Abstract Reasonable energy storage capacity in a high source-to-charge ratio local power grid can not only reduce system costs but also improve local power supply reliability. This paper introduces the capacity sizing of energy storage system based on reliable output power. The proposed model is formulated to determine the relationship between the power capacity and wind energy loss, considering the wind curtailment loss and traditional energy power uncertain reserve. The non-parametric kernel density estimation is adopted to estimate the confidence intervals of wind power prediction error and fluctuation ranges of the actual output of a wind farm under different confidence degrees. The actual historical data of scenery resources in a certain area is used to verify the feasibility of the proposed method. The simulation shows the large-capacity energy storage, the reliable output power of the microgrid wind ensures the feasibility of day-ahead generation plans.