The University of California at San Diego published an interesting report on solar panel cleaning. This article and the comments are quite interesting. To me the main conclusion is you can’t make a clear conclusion if or when to clean your solar panels. I did a survey on the Tesla Motors Forum about the frequency of solar panel cleaning and the results were as follows:
54% Clean their panels less than once a year
20% Clean their panels about once a year
15% Clean their panels a few times a year
11% Clean their panels four or more times per year.
The statistical number is not large as the poll received 35 responses.
The need for cleaning really depends upon where you live, how much rainfall you receive, how much dust, pollen or leaf accumulation you get, and how many bird droppings occur. Also if your panels are very easy to access, you can easily wash them down from the ground with a hose.
The first time I had them cleaned, it was definitely financially worth it, the second time was not a noticeable difference in output as I reported here.