Solar panels are generally selfcleaning during the raining seasons but in particularly dry seasons dust and other substances such as bird droppings can build up over time and impact on the amount electricity generated by a module. Grime and bird poop doesn't need to cover an entirepanel to have an effect. its important to have a professional clean your panels thoroughly.. we offer solar panels cleaning services and free battery and inverter check>up kindly reach us through this line 07062396471