From HeatSpring.com PV Boot Camp Discussion Board
To keep modules operating at their peak efficiency, is there a recommended cleaning solution or is just plain old water the best thing to use? In SoCal I've seen many modules that are so dirty they could probably grow plants on them.
I like using a window analogy. Many people never wash their windows but still get the sunshine.
If you have plants growing out of your PV, that would be shading and time to do something.
I would say that most residential PV systems are primarily cleaned by the rain. If you are in a location where there is a lot of soot, dust, smog, then you will have to do more.
It is usually obvious when the PV needs to be cleaned.
I just saw some pollen on my mom's array. I am hoping it will rain soon and I would expect that she is at a 10% loss of energy until the rain comes or someone sprays a hose up there. Is it worth it for her to pay someone $100 to climb on her roof and do a good window washing quality job? Probably not a good return on investment. I just bought some 250W PV modules for $152 each. I could put one of those on my mom's roof and make more energy without cleaning the PV ever, than if I did spend $1000 in 20 years cleaning them.
If the array is large, there will be a better argument for cleaning the PV.
I live in the California mountains on a dirt road. I used to live on the other side of the dirt road. Due to wind patterns, I used to get a lot of dust and now I get none. Which side of the dirt road you live on, can make all of the difference in the world.
There is a rodeo with PV and often when I drive by, I can't really see the PV. I think someone there is trying to prove that solar doesn't work.
Here is a Solar Pro article on utility scale soiling:
It comes down to.... It is really site specific and usually obvious.