Window heat loss calculator
Compare alternative whole-window areas and U-values for steady heat loss, annual heating energy and cost, with every saving measured against the first option.
Up to 30 uniquely named options and 100,000 characters. Area above 0 to 1 million m² in m² or ft²; whole-window U-value 0 to 20 W/(m² K). Indoor -50 to 100°C and outdoor -100 to 100°C, entered in °C or °F; indoor must be at least outdoor. Degree days 0 to 20,000 K days; annual heating hours 0 to 8,784; heating efficiency/COP above 0 to 20; energy price explicitly entered from 0 to 100,000 per input kWh. JSON report, results CSV and full option comparison CSV. One option per line: name, area in the selected unit and whole-window U-value in W/(m² K). First row is the baseline. Up to 100,000 characters. Review the settings before running.
Your result
How to use Window heat loss calculator
- Enter one option per line: name, area and whole-window U-value in W/(m² K). Select the area unit.
- Choose temperatures plus degree days, temperatures plus annual heating hours, or degree days only.
- Enter the heating efficiency fraction or COP and your energy price per input kWh.
- Review each option and signed savings from the first row, then export the JSON report and CSV comparison.
Useful next steps
Use whole-window U-values
U-value measures heat flow per area and temperature difference. Use a whole-window figure including glass, frame and spacers, not centre-pane performance. U-values must be in W/(m² K); choosing ft² converts area only and does not convert U.S. Btu/(h ft² °F) U-factors.
Snapshot and annual exposure
Indoor/outdoor temperatures give a steady-state outward heat-loss snapshot. Annual degree days in K days estimate heating energy independently of that snapshot. In heating-hours mode the entered temperature difference is held constant for every entered hour. Degree-days-only mode leaves snapshot watts undefined instead of inventing a temperature.
Alternative options and signed savings
The first row is the baseline. Rows are alternatives, not elements to sum. Positive savings mean lower loss or cost; negative values mean higher loss or cost. Different areas trigger a warning. Solar gains, air leakage, thermal bridges, cooling and whole-building interactions are excluded; this is not a compliance or design certification.