This paper analyzes energy cost reduction from peak demand shaving when a CES provider adopts ESS for the CHP-based CES microgrid site in Seoul, Korea. The simulation results show that about 9% of peak shaving can be realized when a 270kWh ESS is used for three thousand CES households. When two or three ESSs are adopted, peak demand shaving increases at most into 12% or 14%, which concludes that ESS capacity enlargement is not so helpful for the peak reduction. It also concludes that peak shaving is appropriate as a supplementary usage of ESS considering that ESS is required to work only a few days for a year to achieve peak shaving.