A question I get all the time from home owner’s, after installing their solar generation system, is “Do I need to clean my solar panels?”. Solar panels will typically be cleaned off during a rain-storm, but to get an idea of how much efficiency and savings would be gained cleaning them yourself we need to look at where you live, how often it rains, the pitch of your roof, and how much your utility is charging (price per kWh).
If you live in an area with regular rain storms like Portland, Oregon or Houston, Texas, it probably does not make sense for you to clean your panels as the added effort your putting in would have a very minimal effect. A study done by Engineer at UC San Diego found that, during a drought, solar panels lost less than 0.05% of their efficiency per day. This means in an area of frequent rain, your panels will likely lose less than 1% efficiency and the savings you get from cleaning your panels will be a couple of dollar (at most) on a typical residential solar system.
Even if you live in an area with infrequent precipitation, it probably won’t make sense for most individuals to clean their solar panels. An Engineer team at UC San Diego tested a 5-kW system that hadn’t received any rain or been clean in 145 days and found that it lost only 7.4 % efficiency in that time which cost the system $20 on their electric rate. If you are the one who would be getting on the roof and cleaning them then this may be worth it, but most any company that would provide this service is going to charge you more than $20.
This study was done in California, so it is important to note that these efficiency losses (due to dust and dirt) could be greater or less depending on where you live. For example, desert areas with lots of wind blown dust will have an increased efficiency loss and for greater periods of time when compared with this study done in California. In these situations, it starts to become more advantageous for homeowners to clean their panels during a drought. Even in this situation, I do not believe it makes sense to pay someone to come by and clean your panels, as I don’t think you would see a return on that investment (average solar panel cleaning services charge $120-$160).
One Situation where it really makes sense to clean your solar panels is when you have them on a very low tilt. When your panels are relatively flat pitched, they will collect a lot more dirt and be much less likely to “self-clean” during a rain storm. A low pitched solar cell will collect puddles of dirt and mud that will make it extremely difficult for the sun to get through and have the panel work efficiently. It is important to note that your solar panels should not have been installed at a near flat pitch (in most regions), but if they are, then it would be very beneficial for you to clean them or maybe even have a company come do it if you are not comfortable being on the roof.
The final factor to consider is your electrical rate. A 2% efficiency loss will have a different financial impact depending on how much your utility charges you per kWh. The higher your rates are, coupled with the other factors discussed, the more it makes sense to clean your panels or hire someone to do it.