My flat panels lose about half their electrical output if I don't clean them for a year.

They end up covered in moss and mud.

Wow. I have panel for 3 years now, never cleaned them actively (even if the installers offered me the service for a nice monthly fee), and never noticed anything like that. Well, let's wait 1-2 months to see if it's still the case, at least (April/May is the best month as far as production is concerned)

They're super easy to clean if you can get to them. Just 30 seconds with a regular household mop and they're shining again.

I'd recommend doing it at night, because I got a nasty zap off mine when presumably some of the mopping water got into the electrics and came up the metal mop pole .. (the electrics are all supposedly fully waterproof, but I guess on my 10+ year old panels stuff has degraded)

They are on the roof, it's a two stores house, I think I will never be able or willing to do it personally :/

You are correct. It’s especially true with a raised frame on the edge of the panels.