Cleaning the solar panels operating for 15 months has made their output of electricity double.

Rain is not a satisfying way of cleaning solar panels. Solar panels that were cleaned professionally had a 12% higher output of electricity compared to those cleaned by rain. This literally means that they will generate 15-25% less electricity, which you will have to source from the utility company at their price rates. This easily adds a couple of years on the payback time. So it is important to clean solar panels in order to protect and maintain your investment.