How Much Do New Windows Actually Save on Energy Bills?

Window sales rely on a big number, usually a percentage, presented next to a monthly payment. The independent figures available are much smaller and much more useful, so it is worth starting there.

What the independent modelling says

Energy Saving Trust, the UK charity that models these figures for government and consumers, puts the saving from upgrading an entirely single-glazed semi-detached home to A-rated double glazing at around £85 a year in Great Britain. In Northern Ireland, where energy prices are higher, the same upgrade is modelled at around £140 a year.

That is for a whole house of windows. It is not nothing, but it is a long way from the figures used in most doorstep presentations.

Indicative savings by house type

The saving scales roughly with how much glass and how much exposed external wall a property has. The ranges below are indicative, based on that modelling, for moving a fully single-glazed gas-heated home to A-rated double glazing.

Two caveats matter. First, these are modelled averages, not a prediction of your bill. Second, they assume the rest of the house is unimproved: if your walls and loft are already insulated, windows become a larger share of what is left, but the total is smaller.

Already got double glazing?

Most people asking this question are not upgrading from single glazing. They have 1990s or 2000s double glazing, often with misted units, tired seals and draughty hinges. Replacing that with new A-rated double glazing produces a much smaller saving, typically £20 to £50 a year, and sometimes almost nothing on the energy line.

That does not mean it is a bad purchase. It means the case for it is about condensation, draughts, noise, security, appearance and the fact that failed units will not fix themselves, not about the bill.

Where the comfort actually comes from

The bigger practical change is not the gas meter, it is the room. Old windows are draughty at the seals, the hinges and the meeting rail, and cold surfaces pull heat off you directly, which is why you can feel a draught in a sealed room.

• Draughts: fixed by new windows, but also by draught-proofing strips for a few pounds a metre.
• Cold surfaces: fixed by better glazing, and partly by heavy curtains closed at dusk.
• Condensation: reduced by warmer inner glass, which stops the water running down the panes.
• Noise: reduced noticeably by double or triple glazing, and by laminated glass in particular.

Cheaper things to do first

Energy Saving Trust models draught-proofing around windows, floors and doors at around £55 a year in Great Britain, for a cost of tens of pounds if you do it yourself. Turning the thermostat down by one degree is modelled at around £120 a year. Heavy thermal curtains, closed before dusk with a proper overlap at the edges, cost very little and cut both draughts and radiant heat loss.

If your only goal is a lower bill, do those first, along with loft and cavity wall insulation. If the windows are also failed, ugly, draughty or single glazed, then you are buying comfort and appearance with an energy saving attached, and the decision makes sense on those terms.

How to get the most saving from a window purchase

• Buy A-rated or better, and check the rating is on the whole window, not the glass alone.
• Make sure the install includes proper draught sealing and insulated reveals; poor fitting wipes out most of the gain.
• Ask for warm-edge spacers and argon fill; they cost little and are part of what earns the rating.
• Fix the cheap things at the same time: loft insulation and draught-proofing elsewhere usually outrank another window upgrade.
• Do not buy triple glazing purely for bill savings on a typical UK home. The cost premium rarely pays back.

If a salesperson tells you new windows will cut your bills by 30%, ask them to put the modelled annual figure in pounds, in writing, on the quote. The honest answer is usually a small number, and that is fine: the reason to replace windows is that they are old, cold, draughty or failing.