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To design a photovoltaic (PV) generation system economically, it is necessary to use data of the total insolation on a horizontal surface. However, such data is only the total daily value and does not represent the power variation caused by the cloud cover. This paper presents the probability method which represents not only the average but also the variance of the PV generation power, and shows simulated results using this methodology. This study's results indicate that the distribution of the PV power divided by the estimated value of the total insolation on a tilted surface is similar to a normal distribution and that a residential (privately-owned) system without storage, whose PV capacity is more than 2 kWp, has little effect upon the reduction of the energy of an average Japanese household.