Applying energy-saving measures in residential buildings is usually constrained by the increase of initial investment.However, if it is analyzed from the view of energy cost and life-cycle cost, the energy-saving benefit can offset the increase of initial investment. An analysis method based on life-cycle concept was developed to calculate the energy cost of residential building flats. Several uncertain factors were included into the model, making it more accurate to reflect practical situation. The model was solved using the software DeST and applied to one residential building project in Shanghai. The case study shows that the initial investment (cost) is paid back during the operational phase through less consumption of energy. It further indicates that the investment recovery period is between 10 and 19 years which are acceptable to households and developers in China.