I heard this statement in my travels recently and thought it was an important issue to explore.
Does rain keep solar panels clean?
If it were correct then theoretically I should be able to just splash purified water on windows to clean them but I know this doesn’t achieve a professional result! The common denominator when cleaning windows using the traditional method (washer and squeegee) and the latest water fed poles is the need to agitate the dirt into solution (with a washer in the case of the former and a brush with the latter) before either pulling this solution off with a squeegee or rinsing it off with purified water. In any case the agitation phase is critical.
Solar panels attract an enormous amount of dirt, dust, grime and droppings. When the sun is shining it essentially bakes this debris onto the glass. Panels may look clean from a distance but it can be a different story when you take a closer look. Also, when cleaning panels I am often amazed at how much dirt emerges from under the frame at the edge of the panel.
Rain, while water in its purest form, does not agitate panels sufficiently to remove baked on stains. It is this build up of dirt that can effect the efficiency and possibly the life of the panel.