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Insulation payback calculator

Insulation savings follow the change in U-value, and the first few centimetres save far more than the last.

W/(m2 K)
W/(m2 K)
%
per kWh
Payback period
1.47years
= Exact Excellent — do it now
Annual saving
817.09
Energy saved per year
11 673 kWh
Reduction in heat loss through this element
93.0 %
Net saving over twenty years
15 141.82
Carbon saved per year
2136
Cost per kWh saved over twenty years
0.0051
Show the working
  1. Area treated = 80
  2. Existing U-value = 2.3
  3. Improved U-value = 0.16
  4. Heating degree days = 2500
  5. Heating system efficiency = 88
  6. Fuel price = 0.07
  7. Installation cost = 1200
  8. u_saving = 2.3 - 0.16 = 2.14
  9. Energy saved per year = 2.14 * 80 * 2500 * 24 / 1000 / (88/100) = 11 672.7
  10. Annual saving = 11 672.7 * 0.07 = 817.091
  11. Payback period = 817.091 > 0 ? 1200 / 817.091 : 0 = 1.468 62
  12. Reduction in heat loss through this element = 2.3 > 0 ? 2.14/2.3*100 : 0 = 93.0435
  13. Net saving over twenty years = 817.091 * 20 - 1200 = 15 141.8
  14. Carbon saved per year = 11 672.7 * 0.183 = 2136.11
  15. Cost per kWh saved over twenty years = 11 672.7 * 20 > 0 ? 1200 / (11 672.7 * 20) : 0 = 0.005 140 19

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

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    The formula

    • annual heat saved = delta U x area x degree days x 24 hours
    • degree days measure how cold the year was relative to a base temperature

    Loft insulation typically pays back in one to three years. Solid wall insulation rarely pays back on energy alone, though comfort and condensation are real benefits.

    Diminishing returns are steep: going from no insulation to 100 mm saves far more than going from 100 to 300 mm.

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