Some exceptions might be if you notice excessive bird droppings, after heavy dust storms in places like Arizona or when you live directly next to or downwind from major highways, factories or agricultural fields. In these instances, it's possible to generate enough grime to call. 1. Be sure to check your panel manufacturer instructions for shutting down the system before cleaning, if necessary. Try not to go on the roof to clean your system. It's risky both for you and your panels. 2. Brushing any loose dirt off before spraying. Generally, cleaning your solar panels isn't something you need to bother with. The best way to figure out if your panels need cleaning is to assess the above conditions. For the most part,. Early morning or in the evening (ideally on an overcast day) is best. Too much sun on wet panels can cause them to dry too quickly and smudge.