What sort of cleaning regime should I have for a solar energy system, and do PV and solar-thermal panels need to be treated differently?
I only have one anecdote and avague idea: An agrarian schooling center (Haus Düsse) in Germany clean their PV System once a year, and clean/dirty means a difference of 10% effectiveness. However, as this is basically a small farm with lifestock, different grain treatments, lots of diesel vehicles etc. you would expect far more dust there than at a lot of other places. 3 years ago, they were starting to systematically analyze what's the most economic cleaning regime.
I work and attend classes at a University in Florida. Our 100kW system has been installed for more than five years without a single cleaning. I have walked the solar field several times and can confirm that no significant dust has collected. (The angle of inclination is about 27 degrees.) But, Florida rains a lot and that helps keep everything clean. I imagine desert areas with wind storms would be much worse. But a duster at best or some cotton rags and dish-soap at worst would be more than enough to keep any system clean.