Window Energy Ratings

Window Energy Ratings

The Window Energy Rating (WER) tells you how energy-efficient your windows are. The rating system is based on a scale of G to A+, with A+ windows being the most energy-efficient. The scheme is similar to those seen on household appliances, white goods and light bulbs.

Considering the fact that up to 25% of the heat within most homes escapes through the windows, it’s worth considering installing new energy-efficient windows in order to save energy and hence money off your energy bills. More and more people are opting for A or A+ rated windows as an energy saving measure.

Choosing windows incorporating a low-emissivity glass such as the well known Pilkington K Glass™ , Pilkington K Glass™ S or Pilkington Optitherm™ S1 Plus in your glazing helps to prevent heat from escaping through the windows, reflecting heat back into your home. These energy saving glass products may be incorporated into double glazing such as Pilkington energiKare™ which is available countrywide and helps windows of any frame type to achieve the highest levels of Window Energy Rating.

The labels below are typical of the labels that you should see on your windows. They are supported by registered bodies which audit the window installers;


Certass Thermal Rating 
Register Scheme


Tel. No. 0845 838 8580

BSI Kitemark Scheme 
for WER


Tel. No. 0845 076 5602

British Fenestration Rating Council WER Scheme


Tel. No. 020 7403 9200

Window Energy Ratings were originally launched in March 2004 by the British Fenestration Rating Council (BFRC), an independent government-supported body established to develop and administer a system of Window Energy Ratings in the UK. The BFRC is run by the Glass & Glazing Federation (GGF) the trade body representing many window companies. Visit the GGF website for more information. 

Since then both BSI and Certass have set up similar schemes and window companies can gain approval and register with any of these.

What is a Window Energy Rating?

A window's rating is determined by a formula which takes into account the total solar heat transmittance, the amount of the sun's heat which can pass through the glass (usually referred to as g-value); the U-value of the whole window (frame and glass combined), how well the window keeps the heat in your home; and the air infiltration through the window seals. To make the Rating standard across all products, a standard window size and configuration is used. The resulting value is then placed into a band on the scale G-A, with A-rated windows being the most energy-efficient. This makes the system of rating windows consistent with other products which have energy performance labels such as those that will be familiar on washing machines, light bulbs and refrigerators.

Who gets a Window Energy Rating (WER)?

A Window Energy Rating and label apply to a whole window (ie the frame and the glass). They do not apply to either the frame or the system or the glass individually. Therefore it is at the point at which all these components come together to produce a whole window that the Rating and label are obtained. Usually it would therefore be the window installer's product which is rated, although in the case of a factory-glazed window it could be the window manufacturer's product. Ask your window installer for more information on this.

Glass specification

Window Energy Ratings take into account both the positive (solar gain) and negative (heat loss) aspects of the glazing used. Products from our low-emissivity glass range have different properties however, when incorparated into Pilkington energiKare™ double glazing, all will meet or exceed the requirements of the building regulations and up to band A+ in the appropriate framing systems.