Based on the concept of the perfect window, the energy saving performances and the corresponding theoretical limitations of the active/passive smart windows regulating the solar spectrum response properties or the long wave thermal emission window regulating thermal emission property were discussed using energy consumption index. It is found out that: ideal active/passive smart window scan realize maximum regulation of near-infrared (NIR) spectrum transmission by adjusting reflection and realize smart energy saving effect in both summer and winter, however, the transmission regulation of actual active/passive smart windows relies on the adjusting of absorption rather than reflection, and their solar spectrum transmissions in both NIR reflection and transmission states are lower than that of ordinary glazing, thus they can only realize energy saving effect in summer, and their performance is worse than that of ordinary glazing in winter; although ideal summer/winter long wave thermal emission window can achieve the best radiative cooling/insulating effect by controlling the radiation heat transfer between the window and the inner surfaces of a room, the energy saving effect of low-e window in summer is because of its low solar spectrum transmission, the contribution of the long wave emissivity is neglectable, but the long wave emissivity is critical for the energy saving performance in winter; the energy saving potential of regulating the solar spectrum response properties is greater than that of regulating the long wave thermal emission property in summer, and the trend is contrary in winter; by properly regulating the NIR spectrum transmission and reflection properties and long wave thermal emission property, the performance of an ordinary glazing can be improved to approach the perfect window for summer or winter. (C) 2013 Elsevier Ltd. All rights reserved.