Calculate heat loss and running cost for windows and doors by U-factor.
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.
Window/Door
Annual loss in kWh = area × U-value × degree days × 24 ÷ 1,000
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.
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.
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.