This paper describes the methodology for quantification of solar irradiance fluctuation in order to analyze the areally totalized PV systems, which consist of a large number of PV systems connected to each other in the optimum size of a certain community area. Two factors quantifying the fluctuation, fluctuation factor and power spectral density, and some examples are presented. Also cross-correlation analysis between nine stations＇ measurements were operated to estimate the extent of the power fluctuation in a certain area size and to determine optimum size of the area of areally totalized PV systems.