Now if we get one of the those periods of two months without any rain
fall with a lot of pollen and dust you may want to hose off the solar
panels but it really is not all that important. (I high pressure hose nozzle
so you can do the washing from the ground and never up on the roof.
Dusk is the best time of day so minerals do not flash dry onto the panel
glass creating a greater haze.) The future maintenance cost of a solar PV
system is virtually zero. You may want to have the panels professionally
cleaned every now and then, but usually the rain will do most fo the
work required to keep the panels clean.