The energy-saving effect of energy conversion systems installed in a residence is evaluated by exergy utilization efficiency (EUE). Conventionally, energy utilization efficiency is used to evaluate energy-saving effect. However, this efficiency can evaluate only under the same heat-to-power ratio. Hence, for the different demand, the energy saving effect cannot be estimated. The EUE proposed is that the exergy demand is divided by the chemical exergy of primary energy. The exergy demand is the total of electricity demand and heat demand converted into exergy from enthalpy. Since the quality of energy for the heat demand is considered, the EUE evaluates the energy saving effect quantitatively for different demand as long as the same fuel is used. To validate this concept, the annual energy consumption was simulated for a residence with Solid Oxide Fuel Cell (SOFC) cogeneration. When the electricity demand is saved under the same hot-water demand, the primary energy consumption decreases obviously. However the EUE deteriorates in a residence with the CGS where a reverse power flow is not achieved. This is because the output of a water heater increases when the electricity demand is saved. When the CGS where reverse power flow is achieved is installed in the residence, the saving electricity demand leads to the decrease in the primary energy consumption and the improvement of the EUE.

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