Window U-Factor and SHGC Savings Calculator
Estimate annual heating and cooling energy savings when upgrading to windows with improved U-Factor (heat loss) and Solar Heat Gain Coefficient (SHGC) ratings.
Window Properties
Climate & Building
Typical range: 30–80 BTU/hr·ft². US average ≈ 50.Energy Costs
Formulas Used
Heating Conduction Savings (BTU/yr):
Qheat = (Uold − Unew) × Area × HDD × 24
Heating Fuel Saved:
Fuelheat = Qheat / ηheat → Cost = (Fuel / 100,000) × Rateheat
Solar Gain Offset in Heating Season (BTU/yr):
Qsolar,heat = (SHGCold − SHGCnew) × Area × Isolar × (HDD × 24 / 17.5)
Cooling Conduction Savings (BTU/yr):
Qcool,U = (Uold − Unew) × Area × CDD × 24
Cooling Solar Gain Savings (BTU/yr):
Qcool,SHGC = (SHGCold − SHGCnew) × Area × Isolar × (CDD × 24 / 15)
Cooling Electricity Saved (kWh/yr):
kWhcool = (Qcool,U + Qcool,SHGC) / (3,412 × COPcool)
Net Annual Savings:
Savings = Net Heating Savings + Net Cooling Savings
Assumptions & References
- Heating degree days (HDD) and cooling degree days (CDD) are base-65°F values from NOAA or ASHRAE climate data for your location.
- Average indoor–outdoor temperature difference assumed as 17.5°F during heating season and 15°F during cooling season to estimate seasonal hours.
- Solar irradiance (Isolar) represents average incident solar radiation on vertical glazing surfaces. US average ≈ 50 BTU/hr·ft²; varies by orientation and climate.
- Heating efficiency (ηheat): use AFUE for gas/oil furnaces (e.g., 0.80), or COP for heat pumps (e.g., 2.5–3.5).
- Cooling COP: SEER ÷ 3.412. SEER 10 ≈ COP 2.93; SEER 16 ≈ COP 4.69.
- Heating fuel cost is expressed per 100,000 BTU (1 therm ≈ 100,000 BTU for natural gas).
- Model follows ASHRAE 90.1 and DOE methodology for fenestration energy performance (NFRC 100/200 standards).
- Typical single-pane: U ≈ 0.87, SHGC ≈ 0.86. Double-pane low-e: U ≈ 0.27–0.30, SHGC ≈ 0.25–0.40.
- Results are estimates. Actual savings depend on window orientation, shading, infiltration, and occupant behavior.
- References: ASHRAE Handbook of Fundamentals; DOE Building Technologies Office; NFRC; ENERGY STAR Window Criteria.