A new model is established to obtain the building energy with a gradual change process of VO2 optical properties. Based on this method, three types of existing VO2 films are employed to simulate the heating and cooling energy consumptions in five typical Chinese cities. The results show that the smart characteristic is not evident in VO2 glasses because these films are almost in the cold state or cold transition state in the entire year. To improve this situation, the concept of an ideal thermochromic intelligent glass is put forward. Unexpectedly, the level of energy consumption increases with the film phase transition temperature decreasing in the heating period. However, the energy-saving effects will be effectively improved both in the cooling and heating periods when the phase transition process interval is shortened. The cooling energy consumptions for the ideal glass decrease by at most 81.7% and 70.5% compared with white glass and Low-E glass, respectively. In the heating period, the level of energy consumption for the ideal glass is better than Low-E glass. (C) 2015 Elsevier Ltd. All rights reserved.