Window U-Factor and Solar Heat Gain Coefficient Savings Calculator
Estimate annual heating and cooling energy savings when upgrading windows to lower U-Factor and SHGC values.
Formulas Used
Conduction Heat Transfer (Heating & Cooling):
Qcond = U × A × DD × 24 [BTU/year]
where U = U-Factor (BTU/hr·ft²·°F), A = window area (ft²), DD = degree days (°F·days), 24 = hours/day
Solar Heat Gain:
Qsolar = SHGC × A × Isolar × Season Fraction [BTU/year]
where Isolar = annual solar irradiance (BTU/ft²/year), Season Fraction = HDD/(HDD+CDD) for heating, CDD/(HDD+CDD) for cooling
Net Heating Savings:
ΔQheat = (Uold − Unew) × A × HDD × 24 − (SHGCold − SHGCnew) × A × Isolar × HeatFrac
Source energy saved = ΔQheat / ηheating
Net Cooling Savings:
ΔQcool = (Uold − Unew) × A × CDD × 24 + (SHGCold − SHGCnew) × A × Isolar × CoolFrac
Electricity saved = ΔQcool / (3412 × COP) [kWh/year]
Cost Savings:
Heating savings ($) = (Source BTU saved / 1,000,000) × Heating cost ($/MMBtu)
Cooling savings ($) = kWh saved × Electricity cost ($/kWh)
Assumptions & References
- Conduction losses use the standard degree-day method: Q = U·A·ΔT·hours (ASHRAE Fundamentals).
- Solar heat gain is split between heating and cooling seasons proportionally by degree days.
- In heating season, solar gain is beneficial (offsets heating load); lower SHGC reduces this benefit.
- In cooling season, solar gain is detrimental (increases cooling load); lower SHGC reduces this penalty.
- Heating degree days (HDD) and cooling degree days (CDD) base temperature: 65°F (18.3°C) — NOAA standard.
- Typical U-Factors: single-pane ≈ 0.87–1.30, double-pane ≈ 0.30–0.50, triple-pane ≈ 0.15–0.25 (NFRC ratings).
- Typical SHGC: single-pane ≈ 0.86, standard double-pane ≈ 0.70, low-e double-pane ≈ 0.25–0.40 (ENERGY STAR).
- CO₂ emission factors: natural gas ≈ 117 lbs CO₂/MMBtu (EPA); grid electricity ≈ 0.386 kg CO₂/kWh (US average, EPA eGRID 2022).
- Solar irradiance varies significantly by location and window orientation. Use NREL's PVWatts or local TMY data for precision.
- Model does not account for infiltration, frame conduction, or interior shading devices.
- References: ASHRAE 90.1, NFRC 100, ENERGY STAR Window Program, DOE Building Technologies Office.