In this study, we have estimated energy storage capacity to realize stable power output with 100% wind and/or solar in electricity supply system using the actual output data of wind power generation and solar power generation in Japan. We conducted numerical calculation and clarified the battery capacity necessary to compensate for the unstable supply, which is a weak point of VREs, and discussed the concrete solutions and tasks to realize stable supply with 100% VREs systems. We simulated some cases when we use the VREs as a baseload electricity source for the first step of our research. According to the results of numerical experiments, for keeping the constant output 1MW, it is necessary at least about 42 days' worth of battery capacity in the case of the solar power-only generation. In the case of the wind power-only generation, it is necessary at least about 60 days' worth of battery capacity. On the other hands, it was found that the battery capacity can be reduced to about 18 days' worth if the electric power generation of solar power and wind power are integrated with the best mixture ratio which we found.