A sizing method of photovoltaic power systems is presented by using the measured quantitative efficiency and parameters in order to determine the optimum capacity of the units. A number of experimental units (total capacity 112 kW) including 5 stand-alone, 2 water pumping, and 8 grid-connected systems were constructed at the Hamamatsu Solar Energy Laboratory of the Japan Quality Assurance Organization (JQA), to simulate several types of systems. Here the parameters can be analyzed and evaluated on the basis of the data gathered from these systems. An evaluation technology of photovoltaic power systems is being developed, through the construction and data acquisition of PV power systems of various patterns, and through identification and formulation of design parameters, as well as establishment of database and simulation techniques.