Windows allow daylight to enter a space but they also allow for the transfer of heat gains and losses that affect the energy consumption of a building. This work optimises the relationship between window size, space dimensions and daylight to the energy consumption of the space. Models comprising of different room ratios and different room sizes were simulated using VisualDOE. The glazed areas of the rooms ranged from zero to 100% of the façade area. Energy consumption as a function of window area and room size was predicted for each model. Seven cities in Brazil and one in the UK were simulated to show the effect of climatic conditions on daylight provision and energy consumption. Resulting from the work, Ideal Window Areas for optimum energy efficiency were predicted.