Reasons to clean solar panels:
The cleaner the surface of the panel, the more unobstructed surface, the more the sun has to "work with." How often should you clean your solar panels? That depends on where you live-if there is a lot of construction in your area, if you live in a windy desert or if you have lots of particulates in your air you will probably need to clean them more often. Some areas need a monthly washing, while other areas can get away with twice a year.
The more often you clean, you might even be able to get away with just rinsing maybe with some "soft water," assuming you don't have built up bird droppings or caked on dirt. A good scrub down and that meter may be spinning backwards. If your meter is spinning forwards in the middle of the day with the sun brightly shining, it means either you have a lot of appliances on or your panels are not doing their job. If the panels aren't doing their job, then you're losing money.
Dust on solar panels does a lot more than just make them look bad. According to Larry Greenemeier writing in Scientific American, "[a] dust layer of 4 grams per square meter can decrease solar power conversion by 40 percent," citing research conducted at Boston University.
Next time I blog... "Some cleaning options"