The European Commission has proposed to expand the labelling directive to include energy saving products like windows. This report presents a proposal for such an energy rating system of windows in EU. The energy rating system includes vertical façade windows and sloped roof windows. The rating system is based on the net energy gain for windows used in reference houses in three zones in EU. Conclusion: The study in this report shows that it should be possible to develop an European scheme for windows where Europe is divided into zones as the performances of windows for the heating season do not differ significantly in the zones; therefore the evaluation of windows can be decided on the basis of the energy performance proposed in this report as the solar radiation becomes high in the summer period, it is necessary to include summer conditions for windows in a labelling scheme where dynamic solutions for summer condi-tions could be used also the energy performance of sloped windows differs from vertical windows, where the passive solar radiation for sloped windows is much higher than for vertical windows and thereby the energy performance of sloped windows is better than for vertical windows the best performing façade window for replacement in the northern part is low energy win-dow with U-values between 0.8 and 1.2 however the difference between the 3 best windows is below 10 kWh/m² in northern climate. The best performing façade window on an overall evaluation will be a window with a U-value of approximately 1.2 W/m²K and a g-value of approximately 0.48 for the whole window the performance of best sloped windows for replacement is the same for all Europe, with a U-value of 1.2 W/m²K and a g-value of 0.48 for the whole window by replacing the windows in the existing building stock an energy saving in Europe can be up to 134,749 GWh/year if existing old windows are replaced with new windows with a U-value of 1.2 W/m²K and a g-value of 0.5 for the whole window further detailed studies for the individual zones are recommended in order to define the ex-act values for the energy performance for the heating season.