I’ve not been on the roof to clean my panels since I put them up last July. This past Spring, I had to wash my car twice to get rid of a sticky yellow buildup of pollen, but it didn’t occur to me that this same pollen could be on my panels. Imagine my surprise when I went up there and found the entire surface of all the panels to be COVERED with pollen – it was like 150 grit sandpaper! This most certainly caused the light to diffuse and reduce the amount of solar energy impacting directly on the solar panel glass!! About 15 minutes with a bucket of soapy water and a cleaner/squeegee made them good as new. Solar output increased immediately by about 5% compared the the day before (both days equally sunny with no clouds). Moral of the story: if you want max power, you need clean panels! If you can reach them with a stream of water from a hose, do this regularly to prevent buildup of dirt and pollen!