There is free design software available from almost all of the major inverter manufacturers. Each is designed for the particular manufacturers inverter, but you might find you have to manually input panel spec. If the particular panel your using is not on the list. Under the MCS you have to use the irradiance levels specified from SAP 2005-9 to calculate kWh/year generated by an array. So working out SAP calcs, Surveying the proposed site to know amount of panels required and then running that through inverter manufacturers software will give you everything you need, to design a system and sell a system. If you run the amount of panels, orientation etc through every manufacturers software you will find one inverter that matches perfectly.
