.svg){kind=small}
Window Energy Ratings Explained: A++ to G Scale UK Guide
Quick Answer
Window Energy Ratings (WERs) grade windows on an A++ to G scale based on their overall energy performance. The rating combines three factors: heat loss (U-value), solar heat gain (g-value), and air leakage (L-value). An A-rated window is a net energy gainer, meaning it allows more energy into the home through sunlight than it loses through heat transfer, effectively contributing to heating rather than draining it.
At a Glance
Window Energy Ratings are the simplest way for UK homeowners to compare the energy performance of different windows at a glance. Developed by the British Fenestration Rating Council (BFRC), the rating scheme uses a familiar traffic-light style label similar to those found on fridges and washing machines. Windows are graded from A++ (the most energy efficient) down to G (the least efficient). The rating takes a holistic view of window performance, combining heat loss through the glass and frame, free solar energy gained through the glass, and heat lost through air leakage around the window. This means the rating tells you more than a simple U-value, as it accounts for the beneficial warming effect of sunlight. Understanding how these ratings work helps you choose windows that will keep your home comfortable and your energy bills low.
How Window Energy Ratings Are Calculated
The Window Energy Rating is calculated using a formula that balances three key factors. The first is the thermal transmittance (U-value), which measures how much heat escapes through the window. A lower U-value means less heat loss. The second factor is the solar factor (g-value), which measures how much solar energy passes through the glass into the home. A higher g-value means more free heating from sunlight. The third factor is air leakage (L-value), which measures the amount of heat lost through gaps and imperfections in the window seal. The formula effectively subtracts the energy losses (from U-value and air leakage) from the energy gains (from solar heat). A window that gains more solar energy than it loses through heat transfer achieves a positive energy balance and qualifies for an A rating or above. The calculation is performed by the BFRC using standardised conditions to ensure fair comparison between different products and manufacturers.
Understanding the A++ to G Scale
The Window Energy Rating scale runs from A++ (the most energy efficient) to G (the least energy efficient), using a colour-coded label. Ratings of A and above (shown in dark green) indicate windows that are net energy gainers over a heating season, meaning they contribute more heat through solar gain than they lose. A B rating (light green) indicates near-neutral performance. C-rated windows (yellow) are the minimum recommended for good energy performance. Ratings of D and below (orange through to red) indicate progressively poorer performance with significant net heat loss. In practical terms, almost all modern double-glazed windows from reputable manufacturers achieve a minimum of C rating, with the majority reaching A or A+. Triple-glazed windows frequently achieve A+ or A++ ratings. Single-glazed windows and very old double glazing would typically fall in the E to G range, though these products are rarely tested as they predate the rating scheme.
What A-Rated Actually Means
An A-rated window has an energy index of 0 kWh/m2/year or above, meaning it allows at least as much energy into the home through solar gain as it loses through heat transfer and air leakage over a typical UK heating season. In effect, an A-rated window acts as a passive solar collector, using free sunlight to offset its heat losses. This does not mean the window produces energy or that it is perfectly insulated. It means that over the course of a year, the solar energy entering through the glass compensates for the heat escaping through it. A-rated windows achieve this balance through a combination of good thermal insulation (typically a U-value of 1.2-1.4 W/m2K), high solar gain glass (g-value above 0.5), and low air leakage (well-sealed frames). Most quality modern double-glazed windows achieve an A rating as standard, so this should be considered the baseline for any new window purchase rather than a premium feature.
Building Regulations and Energy Ratings
Building Regulations in England offer two routes to demonstrate compliance for replacement windows. The first route is to achieve a whole-window U-value of 1.4 W/m2K or better, as specified in Approved Document L. The second route is to achieve a minimum Window Energy Rating of C. Either route is acceptable, and your installer can choose whichever is more convenient for the products being fitted. In practice, most modern windows comfortably satisfy both criteria. For new-build dwellings, windows are assessed as part of the overall SAP energy calculation, where U-values are used rather than energy ratings. However, specifying windows with higher energy ratings will improve the overall SAP score. It is worth noting that the Building Regulations set minimum standards. Choosing windows that significantly exceed these minimums, such as A-rated or A+-rated products, will deliver noticeably better comfort and energy savings. The difference in cost between a C-rated and an A-rated window from the same manufacturer is often minimal, typically £0-£50 per window.
