The rain effect – research (link)
Accumulation of dirt on solar panels impact the functioning of the PV system in areas with little rain and long dry periods such as California and Southwest U.S. This article describes the effect of dirt on large solar systems where panels are connected, and describes a model that will predict loss of profit as a result.
The article concludes that in dry places energy output is reduced by 0.2% per day. Annual electricity output loss is 1.5 to 6.2% per year.