Demand side management is an important part of the smart grid for alleviating the uncertainty in renewable energy generation including such as wind power generation and photovoltaic generation. The nuclear accident caused by the 2011 Tohoku Earthquake forced a re-evaluation of the future power generation system in Japan. As a result, demand side management has become critical for future Japanese power systems with its large integration of renewable energy sources, and the evaluation of the impacts of this management is needed for planning purposes. This paper proposes a new energy system model that expands the function of MARKAL (Market Allocation) and considers the characteristics of the total power system in more detail. The relationship between demand side management and the future power generation mix was evaluated using energy system analysis as part of the proposed model. The electric water heaters and heat pump water heaters are conceived as the controllable loads in the demand side management.