Windows are a valuable source of daylight, on the other hand they influence to a large extent the heating/cooling demand of buildings. Reduction of the energy consumption is one of the key elements of the strategy of the European Union member states. New low-energy buildings should be built using advanced technology and design techniques while the energy-related products used for their construction should be labeled for their energy efficiency. There are several windows energy labeling systems in Europe; however, these systems concentrate on the heating and/or cooling purposes and do not take into account the energy impact of windows on lighting demand. In this article analysis of possibilities and limitations of energy performance of windows evaluation is presented. The study includes various factors that affect energy characteristic of windows as well as their impact on the energy demand for heating, cooling and lighting of a building in different operating conditions. The results indicate the importance of the daylighting in the energy performance assessment of windows as well as a need to develop an appropriate set of reference data for a given location or main operational parameters of the building. Windows energy certification system can provide accuracy and easy to use information for customers. It may also improve the quality of products available on the market. However it may lead to abuses and promotion of solutions beneficial for specific groups of interest. (C) 2015 Elsevier Ltd. All rights reserved.