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 g-factor, U is the U-value 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 into account the building type and that uses hourly climate files shows good fit with building simulation programs.
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