Cleaning Solar Panels Uneccesary?

Yup, I'm still on my solar tangent. Getting close to deciding on a contractor, but not yet. I ran across this Google study on their solar installation. Their study shows that tilted panels, like virtually all residential rooftop mounted systems, don't need to be regularly washed. Rain does a good job of cleaning off the dust. This would discount SunRun's solar panel washing benefit. Panels that are installed horizontally do benefit greatly from a cleaning, however. To see more details on the Google study, check out their slides.