The Window Energy Rating (WER) is the British Fenestration Rating Council's label system for window efficiency, running from A++ at the top down to G at the bottom. It pulls together three separate technical measures into one band that ordinary homeowners can actually use. If you want to know whether a window will keep your home warm and cut your bills, the WER band is the quickest starting point. Use the energy savings calculator to see what a more efficient window could save on your specific house, then come back here to understand what the band on the label actually means.
What a WER actually measures
The WER band is not a single measurement. It is a calculated energy index that combines three factors, each of which affects how much heat your home gains or loses through the window over a typical year.
The first factor is heat loss, expressed as a U-value. Lower is better. A window with a low U-value holds heat in more effectively. The U-value accounts for the glass, the spacer bar and the frame together, as a whole-window figure. This is the factor most people have heard of.
The second factor is solar heat gain, captured by the g-value (sometimes called the solar factor). A higher g-value means the glass admits more solar energy, which adds free heat in winter. On south-facing windows this can be a significant contribution. The WER formula gives credit for this gain, which is why two windows with identical U-values can land on different WER bands if one passes more solar heat through.
The third factor is air leakage through the window frame and seals. A draught-y window wastes energy regardless of its glass spec. The BFRC takes account of this in the energy index calculation, rewarding well-sealed frames.
The three factors are combined into the energy index, measured in kilowatt-hours per square metre per year (kWh/m²/year). A positive number means the window is a net energy contributor across the year, gaining more solar heat than it loses. A negative number means it is a net loser. The higher the index, the better the window performs. The result is then translated into a lettered band, just like the coloured energy label on a domestic appliance.
Most homeowners focus entirely on the U-value, which is fair because heat loss is the biggest part of the story. But the WER label is more honest about real-world performance because it also gives credit for solar gain. A south-facing living room window with good solar transmittance can effectively heat itself in winter. That is worth knowing before you specify heavily tinted or solar-control glass.
Tom Bradley, FENSA-registered installer
The A++ to G scale and the energy index
The BFRC energy index runs from roughly -40 at the lowest end, where windows lose the most energy, up to +20 and above at the top, where A++ windows sit. Each lettered band corresponds to a range of index values. Band G represents the worst-performing windows, broadly equivalent to old single glazing. Bands D, E and F cover older double glazing and poor-quality modern units. Band C was the old regulatory floor before 2023 but has since been superseded. Band B is now the minimum under Part L for replacement windows in existing homes.
Band A, with an energy index of 0 or above, is where most quality double-glazed windows now sit. It represents the practical baseline that any reputable installer should be offering as standard. Bands A+ and A++ represent a step further, typically only achievable with triple glazing or a very high-specification double unit. You can read the detailed A-rated vs A+ glass comparison to see when the extra investment makes sense. The table below sets out the bands side by side.
| WER band | Energy index | Typical U-value | Notes |
|---|---|---|---|
| A++ | Above +20 | 0.8 to 1.0 W/m²K | Highest efficiency; generally requires triple glazing |
| A+ | Above +10 | Around 1.2 W/m²K | High-specification double or triple glazing |
| A | 0 or above | 1.2 to 1.4 W/m²K | The modern standard; most quality double glazing |
| Band B or better | Threshold below A | 1.4 W/m²K or lower | Part L minimum for replacement windows (either route) |
Source: web/lib/research/wer-bands.md and u-values-wer.md. The Part L requirement (band B or better, or U-value 1.4 W/m²K or lower) has applied since 15 June 2023.
A, A+ and A++ in practice
For the majority of UK homes replacing old double or single glazing, A-rated is the right target. It means an energy index of 0 or above and a typical whole-window U-value in the range of 1.2 to 1.4 W/m²K. Most quality uPVC and aluminium windows now arrive at this standard as the base specification, using Low-E glass, argon fill and a warm-edge spacer. It clears the Part L requirement by a clear margin and delivers a meaningful improvement over older glazing.
A+ windows, with an energy index above +10 and a typical U-value of around 1.2 W/m²K, sit one step higher. High-specification double glazing with a premium glass package can sometimes reach A+, but it is more commonly a triple-glazing territory. If you are renovating a home to a high energy standard, or a room faces north and receives minimal solar gain, A+ is a reasonable upgrade to discuss with your installer.
A++ windows require an energy index above +20 and typically have a U-value as low as 0.8 to 1.0 W/m²K. Reaching this level almost always means triple glazing, which brings extra weight, thicker frames and a higher cost per window. A++ is the specification for passive-house builds and ultra-low-energy renovations. For a standard Victorian terrace or 1980s semi, the payback period rarely justifies it unless you are pursuing Passivhaus certification or a very high EPC target.
