The short answer
Window energy ratings run from A++ (best) down to E on a colour-coded BFRC label, similar to the scale on a fridge or washing machine. The rating combines three things: the U-value (how much heat escapes — lower is better), the g-value or solar gain (how much free heat the glass lets in), and air leakage. For replacement windows, Building Regulations require a minimum standard, and most modern double glazing is rated A or above. When comparing quotes, check the energy rating and the U-value, because two A-rated windows can still differ. See double vs triple glazing for how these numbers play out in practice.
Window energy ratings (WERs) give you a single, comparable measure of how thermally efficient a window is. They are run by the British Fenestration Rating Council (BFRC) and shown on a familiar coloured label. This guide explains the scale, the underlying U-value and g-value, what Building Regulations require, and how to use the rating sensibly when comparing quotes.
Energy ratings at a glance
- Scale A++ (best) to E
- Run by The BFRC
- U-value Lower = less heat lost
- Good double glazing ~1.2 W/m²K
- g-value Solar heat gained (free warmth)
- Regs minimum Set by Building Regs Part L
The A++ to E scale
The BFRC label rates a whole window — frame and glass together — on a scale from A++ at the top down to E. Like the energy label on a household appliance, the colour and letter give an at-a-glance sense of efficiency. Most modern replacement windows are rated A or higher; older single glazing would sit far below the bottom of the scale. The rating is calculated from the energy that flows through the window over a year, balancing heat lost against free solar heat gained.
| Measure | What it tells you | Rule of thumb |
|---|---|---|
| Energy rating (WER) | Overall efficiency, A++ to E | Aim for A or above |
| U-value | Heat lost through the window | Lower is better (~1.2 double) |
| g-value | Solar heat let in | Higher means more free warmth |
| Air leakage | Draughts around the seals | Lower is better |
U-value vs g-value
Two numbers do most of the work. The U-value measures how readily heat escapes — a lower figure means better insulation, and good double glazing sits around 1.2 W/m²K, with triple glazing nearer 0.8. The g-value (solar factor) measures how much of the sun’s heat the glass lets through; a higher g-value brings more free warmth in winter but can add to summer overheating on south-facing windows. The energy rating combines both, which is why a window with a slightly higher U-value can still earn a good rating if it lets in more useful solar heat.
What Building Regulations require
When you replace windows in England and Wales, Building Regulations (Approved Document L) set a minimum thermal standard the new windows must meet. A FENSA or CERTASS registered installer self-certifies that the work complies, so you do not need to involve the council separately — see FENSA, CERTASS and Building Regulations. In practice almost all reputable replacement double glazing comfortably meets or beats the minimum, so the rating becomes a way to compare quality rather than just a pass/fail. This is general guidance; current standards and exact figures are set by the regulations in force at the time of your work.
Compare double glazing quotes
Comparing quotes on the same energy rating and U-value helps you judge value fairly. Use our service to reach FENSA or CERTASS registered installers in your area.
Frequently asked questions
What is a good window energy rating?
Aim for A or above. The scale runs from A++ at the top down to E, and most modern replacement double glazing is rated A or higher. A higher rating means the whole window — glass and frame — loses less heat over a year.
What is a U-value?
The U-value measures how much heat escapes through the window. The lower the number, the better the insulation. Good double glazing is around 1.2 W/m²K and triple glazing nearer 0.8.
Is the energy rating or the U-value more important?
Both matter. The energy rating gives an overall A++ to E score, while the U-value is a direct measure of heat loss. When comparing similar windows, check both, as two A-rated windows can have different U-values.
Who runs window energy ratings?
The British Fenestration Rating Council (BFRC) runs the UK window energy rating scheme and issues the coloured A++ to E label you see on rated windows.
Sources & further reading
- BFRC — Window Energy Ratings, the A++ to E scale and U-values explained
- Energy Saving Trust — energy efficient glazing and what the ratings mean
- Glass and Glazing Federation (GGF) — glazing performance and consumer guidance
- GOV.UK / Building Regulations Approved Document L — thermal standards for replacement windows
This is general information, not advice for your specific property or installation. Current standards and figures are set by the regulations in force at the time of your work. Replacement windows should be fitted by a FENSA or CERTASS registered window installer. We are an independent information and introduction service, not an installer.