In the design, operation and evaluation of a photovoltaic power generation systems (PV systems), it is particularly important to accurately estimate the total amount of annual electric power generated by PV system. The authors applied a new I-V curve construction method developed for the calculation of output power from a photovoltaic array in order to develop a practical “simulation program for the calculation of the amount of electric power generated by PV system.” The following items were clarified by this research.