Reduction of environmental impact and an achievement of highly efficient utilization of energy resources are important issues for developing a sustainable energy society. In this study, we basically study on the system-design and operation of cogeneration system (CGS). Several facilities should be linked with an energy network and share the thermal energy and the electric power produced from CGS. There is a necessity to predict and control demand of balance on thermal and electric power energy and to operate CGS and other facilities at effective condition through a year. To compute such a complex system, we propose to apply Unified Modeling Language (UML), which is a language for programing object-oriented modeling, to express energy utilization system. As an example of calculation results, we calculated the fundamental simulation for a community including a hotel, hospital, office, shopping, and residential buildings at a climatic condition of Tokyo. This result shows that the present model makes it possible to design appropriate system and to operate of CGS at any load condition, which results to be able to reduce environmental impact.