When they have snow on them they don't produce at all. When they are partially covered with snow they don't work either. And when it is really cloudy they barely produce. They are very sun dependent. I know that is what solar means but I was a little surprised at how much the clouds affect them. I am very glad I had them installed though. Each one is independent so If one has snow on it but the one next to it is clear, only the snow covered one is not producing.