According to a recent study conducted by engineers at the University of California, San Diego, cleaning dirt and dust from solar panels is more effort than it’s worth, Phys.org reported. The study found that panels left uncleaned or rained on for 145 days lost only 7.4 percent of their efficiency, leading to about a $20 loss over the course of the dry season. The study found larger financial losses for commercial rooftop systems, but the losses were not large enough to warrant hiring a professional cleaner. For very large installations, however, researchers found that cleaning the panels could be a good investment, depending on the size and location of the facility.