My Australian brother in law is the owner of many solar panels. He says that cleaning these causes a significant increase in electricity generated.
People often drive about without realising that their view or their lights are somehow impaired.
Today’s invention is to apply a small solar cell to the inside surface of all of the lights, window glass and indicators on a car.
When any of these generates a lower current than a calibration cell (placed somewhere high on the vehicle), it indicates that the local surface is obscured eg by mud, ice, grime or crazing. Time to take some action.
You could also use these sensors to rate different car washes for cleaning efficiency.