this question and concluded that (with some exceptions) since less than 0.05 percent of energy could be lost per day due to dirt, it was not worth the effort or the expense to clean solar panels. Even a mild rain rinses off most of the obstructing dirt, pollen, etc., which led them to conclude that hiring someone to clean the panels does not pay off. 

There are, however, a few exceptions that can make cleaning the panels worthwhile. If your panels are installed at less than a 5 degree angle, then they should be washed periodically because rain alone might not carry away the accumulated dirt.

Engineers also found a few sites where photovoltaic panels were dirty enough to warrant cleaning due to specific, localized circumstances. For example, being directly next to and downwind of a highway, factory, or agricultural field may generate enough dirt to warrant periodic cleaning.

A third issue is bird droppings, which, if heavy enough, could block enough sunlight to make a cleaning necessary. 

If you do find it necessary to clean the panels and wish to carry out the task yourself, contact the installer to find out if you should take any special precautions aside from not falling off the roof. Two obvious measures: Don’t spray a hard blast of water on the panels and don’t use an abrasive sponge, as these can actually damage the panels, reducing their efficiency.