Updated: May 4
Panels just to the North of West, giving the sun the opportunity to heat your hot water for free in the afternoon.
The dirt or tree sap layer would be stopping the panel from collecting heat. It would be acting like constant shade on the panels so affecting the amount of hot water this panel can make.
In the winter months the combination of the way the panels are facing, and the shade caused from the muck on them, would affect the amount of free hot water supplied by the sun,
Cleaning of your panels once a year, around April is a really good idea. We will always help you remember this when part of the Sears Solar Community by sending you a reminder to give them a clean.