Most installers will inform people that their Solar panels are self-cleaning, to a degree this is correct, the angle of the panel and the rain fall we experience in this country does keep the panels clean.
The pitch of the panels on your roof are similar to the pitch of a conservatory roof so do conservatory roofs never need cleaning?
I would suggest that you consider having Solar panels cleaned at least once a year,
The service I can offer takes advantage of the pure water technology that I use for cleaning windows, this means that no chemicals are used, water is dispensed via a soft bristle brush and I do not need to climb onto the roof possibly damaging tiles and panels,
Any price quoted would remain the same for three consecutive years (on condition the panels are cleaned on a 12 month rota)