photovoltaics – Is washing solar panels useful/needed/mandatory? – GWC Mag

It really all depends on the local climate.
In dry regions, dust will build up over time on the panels and that will slowly have an effect on output.
In wet regions, like Ireland, the panels get washed far more than they need so other cleaning is unnecessary.
In hot, wet regions you can get growth in any part of the panel that retains water, i.e. where the laminate meets the aluminium frame, and if left uncleaned this will in time have an effect.

So I thinkg it would be rare that the panels need cleaning every year but definitely keep an eye out for dust buildup and/or growth.