Rainwater alone cannot clean your solar panels much in the same way that rainwater will not wash your car. Some of the debris may be rinsed off but a layer of dirt, grime, salt residue and pollen will still accumulate. Without a clean surface sunlight doesn't fully reach the solar cells. A professional cleaning with pure deionized water is the only way to get the panels completely clean and operating at full energy efficiency.

Regular tap contains trace amounts of minerals. While this is ideal for drinking, it is these same minerals that lead to streaks and residue when cleaning your solar panels. When water is purified, these same minerals are removed. Deionized water’s ability to attract dirt, eliminates the need for a chemical cleansers. Soaps can leave a film or residue that not only shades panels like the dirt that was just washed off, but it can also encourage dirt to stick and build up faster. We use a three stage RO/DI system. First, the water goes through a carbon filter to remove any sediment. Next, it hits the RO membrane, which removes 95% of the remaining contaminants, and then the DI resin canister removes the rest. We use a TDS meter (totally dissolved solids) to ensure that the filtered water is pure down to a measurement of 0 PPM (parts per million). A soft brush is used to agitate the dirt from the glass and then it is rinsed completely with the deionized water. Because there are no minerals in the water when the panels dry they are spot and streak free. Pure water is completely harmless to both your property and the environment around it 

Solar panels are most efficient when they are clean. Having them cleaned on a regular basis maximizes the systems ability to generate electricity. For optimal performance it is generally recommended to have solar panels cleaned every 6 months to a year.