In short, YES, cleaning your solar panels can definitely make a difference in efficiency
having seen and proven this ourselves many times over the years we can definitely confirm that it does.
In general you would expect about a 15-20% improvement on performance, in some cases where there is a lot of airborne contaminants, high humidity causing mould to build up and bird droppings there will be up to 40% improvement once they are cleaned.
“a drop in the efficiency of a solar PV panel is not desired. One of the contributing factors in the drop of efficiency in solar PV panels is the accumulated dust on the solar panel. In practice, dust must be removed from the surface of solar PV panels in order to ensure highest performance.” Solar Energy Power Association notes that dirty solar panels can lose 20% of their energy output. The National Renewable Energy Laboratory puts that figure even higher, at 25%. Recent university research has shown that a dirty solar panel can lose 50% of it’s efficiency compared to a clean solar panel.”
~ The World Academy of Science, Engineering & Technology