Since I've had more solar panels on my Roadtrek RV longer than anyone else, I get asked occasionally what i have to do to keep them working. The answer is… hardly anything. They really take care of themselves, pretty much. I'm the sort that likes to do all kind of unnecessary maintenance, and all I do is clean them once or twice a year.
Don't stay up at night worrying about dirt on your panels cutting down on the amount of electricity. The available research on static panels in heavily urbanized areas indicates that dirt only cuts down on yield about 3-5%, and that's in rain-deprived southern California where they go months with no cleansing showers. Driving helps keep the panels clean, especially driving in the rain, and a decent hailstorm will REALLY clean them off well – even gets the bug splatter off the front edge. I would estimate that the amount of dirt on my panels when they come due for their semi-annual cleaning isn't enough to knock off a percent or two from the yield – I recently cleaned them and didn't detect any measurable difference in yield before and after, even with my fancy meters.
If you want to go to the trouble to clean your panels anyway, make sure you have the proper equipment to get up there and move around without endangering yourself. I use a seven foot step ladder, but I'm kinda tall and have long arms. As far as cleaning equipment, use only a terrycloth rag or soft sponge – no scratchy brushes on long handles.
The top surface of solar panels is tempered glass, which you want to go to great lengths not to scratch. No bristle brushes, no abrasives, nothing like that. Tempered glass gets its strength from the tension created by cooling the outside surface while the inside is still molten, creating hundreds of pounds of pressure. Any scratch weakens the surface, which is under very strong tension. It's much stronger than regular glass, but once it goes, it shatters into pebbles. The side windows of your car and the glass tops on your sink and cooktop are also tempered glass.
I don't use soap to clean mine – they don't pick up oily road grime being so high up, and anything you use in addition to clear water might leave a film, so the minimum necessary to loosen the dirt and float it off with a stream of water is desirable. I usually just use a terrycloth rag and a hose, reaching in halfway from each side and moving the ladder down the side and around to the other side until I'm done. Moving the ladder frequently is a lot easier than recovering from a broken hip or head injury, so don't lean out far enough to get your center of gravity outside the ladder footprint.
That's about it – i just cleaned mine after parking under a tree for the last three months, and it probably did more to assuage my desire for unnecessary maintenance than it actually increased the solar harvest. But I'LL know it's clean, so now I can sleep at night 😉