A linear programming formulation is presented for the optimal synthesis of simple cogeneration systems for multiperiod operation with varying demands. The proposed design problem consists of determining the optimal configuration of the plant equipment taking into account the optimal operational policy of the plant for the annual energy demands of electric power and heat previously known. The objective function to be minimized is the annual total cost. The multiperiod formulation includes binary variables for the choice of units as well as binary variables for operational conditions of the existing equipment, during each period. As a result, both the configurations of the plant equipment and the operational policy are optimally determined. The proposed model is applied to a real system to verify its validity and efficiency.