Not sure. My experience is with a nursery I go to for native plants. They have no power other than solar. Their old panels recently replaced did not work at all when overcast or barely. The new panels keep on cooking along streaming power all day long. Those new ones work on a dark cloudy day just as good. Seems as long as there is visible light they produce power. So, I would think like Besarien that a light to moderate dusting may only act like a filter especially with a sunny day.