In order to evaluate the energy saving performance of various windows on a common basis, three conceptual window models are presented, and an energy consumption index is defined as the ratio of the energy consumption of a given window to the corresponding value of the perfect window. A building energy analysis program, "BuildingEnergy", was used to evaluate the energy consumption value of different window models. The following results are obtained: the energy saving potential of regulating the emissivity of the window is greater than that of regulating the solar transmissivity, the optimized phase transition temperature of the ideal near infrared solar spectrum regulating window is between 16 and 21 degrees C. and due to the high absorptivity in the metal state, the single vanadium dioxide (VO2) glazing discussed here behaves differently than the ideal near infrared solar spectrum regulating window, and it shows no obvious solar control advantage in energy savings over the ordinary window, and the phase transition process has no contribution to the energy saving effect of the single VO2 glazing in the summer. (C) 2012 Elsevier B.V. All rights reserved.