Solar panels are typically made of two or more layers of crystalline silicon wafer cells which have been chopped to allow better light transmission. The cells bring in energy during the day in either clear sky conditions or with low-hanging clouds. When exposed to raindrops (or any other type of liquid), they lose efficiency because they block out sunlight penetration--in addition to causing puddles near the panel that can cause mineral buildup over time. That being said, solar panels can still produce electricity under cloudy cloudy skies although production is diminished.