In this paper different window energy rating systems (WERS) are evaluated and compared. The comparisons are made for different European climates, types of buildings and orientations. The purpose of the paper is to evaluate how complex a WERS needs to be in order to be able to provide a reliable energy rating which can be applied to different buildings in different climates and orientations. The results indicate that Europe needs to be separated into several different climate zones. A simple linear heating energy-rating model of the form Ag-BU (where g is the total solar energy transmittance, U is the thermal transmittance of the window, and A and B are empirical coefficients), may be sufficient within certain regulations. For warm locations a linear cooling energy rating, depending only on the g factor may also yield an acceptable rating. More advanced models that take the building type into account and that use hourly climate files showgood fit with building simulation programs. (C) 2003 Elsevier Ltd. All rights reserved.