Demonstration project named “Verification of Grid Stabilization with Large-scale PV Power Generation” has been conducted by the New Energy and Industrial Technology Development Organization (NEDO). In the Wakkanai PV power station, Sodium-Sulfur battery system is introduced to realize some additional functions. Since fluctuation in output of MW-class PV system may disturb the stable operation of power station, it is important to develop a fluctuation suppression technology. The authors have developed a fluctuation suppression using a battery system. In this paper, the relation between available battery capacity and fluctuation suppression performance is analyzed by computational simulation. This paper also estimates and discusses required battery capacity for fluctuation suppression.