Energy Savings and Payback: Doing the Sum Honestly
Payback claims in this industry range from three years to twenty, which tells you the number depends entirely on assumptions. Here is how to work out your own rather than accepting anyone's.
The four inputs that decide it
Your glazed area, the U-value improvement achieved, the number of heating degree days where you live, and your fuel tariff. The last two dominate. An electrically heated house saves several times what a mains-gas house saves for the identical physical improvement, which is why a single national payback figure is meaningless.
- Total glazed area upgraded
- U-value before and after
- Local heating degree days
- Fuel type and unit rate
A worked approach
Heat loss through a window is roughly the U-value multiplied by the area multiplied by the temperature difference. Take the U-value improvement — commonly around 5.4 down to about 1.87 — multiply by the square metres, multiply by degree days for the heating season, convert to kilowatt hours and price at your tariff. Do it with your own numbers and you get a figure you can trust.
Why comfort usually outweighs the arithmetic
Almost every client we survey names the cold seat by the window, the noise, or the mould before they mention the bill. Removing the downdraught frequently lets people turn the thermostat down by a degree because the room feels warmer at the same air temperature — a saving the U-value calculation does not capture at all.
Be sceptical of these claims
A single payback figure quoted without asking your fuel type. A percentage saving on your whole bill rather than on window heat loss. And any figure that appears without the assumptions behind it. Ask what was assumed, and if the answer is vague, discount the number.
Frequently asked questions
It depends heavily on fuel type and glazed area. Electrically heated period properties with large windows see much faster returns than gas-heated homes with modest glazing. We will run the numbers for your actual property rather than quote a generic figure.
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