Daylightable floor area can be determined by the geometries such as window area, perimeter area, and floor area. The daylightable floor area ratio (DFR), which is defined as the ratio of the daylightable floor area to the total floor area, decreases with an increase in the floor area. In this study, the heating and cooling energy consumption of office buildings was simulated by using eQUEST(DOE-2.2). The energy performance of the buildings applying dimming control of lighting with daylight was estimated by varying the floor area (six cases) and window-to-wall ratio (WWR) (four cases). The relationships for the reduction in the annual heating and cooling energy according to daylight and DFR in office buildings were developed by using a multiple regression analysis. As a result, the annual cooling energy in the buildings decreased, but the annual heating energy increased by applying daylight as lighting. The reduction rate in the annual cooling energy of the buildings with large floor area was smaller than that of the buildings with small floor area. The buildings having high WWR showed smaller energy reduction rate due to an increase in infiltration and thermal heat transfer through window. The increasing rate in the annual heating energy of the buildings with large floor area was larger than that of the buildings with small floor area. The buildings having high WWR showed smaller increasing rate of the annual heating energy due to an increase in radiation heat transfer through window.