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Power output fluctuation of photovoltaic power generation systems (PVSs) may cause negative impact on the existing electric power system when the penetration of PVSs is quite large. A micro-grid consisting of clustered PVSs and a battery system would be one of the promising measures to the negative impact of clustered PVSs, while the maximum power and storage capacity of battery system should be reduced as much as possible from the economic point of view. Because of the difference in output fluctuation among PVSs in the various locations, the total output fluctuations of PVSs would be relaxed due to the so-called "smoothing-effect". Considering such the effect, the required performance of battery system can be reduced, even though small output fluctuation of each micro-grid still remains. By using data on solar irradiance observed in five points simultaneously, this study evaluates the total output fluctuation of several micro-grids and the required performance of battery system, taking the smoothing effect into account.