An optimal structural design model of a residential cogeneration system, known as combined heat and power, considering various kinds of operating restrictions is developed from the energy-saving viewpoint. As principal operating restrictions of cogeneration units, a constant power output operation, a daily start stop operation, and a continuous operation are focused on. The developed model results in a mixed-integer linear programming problem and the selection and multi-period operation are simultaneously optimized. Moreover, the model is applied to the structural design of a residential cogeneration system, consisting of a cogeneration unit and its peripheral devices, for simulated energy demands in a Japanese residence. The candidates for a cogeneration unit are a gas engine employing a constant power output operation, a polymer electrolyte fuel cell employing a daily start stop operation, and a solid oxide fuel cell employing a continuous operation, and the candidates for peripheral devices are an electric water heater and an air-cooled heat exchanger. The optimization results reveal that the selection of the cogeneration unit is influenced more by their operating restrictions than by the consistency in the heat-to-power ratios of the cogeneration unit and energy demands. In addition, it is found that the selection of the peripheral devices varies with the selected cogeneration unit and energy demands. (C) 2013 Elsevier Ltd. All rights reserved.