One practical note: do not rely on the band letter alone when comparing quotes. Ask for the BFRC certificate or the energy index number itself. Two windows both labelled A can sit at different points within the same band, and the index tells you exactly where each one lands.
I would not tell most of my customers to chase A+ or A++ unless they have a specific reason, like building to Passivhaus or replacing windows on a very exposed north elevation where solar gain is essentially zero. For the average semi, A-rated double glazing with proper glass and a warm-edge spacer does the job. Spending more on the band rarely gives you the savings to break even in a sensible timeframe.
Tom Bradley, FENSA-registered installer
WER versus U-value: which one to look at
This is a question many homeowners get confused by, and understandably so. The U-value is the more precise technical figure for heat loss, and it is the number Building Control uses when checking compliance via the U-value route. The WER band is broader and more consumer-friendly, but it captures more of the real-world picture because it includes solar gain and air leakage alongside the U-value.
If you are comparing two products, the WER band is a useful first filter. If both sit at A-rated, dig into the energy index to see which scores higher. If you want to know whether a window meets the legal standard, the U-value is the cleaner check: is the whole-window figure 1.4 W/m²K or lower? If yes, it passes. You do not need both numbers, but having both gives you a fuller picture of what you are buying.
There is one important caveat: solar gain is only a benefit in north European climates where heating dominates. For most UK homes, more solar gain is a good thing in winter. If you are fitting solar-control glass to reduce summer overheating, for example on a south-facing conservatory or a large glazed extension, the glass will deliberately have a lower g-value. That will reduce the WER energy index but may be the right choice for comfort. In that context the U-value matters more than the WER band.
For most standard replacement windows the two measures point in the same direction. You can explore the full cost implications of different glass specifications on the glass specification costs page.
What Part L requires
Approved Document L governs energy efficiency in buildings. For replacement windows in existing homes, the full standard that has applied since 15 June 2023 sets two alternative compliance routes, and a window meeting either one passes.
Route one: a whole-window U-value of 1.4 W/m²K or lower. This is the figure assessed across the whole window assembly, including frame and spacer bar, not just the glass centre-pane. It is the route most commonly used by Building Control when checking paperwork, and it is the figure a FENSA-registered installer certifies against.
Route two: WER band B or better. A window with a BFRC energy label at band B or higher meets Part L via this route regardless of the specific U-value. Most modern A-rated windows clear band B comfortably, so in practice an A-rated window can be confirmed as compliant against either route.
It is important to be clear about what Part L does not say. It does not require band A, and it does not require A+ or A++. Band B is the statutory floor. The previous floor was band C, which applied before 2023, so if you are reading older guides or leaflets the requirement may have changed since they were written.
New-build and extensions under the Future Homes Standard face a stricter requirement that goes beyond the replacement window floor, but that standard is separate from the route described here. If your project involves an extension or new build rather than straight window replacement, your installer or architect should advise on the applicable route. You can also check what a better-performing window means for your energy bills with the energy savings calculator, or price the windows themselves in the double glazing cost calculator.
Frequently asked questions
WER stands for Window Energy Rating. It is the label system run by the British Fenestration Rating Council (BFRC) and rates windows from A++ (most efficient) down to G. The rating combines heat loss, solar heat gain and air leakage into a single band so homeowners can compare windows at a glance without working through separate technical figures.
Part L (Approved Document L, full standard since 15 June 2023) requires replacement windows in existing homes to achieve WER band B or better as one compliance route. The alternative is a whole-window U-value of 1.4 W/m²K or lower. A-rated windows clear band B comfortably, but band B is the regulatory floor, not band A.
An A-rated window has a BFRC energy index of 0 or above. A+ windows must score above +10, and A++ windows must score above +20. Bands lower than A carry a negative index, meaning they lose more energy than they gain from solar heat. The index runs from roughly -40 at the bottom to +20 and above at the top.
No. Part L requires band B or better (or a U-value of 1.4 W/m²K or lower) for replacement windows. A+ and A++ go well beyond the regulatory requirement. They are worth considering if you want the best possible energy performance, but A-rated double glazing already meets the rules and suits most homes.
A++ windows typically have a whole-window U-value of around 0.8 to 1.0 W/m²K. Reaching that level generally requires triple glazing. A+ windows sit at around 1.2 W/m²K. Standard A-rated double glazing falls in the 1.2 to 1.4 W/m²K range and is the modern baseline for quality double-glazed units.
No. The U-value measures only heat loss. The WER energy index also factors in solar heat gain (g-value, which adds warmth in winter) and air leakage, then combines them into a single band. A window with good solar gain can score a higher WER band than its U-value alone would suggest, which is why south-facing windows often rate well.