The solar rating is a measure of the average solar energy (also called "Solar Irradiance") available at a location in an average year. Radiant power is expressed in power per unit area: usually Watts/sq.meter, or kW/sq.meter
The total daily Irradiation (Wh/sq-m) is calculated by the integration of the irradiance values (W/sq-m).
Click here for the Solar Radiance map of the USA.
Shading: If your solar collectors experience any shading during the day the output of your solar system can be dramatically reduced. This is especially true of photovoltaic (PV) solar panels since a partially shaded PV panel can result in a loss of power across the PV array. Some solar incentive programs, such as the California Solar Initiative (CSI) reduce the incentive available to you if your solar system is impacted by any shading.
Go to www.solar-estimate.org