Nowadays, due to the huge energy consumption, the energy-saving problems of the product have been emphasized with many designers. In this paper, informed by the modular design method, the total energy performance in modular product can be analyzed and separated into the energy performances of basic modules. And with the physical analysis of basic modules, the energy equations are established with the band graphs theory. Then, the physical parameters, which could influence the energy consumption, are identified as "energy factor". Thus, the energy consumption of the modules could be optimized with adjusting design factors, and the energy-saving design scheme for the whole product is obtained in the optimized model. Finally, the model and the method in this paper are demonstrated by an instance of the crank block pump.