The energy performance of dwellings depend of the windows characteristics with respect to the orientation, size and energy performance of the windows. In order to make it easy to select windows for use in dwellings a program has been developed to calculate the energy performance of individual windows in dwellings. The basis of the program is the simplified method desribed in EN 13790. The thermal transmittance and total solar gain coefficient for the windows are calculated for the actual configuration and sizes of the window based on data on the glazing and the frame of the window. The energy performance of the window during the heating season and the sommer period is calculated for the actual orientation and for the utilisability of solar gain for the actual building. In the program an overview of the energy performance of different window products in different configuration and sizes are set up for all the relevant orientations of the building. Based on the overview the best energy performing window design with respect to configuration, size and window product data can be found for the actual building. In this way the full potential of using windows to save energy in buildings can be used in an easy way.