The answer is yes.  Like anything else, the better you take care of your solar panels, the better they will work for you.  Developing a regular maintenance routine for your solar panels will make sure that you are getting the best value out of your system.   ​ Dust, leaves, bird droppings and other environmental deposits will keep your panels from functioning at their full capacity. Clean Technica's report showed that the US National Renewable Energy Laboratory (NREL) showed a potential 30% energy yield loss per year for solar panels that were not cleaned regularly.

The answer to this is: "It depends".    Kipp and Zonen created a whitepaper ​that discusses the effects of soiling on solar panel arrays.   ​ There are many factors that go into deciding how often your solar panels should be cleaned.  Some of these factors include geographic location, environmental factors such as wind, dust and humidity, distance from trees, and angel of your array.   ​ As an example, dirt tends to be worse in desert areas like Southern California because the air tends to be dry and dusty.  ​ The experts at Sunshine Windows can help you determine how often your panels need to be cleaned and get you set up on a regular cadence to make sure your system is working at it's top efficiency.

In a short answer, No.  Large accumulations of dirt, dust, bird droppings and other environmental deposits make it so that a simple rain storm with not give your solar panels the cleaning that they need in order to work at their optimal efficiency.

While you may be able to wash away some of the topical dirt that is on your solar panels, simply spraying them with a garden hose is not going to provide enough cleaning to make a large difference in you system's efficiency.  Furthermore by using regular water on your solar panels, it is likely that they will develop hardwater (calcium) buildup over time.  The thick layer of calcium allows for less light to penetrate and reduces the output of your solar panels.  Sunshine Windows uses deionized water for solar panel cleaning.  This is accomplished by running our water through a 45" 135lb mixed bed deionization tank prior to cleaning your solar panels.