ABSTRACT The percentage of energy consumed by buildings is very large in industry. Recently, the cases of applying smart technology to efficiently improve the energy waste of these buildings are increasing, and this study targets Electrochromic (EC) smart windows. Smart window means a window that can freely change Visible Light Transmittance (VLT) and Solar Heat Gain Coefficient (SHGC). Since windows are the largest medium that can cause energy loss in buildings, the application of smart windows enables efficient energy management. However, this technology currently has a high-cost problem, so a partial application to windows is proposed. This study aims to analyze the relationship between partially applied smart windows and energy loads. Smart windows require an environment control strategy because windows are primarily to provide a comfortable indoor environment. An environmental control strategy was established according to the application area of the smart window, and Useful Daylight Illuminance (UDI) was used. In addition, the energy load of the smart window in which the environmental control strategy was established was derived, and the relationship with the partially applied area was analyzed. This study provides information on how much the partial application of smart windows can save energy wastage in buildings.