Contribution of system technology applied to energy system is expected to realize environmental conscious social infrastructure. To design sophisticated and efficient energy system, a unified evaluation metric for energy efficiency is required. In this paper a unified metric 'Total energy efficiency' for general energy system is proposed. Then a design and evaluation case study is shown, applied to a smart building based on the high performance energy equipments, called net zero energy building (ZEB). The simulation example evaluated the total energy efficiency for various types of smart building with photovoltaic (PV) system, LED lighting, high performance HVAC system, battery and heat storage, etc. The case study shows the effectiveness of the proposed metric 'total energy efficiency' for designing various types of energy systems.