A power interchange operation, in which electric power generated by multiple residential solid oxide fuel cell cogeneration systems (R-FCGSs) is shared among residences in a housing complex with no reverse power flow to a commercial electric power system, has a high energy-saving effect as compared with a stand-alone operation of individual systems. To further improve its energy-saving effect, the optimal sizing of the R-FCGS for this power interchange operation is discussed from the energy-saving viewpoint by conducting optimal operational planning based on mixed-integer linear programming. The energy-saving effect of the power interchange operation is analyzed for 14 different scales of the R-FCGSs with the rated electric power output ranging from 0.2 to 1.5 kW per 0.1 kW. The analysis for a housing complex with 20 residences located in Japan reveals that the power interchange operation has an advantage over the stand-alone operation in terms of energy saving for any scale of the R-FCGS investigated in this study. Furthermore, it is found that the 0.6-kW R-FCGS is the optimal scale for the power interchange operation in terms of the energy saving and saves the annual primary energy consumption of 17% as compared with the conventional energy supply with no R-FCGS. (C) 2011 Elsevier Ltd. All rights reserved.