This paper proposes a method to analyze the errors caused by the prediction of renewable energy generator and suggest solution using energy storage system. In the case of renewable energy, it is difficult to predict the generation of electricity as it responds sensitively to environmental impacts. By using the energy storage system as a back-up device, it is possible to solve this problem more efficiently, and minimize wasted power through storage of surplus power and to supply stable power by receiving power from the storage device. In this paper, we propose a method of minimizing the electricity cost of consumers by preliminarily setting reserve portion of storage system and proper operating method of energy storage system based on advance analysis of power prediction error. Based on this method, it is confirmed that the electricity rate of consumer is reduced by 10% per day according to the use of proper operation method in energy storage system.