Cloudy days might sound like the Achilles heel of solar panels, but this is simply not the case. Even on the cloudiest of days, a single solar unit will still work at 25% efficiency. As mentioned above, solar panels convert ultraviolet rays from the sun into a usable energy output. These ultraviolet rays are so energized that they easily pass through cloud cover allowing your solar panels to work efficiently. This is the same reason why you should wear sunblock during the summer even on cloudy days because of how powerful ultraviolet light is.