How to calculate how much energy your windows will save from its U-value

The number assigned to a building fabric that represents its thermal resistance is called its U-value. A U-value tells us how quickly or slowly it takes for heat to pass through it. Fabrics with lower U-values are better insulators than those with higher U-values.

Part L of the UK Building Regulations (Conservation of Fuel and Power outlines all of the legal requirements for insulating modern homes. In newly-built homes in 2016 the construction industry has to install windows which have a maximum Uw-value of 1.4W/m2K (Watts per metre-squared Kelvins) or lower, or a Window Energy Rating of ‘B’, which is pretty much the same thing.

Triple-glazed windows in a stunning home in Iceland

Notice the little ‘w’ after the U in U-value. It’s important because it represents the combined U-values of the fabrics within the window, ie, the FRAME (Uf-value), the GLAZING (Ug-value) and the SPACER (psi-value). The spacer is the material between the glazing panes. By calculating all of those fabrics together based on their respective area and thickness you have the Uw-value.

The Uw-value is what you should request from your window manufacturer. Also, consider that this is the uninstalled value. A triple or double-glazed window is only as good as its installation. Poor installation can reduce an expensive window’s Uw-value, as heat is lost through the frame, points where installation doesn’t meet, thermal bridges or cold air gaps.

Triple-glazed windows, installed correctly, should have a Uw-value of 0.85W/m2K or lower.

How do you calculate how much money your triple-glazed windows will save from the U-value?

Let’s say you have a double-glazed window in your living room that is 2 metres wide and 2 metres high. It has a Uw-value of 1.5Wm2K and you are considering replacing it with a triple-glazed window that has a Uw-value of 0.7Wm2K.

To calculate how much money you will save on heating lost through the double-glazed window you follow the steps below.

  1. Work out the difference between the two window U-values (1.5 – 0.7 = 0.8Wm2K)
  2. Multiply that number by the window area (0.8 x (2 x 2) = 3.2)
  3. Work out temperature difference inside and outside your home (21DegC – 4DegC = 17)
  4. Multiply the temperature difference by the U-value area (3.2 x 17 = 54.4)
  5. Divide that number by 1000 to convert it into kilowatts per hour (54.4 / 1000 = 0.0544kWh)
  6. Multiply that by 24hrs in a day and 365 days in a year to get your annual kWh (0.0544 x x 24 x 365 = 476.54kWh) (with thanks to J Dixon and Jason for spotting my original error)
  7. Multiply that figure by the cost of a kWh of gas (476.54 x 2.78p =£13.25)

This is a slightly crude calculation as there are always other factors involved, but to replace that single double-glazed window in your living room with a triple-glazed version will save you perhaps £7 per year in heating bills, assuming you heat up your home for six months of the year in the cold months. In short, there’s only a small payback in triple-glazed windows compared to the paybacks in solar panels or biomass heating systems unless you’re looking at a whole house PassivHaus approach, in which case you can get your annual heating bills down to £40 per year, and that feels very good.

Why choose triple-glazing, then?

Because it’s way more comfortable to live in a triple-glazed home. Stand by a single-glazed window when it’s snowing outside and the air temperature will be about 1DegC. Stand next to a triple-glazed window, with a centre-pane U-value of around 0.65Wm2K and the temperature will be about 18DegC. We humans sense cold radiation very easily, and sitting near a window when it’s cold outside makes us shiver.

Also, it’s impossible for condensation to form on a surface with a temperature of 18DegC, so you won’t be troubled by excessive moisture, which can cause black mould and subsequent health problems.

Triple-glazed windows are expensive, but if you’re considering replacing your current windows it’s worthwhile to pay a little more for the triple-glazed versions because you’ll be more comfortable and healthier with triple-glazed windows in your walls.

Patrick

4 Comments

  1. Let’s say you have a double-glazed window in your living room that is 2 metres wide and 2 metres high. It has a Uw-value of 1.5Wm2K and you are considering replacing it with a triple-glazed window that has a Uw-value of 0.7Wm2K.

    To calculate how much money you will save on heating lost through the double-glazed window you follow the steps below.

    1.Work out the difference between the two window U-values (1.5 – 0.7 = 0.8Wm2K)
    2.Multiply that number by the window area (0.8 x (2 x 2) = 3.2)
    3Work out temperature difference inside and outside your home (21DegC – 4DegC = 17)
    4. Multiply the temperature difference by the U-value area (3.2 x 17 = 54.4)
    5. Divide that number by 1000 to convert it into kilowatts per hour (54.4 / 1000 = 0.0544kWh)
    6. Multiply that by 365 to get your annual kWh (0.0544 x 365 = 19.856kWh)
    7. Multiply that figure by the cost of a kWh of gas (19.856 x 4.18 =£16.48)

    I agree with you from step 1 to 5. But step 6, you mean 0.0554 is the kWh you save per day? I don’t think so.

    • Hi Jason,

      I’m happy to be corrected – why do you think Step 6 is incorrect?

      Thanks,
      Patrick

  2. Google calculator is great for these sorts of things:
    https://goo.gl/4qLKSf
    (54.4 watts) * 1 year = 476.860215 kilowatt hours

    https://goo.gl/xA9uSs
    (54.4 watts) * (1 year) * (0.0278 (British pounds / kwh)) =
    13.256714 British pounds

    (I’m assuming 2.78 pence per kWh there, based on https://www.ukpower.co.uk/home_energy/tariffs-per-unit-kwh — needs VAT adding)

    Long hand, this is (54.4 * 60 * 60 * 24 * 365.25 / 1000 / (60 * 60)) * 0.0278
    – obviously the 60 * 60 terms can factor out, thus:
    (54.4 * 24 * 365.25 / 1000) * 0.0278

    tl’dr: you missed a “multiply by 24” in step 5, but your final multiplication (19.856 x 4.18 =£16.48) is not at all valid, but by chance(?) almost canceled out the prior error. (Gas would need to be 83p/kWh for this calculation to work)

    • Hi Jonathan,

      Thanks for the correction, you’re completely right. I’ve now corrected it and name-checked you with thanks.

      Best wishes,

      Patrick

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