Read an interesting article the other week. Wish I could remember where but I've reached that age.
Chap was pointing out that the reason you need your solar panels cleaned every year isn't for the immediate gain in output, which is fairly minor on rooftop panels. Instead, it's to prevent dirt being left on them that would encourage the formation of lichen that, in the long term, can severely reduce the output of panels and which, as we all know, is pretty resistant to cleaning.
Clean now in order to prevent not being able to clean in the future.
Interesting point, imo.
Vin