It is required for the energy systems to satisfy simultaneous solutions to the problems, such as cost reduction, global warming, assurance of energy security, and resource conservation. To evaluate optimal end-use energy systems from the stand point of CO 2 emissions, we apply a comprehensive approach based on the life cycle assessment. Several combinations of electricity and heat supplying systems are compared. They include the electricity driven heat pump, gas engine co-generation, absorption refrigeration and so on. Calculations represent total CO 2 emission by energy consumption of several operation patterns, based on the actual data of energy demand, CO 2 intensity of the grid electricity and the equipment specifications. The results indicate that CO 2 emissions can be minimized by maximum utilization of electricity from the grid.