Do Solar Panels Need Cleaning?
Before we answer this I should declare an interest: we clean large scale solar plants and our experience tells us that almost every solar site benefits from a clean at least once a year. But don't take my word for it; here's a study that looks at the "Influence of Dirt Accumulation on Performance of PV Panels". If you don't want to wade through the technical stuff, let me summarise it for you:
They used spotlights to control the conditions (simulating solar radiation) and they tested a number of different contaminants: dust, sand, moss etc. One thing they didn't test (disappointingly) is animal waste (birds: we're looking at you!), and in Australia this is actually one of the most common. To cut a long story short they found some contaminants reduced production by as much as 85%, but please note that there is quite some difference in resistance between a light coating of dust and moss, and a great deal will depend on rain and other environmental factors.
The conclusion was that rain helped but was no substitute for a regular maintenance programme, and clearly the moss has to go...