The energy rating on a window quote is one of the easiest places to be sold something you do not need, mostly because the labels imply a ladder you are supposed to climb. In practice band A is where the market already sits, and everything above it is a different product rather than a better version of the same one. This tool tells you which side of that line you belong on. It does not price the job, which is what the cost by glass spec table is for.
The bands, and what each one actually is
The Window Energy Rating is run by the British Fenestration Rating Council and nets three things against each other: heat lost through the window, solar heat gained back through it, and air leaked around it. That is why two windows with the same U-value can land in different bands.
| Band | Energy Index | Typical U-value | What it is in practice |
|---|---|---|---|
| A | 0 or above | About 1.2 to 1.4 W/m²K | Standard modern double glazing. The baseline. |
| A+ | Above +10 | About 1.2 W/m²K | Generally requires triple glazing |
| A++ | Above +20 | As low as 0.8 to 1.0 W/m²K | Triple glazing, best available |
Part L accepts band B or better, or a U-value of 1.4 W/m²K or lower, as alternative compliance routes for a replacement window.
For context on where you are starting from, single glazing sits at roughly 4.8 to 5.8 W/m²K and pre-2002 double glazing at about 2.8 to 3.2. That is the jump that does the heavy lifting. Against single glazing, A-rated double glazing saves up to £140 a year on a typical semi according to the Energy Saving Trust. The step from A to A++ is a much smaller increment on a much smaller number, which is the whole reason this decision needs thinking about rather than defaulting upwards. Our guide to u-values explained covers the measurement itself, and WER ratings explained covers the label.
Why the rest of the house decides this
Glazing is one part of a building fabric and it behaves like one. Putting triple glazing into a house with an uninsulated loft is paying a premium to slow heat down at the window while it leaves freely through the roof. That is why this tool asks about your walls and loft: the answer changes the recommendation more than the glazing you currently have does. If the fabric is not sorted, A-rated glazing plus insulation beats triple glazing alone, every time and for less money.
I fit a lot of triple glazing and I talk plenty of people out of it. If your loft is thin and your walls are bare, the third pane is the last thing on the list, not the first. Get to A-rated, spend the difference on insulation, and you will feel it more.
Tom Bradley, window installer
Spec is one of three decisions on a window quote and the other two have their own tools. What the frames are made of is handled by the window frame material classifier, and whether a cheaper spec that needs replacing sooner really saves you anything is what the 25 year window cost comparator exists to test. If you already have quotes with different specs on them, the quote quality auditor will tell you whether you are comparing like with like.
Frequently asked questions
For most homes, yes. Band A is the baseline rather than a premium feature, because most quality modern double glazing reaches it as standard. It gives a whole-window U-value of about 1.2 to 1.4 W/m²K, which clears the Part L requirement of 1.4 or lower comfortably. Moving up to A from a lower band costs roughly £0 to £50 a window, so there is rarely a good reason to specify less.
They are points on the BFRC Energy Index, which nets solar heat gained through the glass against heat lost through it and air leaked around it. Band A is an index of zero or above, A+ is above +10 and A++ is above +20. In hardware terms A is standard double glazing, while A+ and A++ generally require triple glazing, which is what takes the U-value down to about 0.8 to 1.0.
No, and installers sometimes imply otherwise. Since 15 June 2023 a replacement window in England must achieve a whole-window U-value of 1.4 W/m²K or lower, or a Window Energy Rating of band B or better. Those are alternative routes to compliance and band B is the floor, not band A. A-rated is simply what the market supplies as standard, which is a different thing from a legal minimum.
Not reliably. The Window Energy Rating combines heat loss, solar gain and air leakage, and sound is not one of its inputs. A window can rate A++ and do little for road noise. If noise is your main problem, discuss the glass make-up and the quality of the seal with your installer as a separate specification from the energy band.