1. A computer processor implemented method of developing irradiance mapping using a distributed network of solar photovoltaic systems, said method comprising the steps of: selecting a predetermined geographic area having at least five solar photovoltaic systems to provide a photovoltaic system; calibrating the photovoltaic system; reversing the photovoltaic system performance model using a computer processor to solve the irradiance input value; calculating irradiance according to said irradiance input value, energy output and weather data using said computer processor to provide a single irradiance point; and mapping at least two said single irradiance points to create an irradiance map.