Window Energy Rating Calculator — WER Band Estimator

A window energy rating is a measure of the overall energy performance of a window, combining heat loss (U-value), solar heat gain (g-value) and air leakage into a single rating. The BFRC (British Fenestration Rating Council) administers the scheme in the UK, rating windows from A++ (most efficient) to E (least efficient). It shows the net energy balance in kWh/m²/year.

For replacement windows under Part L of UK Building Regulations, you need either a whole-window U-value of 1.4 W/m²K or better, OR a window energy rating of band B or above. For new build, the standard is higher at 1.2 W/m²K U-value. The WER route can sometimes be easier to achieve because solar gain offsets heat loss.

The g-value (solar factor or total solar energy transmittance) measures how much solar energy passes through the glass. A g-value of 0.5 means 50% of solar energy enters the building. Higher g-values mean more free solar heating in winter but potentially more overheating in summer. Typical values: clear double glazing ~0.63, low-E double ~0.43, triple glazing ~0.35.

The BFRC rating equation, as published in the trade bulletins, is: Rating = (218.6 × 0.9 × g) − 68.5 × (Uw + L), measured in kWh/m²/year. The 0.9 is the Fw solar-gain correction factor from EN ISO 13790 (formerly EN 832): it converts the normal-incidence glass g-value into a time-averaged solar factor that allows for the angle of the sun through the year. It is not a frame allowance — the frame area is dealt with separately inside the BFRC simulator (the proportion of glass in the window). The 218.6 constant represents the solar gain benefit averaged across UK orientations over a heating season, and the 68.5 represents the heat loss; the air-infiltration term L (an effective heat loss in W/m²K, derived from BS 6375 testing) is added to the U-value inside the same bracket. The result is the net annual energy flow across the window per square metre.

Yes. A positive rating (band A or above) means the window gains more solar energy than it loses through conduction and air infiltration over a heating season. It needs both a low U-value and a high solar-gain glass: a U-value around 1.2 W/m²K combined with a g-value near 0.50 or more typically lands in band A or A+. A modest low-E glass (g ~0.43) at 1.4 W/m²K usually rates band B or C, so high solar gain is as important as a low U-value. A++ rated windows achieve +20 kWh/m²/year or better.