Window Energy Loss Calculator
Calculates conductive window heat loss and solar heat gain based on total glass window square footage, U-Factor (rate of heat loss: single pane = 1.10; double-pane Low-E = 0.27), and local winter heating degree days.
Formula
BTU Loss = Area × (U_old - U_new) × HDD × 24; Annual Savings = BTU Loss ÷ Furnace Efficiency × Fuel Cost
1. Total Glass Area Sq Ft = Window Count × 15.0 sq ft average window size (3 ft × 5 ft) 2. Delta U-Factor = U_Existing (1.10 or 0.55) - U_New Low-E Double-Pane (0.27) 3. Annual BTU Conduction Saved = Total Area × Delta U × Heating Degree Days (HDD) × 24 hours/day 4. Annual Heating Dollar Savings = (BTU Saved ÷ 100,000 BTU/therm) × $1.40/therm ÷ 0.85 furnace efficiency 5. Add Infiltration Air Sealing Savings (+25% bonus for draft elimination)
Inputs
- Number of Windows to Replace
- Existing Window Glass Type
- Climate Severity
Worked example
Upgrading 12 drafty single-pane windows to Energy Star Low-E double-pane vinyl replacement windows.
Outcome: Total window area is 180 sq ft. U-factor improves from 1.10 to 0.27 (Delta U = 0.83). Eliminates 19.7 million BTUs of heat loss per winter, saving ~$325 to $410/year on heating bills while eliminating uncomfortable cold draft zones near windows.
What is a window U-Factor and what number should I look for?
The U-Factor measures the rate of heat loss through a window assembly (the inverse of R-value; U = 1 / R). Lower numbers are better. Standard single-pane glass has a terrible U-Factor of ~1.10 (R-0.9), whereas modern double-pane Low-E argon windows achieve 0.27 (R-3.7) or lower.