Given the thermal properties of the sash, frame, and sill, and the dimensions of the glazing and thermal properties of the glass, the heat transfer rate for a given window and set of conditions can be calculated. This can be calculated in kW ( kilowatts ), but more usefully for cost benefit calculations can be stated as kWH pa ( kilowatt hours per annum ), based on the typical conditions over a year for a given location.

The glass panels in double glazed windows transmit heat in both directions by radiation, across the panes by convection, and by conduction around the perimeter seals. The actual rates will vary with the conditions throughout the year, and while solar gain is much welcomed in the winter, it may result in increased air conditioning costs in the summer. The unwanted heat transfer can be mitigated by for example using curtains in the winter and using sun shades in the summer. In an attempt to provide a useful comparison between alternative window constructions the British Fenestration Rating Council have defined a "Window Energy Rating" WER, ranging from A for the best down through B and C etc. This takes into account a combination of the heat loss through the window ( U value, the reciprocal of R-value), the solar gain ( g value ), and loss through air leakage around the frame ( L value ). For example, an A Rated window will in a typical year gain as much heat from solar gain as it loses in other ways (however the majority of this gain will occur during the summer months, when the heat may not been needed by the building occupant). This provides better thermal performance than a typical wall-(wikipedia)