A simple but effective visual interpretation of how solar panels work, and how dirt affects their productivity.
Splodges Of Dirt / Algae / Lichen / Moss / Bird Droppings
If any individual cell on your solar panels is dirty, that cell must be isolated using a bypass diode.
This is necessary because the panels are wired in series; if one cell is not functioning properly, it can cause the performance of all other cells to drop to the same low level.
To address this issue, bypass diodes are incorporated into the system, typically with three diodes per panel. When one or more cells are dirty, the section containing the affected cell is turned off by the bypass diode.
Therefore, if you notice a single spot of dirt on your panel, it indicates that one-third of that panel is inactive, resulting in a significant reduction in power generation.
What About Dust & Traffic Film?
Dust and traffic film has a different affect on your solar panels performance.
Until a single cell, or multiple cells, are bad enough to trip the bypass diode, then they stay active. However, all cells will be operating at a reduced efficiency due to the dust and traffic film (or any other dirt).
How much they reduce in efficiency depends on the amount of light being blocked by the grime. It's never possible to accurately determine the likely efficiency gain from cleaning the panels. Some say between 10% to 30%, but there are too many variables to be accurate.
There is agreement that any dirt will affect the performance of your panels.
As for claims that Solar Panels Are Self Cleaning, you don't need to look at many panels to know that isn't entirely correct.
