Window/Door Calculator
Calculate heat loss and running cost for windows and doors by U-factor.
About Window/Door Calculator
Degree days summarise the heating season into one figure, so annual loss follows directly from area and U-value. Replacing single glazing at around 4.8 typically cuts glazing losses by 70 per cent. Window replacement rarely pays back on energy alone at current prices, so the saving is best treated as a secondary benefit alongside comfort and condensation control.
Formula
Window/Door
Annual loss in kWh = area × U-value × degree days × 24 ÷ 1,000
- U-value
- Heat transfer per square metre per kelvin, lower is better
- degree days
- Cumulative temperature difference over the heating season
Worked example
With the default values below, Window/Door Calculator returns Annual Energy Saving of $1,256.64. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total window and door areaminimum 0 m2
- 22 m2
- Existing U-valueminimum 0.1 W/m2K
- 4.8 W/m2K
- Replacement U-valueminimum 0.1 W/m2K
- 1.4 W/m2K
- Heating degree daysminimum 0 Kd
- 2,500 Kd
- Energy price per kWhminimum 0
- 0.28
Results
- Annual Energy Saving
- $1,256.64
- Existing Annual Heat Loss
- 6,336.0 kWh
- Heat Loss After Replacement
- 1,848.0 kWh
- Energy Saved
- 4,488.0 kWh
- Reduction in Heat Loss
- 70.83%
Step by step
- 1Values used
- 2Window/Door
- 3Annual Energy Saving= 1,256.64
- 4Existing Annual Heat Loss= 6,336.0 kWh
- 5Heat Loss After Replacement= 1,848.0 kWh
- 6Energy Saved= 4,488.0 kWh
- 7Reduction in Heat Loss= 70.83
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Window/Door calculated?
Annual loss in kWh = area × U-value × degree days × 24 ÷ 1,000 Degree days summarise the heating season into one figure, so annual loss follows directly from area and U-value. Replacing single glazing at around 4.8 typically cuts glazing losses by 70 per cent.
Why does Window/Door matter?
Window replacement rarely pays back on energy alone at current prices, so the saving is best treated as a secondary benefit alongside comfort and condensation control.
What values do I need to enter?
This calculator takes 5 inputs: Total window and door area, Existing U-value, Replacement U-value, Heating degree days, Energy price per kWh. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.