The renewable energy resource such as the photovoltaic generation and the wind turbine generation is rapidly spreading into the power system. The forecast of its output is utilized to the balancing operation of demand and supply. Since the forecast error causes the imbalance of demand and supply, it must be compensated in the real-time operation by conventional generators. In this paper, we focus the virtual power plant which is composed by the distributed photovoltaic generations and the battery. The battery charges or discharges the forecast error of output from the photovoltaic generation to reduce the imbalanced energy between the scheduled and the actual output. By the computational simulation, we analyze the relationship between the imbalanced energy and the capacity of battery. In addition, the marginal imbalanced energy is proposed to design the monthly required capacity of the battery.