We know that dust, dirt, pollen, bird droppings, and other debris can negatively affect solar panel efficiency.
Google conducted a groundbreaking experiment at their 1.6 MW solar farm in Mountain View, California. They found that cleaning the solar panels was “the number one way to maximize the energy they produce.” Cleaning solar panels that had been in operation for 15 months doubled their output of electricity.
Google concluded that for flat solar panels, relying on rain alone is not an adequate way of cleaning solar panels.