Large-scale penetration of photovoltaic power generation (PV) causes concerns in keeping supply-and-demand balance of a power system because output power of PVs shows considerable fluctuations due to weather condition. To assess the impacts on the balance, fluctuation characteristics of PVs should be examined for the case of large-scale penetration of PVs. Since fluctuation of total output of many PVs dispersed across an area is dominated by smoothing effects and the smoothing effects are governed by correlation among solar irradiances across the area, correlation in solar irradiances at various locations should be examined. The paper analyzes solar irradiances on the days with large short-term fluctuations in irradiance using cross correlation function, spectral analyses, etc. The results show that the days with large fluctuations can be generally classified into two categories-ie., days with high and low correlations-and the correlation basically depends on horizontal scale of clouds. © 2014 The Institute of Electrical Engineers of Japan.