A Window Energy Rating System (WERS) provides a simple, approximate method to compare the energy performance of the various windows and to determine the different potential savings for the various weather conditions. The main aim of this paper is to obtain a WEIRS for two climatic zones in Spain. For this purpose, the heating loads and energy savings of a residential building with different types of windows were obtained by three ways. Firstly, the energy through the window was evaluated considering only the climatic conditions. Secondly, the study was performed taking into account the energy useful for the heating system considering the climate and the type of building. Finally, the different cases were simulated using TRNSYS16 and WINDOW5. This Study was performed for different European climates. The WERS proposed here is based on the second method. It takes into account the U factor of the window, U factor of the frame, absortivity of the frame, solar heat gain of the glazing and infiltration. (C) 2009 Elsevier B.V. All rights reserved.