When discussing solar chargers, panels or anything to do with the technology of solar energy, possibly the most frequently asked question alludes to its effectiveness on cloudy days. Although it has alluded to a few times in this guide, it is important to fully understand their usefulness on cloudy days. To provide optimal performance, solar panels need the rays from the sun. This is not something you can get around, but they are not rendered useless on cloudy days (depending on the model). They will still produce power but at a less efficient and powerful rate.
To be honest, it does not matter if it is clouds or even a roof or massive tree cover, without direct access to sunlight solar panels does not perform to their maximum capacity.