How to Read a Window Energy Rating Label
The BFRC window energy rating label is designed to be easy to read at a glance. At the top is the BFRC logo and a unique licence number confirming the rating is genuine and audited. Below this is the manufacturer's name and the product name or range. The central feature is the colour-coded energy band, showing where the window falls on the A++ to G scale with an arrow pointing to the achieved rating. On the right side of the label, three performance values are displayed: the energy rating in kWh/m2/year (positive values indicate net energy gain), the thermal transmittance or U-value in W/m2K, and the solar factor or g-value. At the bottom is a QR code that can be scanned for more detailed product information. When comparing labels, focus primarily on the overall band rating for a quick comparison, then look at the individual values if you want to understand why one product performs better than another. The BFRC maintains a searchable online database where you can verify any rated product.
Energy Ratings vs U-Values: Which Matters More?
Both metrics are valuable, and understanding when to prioritise each helps you make the best decision. U-values are the most straightforward measure of insulation quality and are directly referenced by Building Regulations. They are particularly important for north-facing windows that receive minimal sunlight, where solar gain is negligible and thermal insulation is the primary factor in performance. Energy ratings provide a more complete picture by factoring in solar gain, making them especially relevant for south-facing and east or west-facing windows where sunlight provides meaningful free heating. A south-facing window with a moderate U-value but high solar gain may outperform a north-facing window with a better U-value but no solar contribution. For most homeowners, the practical advice is simple: choose windows that achieve at least an A energy rating and a U-value of 1.4 W/m2K or better. This ensures good all-round performance regardless of orientation. If budget allows, specify the lowest U-value available (typically triple glazing at 0.8 W/m2K) for the best comfort and energy savings.
Solar Gain: The Hidden Factor in Energy Ratings
Solar gain (g-value) is the often-overlooked component of the energy rating that can make a significant difference to real-world performance. The g-value measures the proportion of solar energy that passes through the glass into the home, expressed as a decimal between 0 and 1. A g-value of 0.6 means 60% of the sun's energy passes through. Standard clear double-glazed units typically have a g-value of 0.5-0.65, while triple-glazed units are slightly lower at 0.4-0.55 due to the additional pane. Solar gain is beneficial in winter because it provides free heating, but it can cause overheating in summer. Low-E coatings, which are essential for good U-values, also reduce solar gain to some extent. This creates a trade-off: the most insulating glass may not admit as much free solar energy. The BFRC energy rating calculation balances this trade-off over a full year. For south-facing windows, choosing glass with a higher g-value can significantly boost the energy rating and reduce heating costs during winter months.
Common Misconceptions About Window Energy Ratings
Several misconceptions about window energy ratings can lead to confusion. First, an A rating does not mean the window generates electricity or actively heats your home. It means the passive solar gain through the glass offsets the heat lost through it over a year. Second, a higher energy rating does not always mean a lower U-value. A window with a moderate U-value but excellent solar gain can achieve a higher rating than a window with a better U-value but less solar gain. Third, the rating applies to the specific window configuration tested, not to every window in the range. A small bathroom window may perform differently from a large picture window even if they use the same frame and glass system. Fourth, energy ratings do not account for the orientation of the window on your home. The standard calculation uses average UK solar conditions, whereas your actual performance will vary based on which direction the window faces. Finally, energy ratings do not measure acoustic performance, security, durability, or aesthetic quality, all of which should also factor into your decision.
