Recently, it has been concerned about overconsumption of the energy use of the newly-built curtain wall buildings with a large WWR(Window to Wall Ratio). However it is estimated that the influence of the window area on energy is changed according to the building size, preferably, WFR(Window to Floor Ratio) is correlated with building energy. EnergyPlus is used for a building energy simulation in this study. The results of the simulations show a building energy is relative to WWR, but there is a limitation to use for the index because of the building size. However, building energy is mostly relative to WFR, the correlation of this can be expressed a linear equation. Therefore, developing synthetic index according to WFR is acutely needed. And, in case of the solar gains according to a window area, the correlation can be expressed a logarithmic function, the more WWR become the more solar energy gains. The solar gains are about 5.04~5.40 times as large as an annual horizontal global radiation. If the synthetic index is developed through analysis of glazing performances(U-factor, SHGC, VLT), the index should be used for the window design of buildings.