Understanding Energy Performance

You’ve made the decision to upgrade your windows to increase security and help reduce outside noise, but what exactly do all Energy Performance Ratings mean? Well, we’ve put together this simple guide to understanding what the The Window Energy Rating (WER) energy efficient label system means.

The British Fenestration Rating Council (BRFC) use three key factors to decide which letter on the A+ to G scale windows are awarded. These three factors are; G-Value, L-Value and U-Value. Don’t worry we’ve put a definition of each value below.


This value measures how much heat the window glass is able to absorb from direct sunlight. Newer modern glass is able to absorb more heat form the sun than older windows. This effectively gives your home free heat to help keep you warm. G-Value ranges from 0 to 1, the closer to 1 the greater the amount of heat the glass can gain.


The L-Value measures how much air ‘leaks’ from a window. This value determines how airtight windows are and therefore how effective the windows will be at stopping draughts from coming in. For this value, the lower the better! Look for a rating of 0 for the most efficient windows with best performance. 


This value measure how efficiently a window retains warmth. This value determines how well a window stops the heat inside your home escaping. Look for low rating for this one as the lower the rating the more efficient the window is at keeping heat in.

The combination of these three factors will determine The Window Energy Rating. To comply with current building regulations any windows installed must be rated C and above. The higher the rating, the better. Think of it like exam results, you want your windows to be as close to a A-class student as possible!

If you would like to find out more about the windows we offer and The Window Energy Rating of windows we install, please contact our team who will be happy to assist you. You can also find out about the window style we install on our Windows page. 

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