When you are looking to invest in new windows, it can be difficult to understand the pros and cons of different types, particularly when it comes to the WER (Window Energy Rating).

The type of window you choose is important because it can save you money, so it’s important to be as informed as possible before you make this big decision. Understanding what WER is will stand you in good stead when it comes to making the right window choice.

What Are Window Energy Ratings?

The WER is designed to let you know how energy efficient your windows are. The system works on a scale of G to A+ and A+ are the most energy-efficient.

Which Registered Bodies Audit Energy Ratings?

You will find the WER stickers on windows that follow the rating scheme. Registered bodies that audit the energy ratings, which appear on these stickers are:
Certass Thermal Rating
BSI Kitemark
Scheme and
British Fenestration Rating Council

It is important to check that these registered bodies have approved the rating so you can feel assured the information you are getting is officially recognised and unbiased.

Who Gets The WER?

The WER and the associated label apply to the window as a whole, not just one part of it like the glass or frame separately. So it is only when the window is in its complete form that the WER can be obtained. Most often you can expect the window installers product to have the rating, but in some cases, the rating can be applied to the manufacturer’s product. If you want to know where the rating was obtained, enquire with your window installer.

Understanding The Ratings

A window can be rated at highest A+ and at lowest a G. This makes it really easy to understand window efficiency, although it is a good idea to understand more detailed information in relation to the window to check you are selecting the best window for your individual needs.

Energy Index

The energy index is really important and lets you know how much energy the window can either lose or gain. This index uses kilowatt hours per square metre per year which sounds complicated but simply means that when you read the energy index and it is a minus figure, it indicates energy is lost and when it is a plus figure, it means energy is gained. Ideally, a window listing a 0 or above is perfect as that lets you know either no energy is being lost, or energy is being gained.


U-value represents the thermal transmittance through the glass which just like the energy index, is measured in kilowatt-hours per square metre per year and also contributes to the final energy index figure. With the U-value you’re looking for the lowest possible number as the lower the number, the more energy you will be saving.

U-value appears as 1.4Wh/m2.K which means for every square metre of window, 1.4 kilowatts of energy are lost every year.


G-value is where you are gaining energy that you lose through your windows via a solar factor which basically means – the energy you gain through your windows from the sun. This figure also contributed to the index value and is again measured in the same way – kilowatt hours per metre per year. With this figure you want it to be as high as possible – the more energy gained from the sun, the more money you save.


L-value is the windows air leakage which is again measured in kilowatt-hours per metre per year, and again, the L-value contributes to the overall energy index. With this figure, you will want it to be as close to 0 as possible and this isn’t an unrealistic expectation with modern windows.

Combining All The Figures

Combining the U (U-Value), G (G-Value), and L (L-value) amounts together gives you the windows energy index and then the full set of figures creates the windows WER.

An A-Rated Window Can Save You Money

All properties do lose heat through their windows but energy-efficient windows will help keep your home more secure, warm, quiet and reduce your energy bills overall. Energy-efficient windows can be double glazed, triple glazed or secondary glazing.

If you installed energy-efficient windows in every window in a single glazed house you can save huge amounts, particularly with A-rated windows. This ranges from £40-£60 for a flat (per year) to over double that for a detached property (source).

Energy-efficient windows also provide the following benefits:

  • Lower energy bills
  • A lower carbon footprint
  • A warmer home with fewer draughts and cold spots
  • A quieter home with protection against external noise
  • Condensation reduction

Energy-efficient glazing will cost different amounts depending on the home and window installer, and the savings will be different depending on the home as well. Big savings can be made, and what’s even better is the energy-efficient glazing should last you at least 20 years, so overall it’s a really sound investment. If you want more information on how much you could save, take a look at the GGF energy savings calculator.

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