Pros and Cons
Advantages
- Simple A++ to G scale makes comparing windows easy for consumers
- Holistic measure combining heat loss, solar gain, and air leakage
- Accepted as an alternative compliance route for Building Regulations
- Independent BFRC certification ensures ratings are genuine and audited
- Helps identify windows that provide free solar heating benefits
Disadvantages
- Does not account for window orientation on the actual property
- Rating applies to specific test configuration, not all window sizes
- Can be confusing when compared alongside U-values and g-values
- Does not measure acoustic performance, security, or durability
Cost Guide
The cost difference between energy rating bands is surprisingly small. Moving from a C-rated window to an A-rated window from the same manufacturer typically adds just £0-£50 per window, as the improvements are largely in glass specification and weatherseal quality. Achieving A+ or A++ ratings generally requires triple glazing, which adds £100-£200 per window over standard double glazing. For a typical three-bedroom house with eight to ten windows, specifying A-rated double glazing rather than C-rated adds approximately £0-£500 to the total project cost. The energy savings from higher-rated windows depend on the property and the windows being replaced, but upgrading from single glazing to A-rated double glazing can save £100-£200 per year on heating bills. Upgrading from older double glazing (E or F rated equivalent) to modern A-rated windows saves approximately £50-£100 per year. Given the 20-30 year lifespan of modern windows, the energy savings comfortably outweigh the modest additional cost.
Is It Right for Your Home?
Window Energy Ratings are relevant to every homeowner purchasing new windows. They provide a straightforward way to compare products without needing to understand the technical details of U-values and g-values. For most homeowners, the practical recommendation is to specify windows with a minimum A rating, which is standard on virtually all quality modern double-glazed windows and represents excellent performance at no significant cost premium over lower-rated options. If you are particularly energy-conscious or building to high standards, look for A+ or A++ rated products, which typically require triple glazing. Pay particular attention to energy ratings when your windows face south, east, or west, as the solar gain component has the greatest impact on these elevations. For north-facing windows, focus more on the U-value, as solar gain will be minimal regardless of the glass specification. Ultimately, energy ratings are one important factor in your window decision alongside cost, appearance, security, and noise reduction.
Frequently Asked Questions
A Window Energy Rating (WER) is a classification from A++ to G that measures the overall energy performance of a window. It combines heat loss (U-value), solar heat gain (g-value), and air leakage (L-value) into a single rating. A-rated windows are net energy gainers, meaning they contribute more solar heat than they lose.
An A-rated window achieves an energy index of 0 kWh/m²/year or above, meaning it allows at least as much energy into the home through sunlight as it loses through heat transfer and air leakage. Most quality modern double-glazed windows achieve A-rating as standard.
In England, replacement windows must achieve either a minimum C energy rating or a U-value of 1.4 W/m²K or better. Either route is acceptable for Building Regulations compliance. Most modern windows exceed both requirements, with A-rating and U-values of 1.2-1.4 being standard.
WERs are calculated by the BFRC using the formula: Energy Rating = Solar Gain - Heat Loss - Air Leakage. Specifically, it subtracts energy losses from U-value and air leakage from the solar energy gained through the glass. A positive result earns an A rating or better.
No. A U-value measures only heat loss through the window. An energy rating is a broader measure that also includes solar heat gain and air leakage. A window with a moderate U-value but high solar gain could achieve a better energy rating than one with a lower U-value but less solar gain.
BFRC ratings are not legally mandatory, but they provide a trusted, independently verified way to demonstrate Building Regulations compliance and compare products fairly. If your installer uses the energy rating compliance route rather than U-values, the windows must carry a BFRC rating.
The g-value (solar factor) measures the proportion of solar energy that passes through the glass. A g-value of 0.6 means 60% of solar energy is transmitted. Higher g-values mean more free heating from sunlight, which boosts the energy rating. Standard double glazing has a g-value of 0.5-0.65.
Yes. A+ rated windows have an energy index above +10 kWh/m²/year and A++ above +20 kWh/m²/year, compared to A-rated at 0 or above. A+ and A++ windows are significantly better net energy gainers, typically achieved through triple glazing or high-performance double glazing with optimised solar gain.
Energy-efficient windows contribute to a better EPC (Energy Performance Certificate) rating, which can positively affect property value and saleability. Properties with A-rated or better windows demonstrate modern, efficient specifications that appeal to energy-conscious buyers and reduce future utility costs.
Old double glazing installed before the BFRC scheme was established was not rated at the time. However, based on their specification (typically air-filled cavities with uncoated glass), most pre-2002 double-glazed windows would fall in the D to G range, losing significantly more energy than modern A-rated alternatives.
Ready to Get Started?
Get a free, no-obligation quote for your window project. Our experts will help you choose the perfect windows.