A window energy label shows a Window Energy Rating band from A++ down to E, and separately most quotations give a U-value in W/m2K. The band is a single summary figure that nets heat loss against solar gain for a whole window. The U-value measures heat loss only. Lower U-values are better, and a good modern window sits around 1.2 to 1.4 W/m2K.
- The WER band is a whole window figure covering frame, glass, spacer and seals
- U-value measures heat loss only. Lower is better, and it is the number Building Regulations actually uses
- A window can carry a high band because of solar gain rather than because it loses less heat
- A centre pane U-value always looks better than a whole window figure, so check which you are shown
- Ask for the whole window U-value in writing, not a band letter alone
Window quotations are full of numbers that look comparable and are not. One installer offers an A rated window, another quotes a U-value of 1.3, a third mentions a centre pane figure of 1.0 and calls it the best of the three. All three could be describing similar products, or wildly different ones. The label is useful once you know what each figure covers and what it leaves out.
What is a Window Energy Rating?
The Window Energy Rating scheme gives a whole window a band from A++ through to E, in the same visual style as a washing machine label. It combines three inputs into a single energy index: how much heat the window loses, how much solar heat it gains, and how much air leaks through it.
The important word is whole. A WER band covers the complete window as a manufactured unit, so frame, glass, spacer bar and seals all count. That is fairer than a glass only figure, because a superb sealed unit in a poor frame is not a good window. The British Fenestration Rating Council publishes the methodology and a register of rated products.
What does the U-value tell you?
A U-value measures the rate at which heat passes through the window, in watts per square metre per degree of temperature difference, written W/m2K. Lower is better. It is a pure heat loss figure with no credit for sunshine, which makes it the conservative measure and the one Building Regulations uses. Approved Document L sets the maximum for replacement windows, published on gov.uk.
| Figure | What it covers | What to watch |
|---|---|---|
| WER band | Heat loss, solar gain and air leakage for the whole window | A high band can come from solar gain rather than low heat loss |
| Whole window U-value | Heat loss through frame, glass edge and glass together | The figure that matters for Building Regulations |
| Centre pane U-value | Heat loss through the middle of the glass only | Always the flattering number. Not a compliance figure |
| g-value | How much solar heat the glazing lets in | High helps in winter, can overheat a south facing room |
Does a higher band always mean a warmer room?
Not necessarily, and this is where the band can mislead. Because the index nets solar gain against heat loss, a window with generous gain can reach a strong band while still losing more heat than a lower banded window with better insulation. On a north facing elevation there is little useful gain to collect, so the band flatters a window that will not perform as the letter suggests. On a large south facing conservatory the opposite problem arises and too much gain makes the room uncomfortable.
Look at both figures and think about orientation. Use the whole window U-value as your baseline test of insulation, then consider whether you want solar gain on that elevation. Where overheating is the risk, the glazing specification matters more than the band, as we covered in our guide to solar control glass.
What actually moves the numbers?
Do I have to upgrade to the current standard?
If you are replacing windows, yes, the replacements must meet the current requirement rather than matching what was there before. That is checked through building control or, far more commonly, a competent person scheme certificate from the installer. Listed buildings and conservation areas can be treated differently, as our article on window replacement in conservation areas explains alongside our heritage windows range. Where the work is a like for like repair, such as a single sealed unit swapped into a sound frame, the whole window requirement does not bite the same way. Specify the best unit that fits regardless, because the difference between an ordinary and a good sealed unit is small against the labour.
What drives the cost of a better rating?
Moving up a band is rarely a single line item. Going from a basic double glazed unit to a warm edge, argon filled, soft coat low emissivity unit is a modest uplift on the glass and often the best value change available, because it improves the figure without touching the frame. Triple glazing is a bigger step, and the extra pane brings weight that can mean heavier hinges and, on large openings, a different frame system. On timber windows the sections may need to grow to carry a thicker unit.
The judgement worth making is where the marginal money buys most comfort. On a draughty older property, sorting air leakage and getting a whole window U-value in the 1.2 to 1.4 range usually transforms how the room feels. Chasing the last tenth beyond that delivers steadily less, and the same budget spent elsewhere in the house often does more.
Frequently asked questions
Is an A rated window always better than a C rated one?
What U-value should I be asking for?
Is triple glazing worth it in the UK?
What is a warm edge spacer bar?
Does the frame material change the rating much?
Who certifies that my new windows comply?
Ask for the numbers in writing
The simplest protection against a confusing quotation is to ask every installer for the same two things: the WER band and the whole window U-value for the exact product being offered. If those are supplied clearly, comparison becomes straightforward. Our team is happy to set out both for any window in our range and to explain what each will mean for your particular elevations.