A good way to see why rainwater will not clean solar panels is by looking at your car windscreen. If you didn’t clean your windscreen for 6 months, how much visibility would you lose?
If left, the grime on your windscreen would accumulate and eventually ‘green’ over. Even if the car is rained on, the rain contains airborne dust particles that settle and attach to the surface after the rainwater evaporates. When we use our windscreen wipers, we clean the centre of the screen, but where the wipers do not reach, there is a film of dirt. The whole of your solar panel is in exactly the same condition as the edge of your car windscreen because they are both exposed to the same elements. Visibility through your windscreen is lost due to the loss of light. Therefore, the same principles hold true for solar panel cleaning. It is very important to maximise the light getting into the solar panel and in turn, increase your financial reward.