Compare two glass specifications and estimate the annual energy cost difference for heating and cooling, by climate zone.
Step 1: Enter glass specifications (NFRC whole-window values)
Spec A, Baseline
e.g. existing glass or lower-performance option
Typical clear double IGU, air fill: ~0.47. Single pane: ~1.04.
Clear double IGU, no coating: ~0.57. Single pane clear: ~0.82.
VS
Spec B, Upgraded
e.g. Low-E coated or triple-pane option
Typical soft-coat Low-E, argon fill double IGU: ~0.27–0.30.
Solar control Low-E: ~0.19–0.27. Balanced Low-E: ~0.27–0.40.
Step 2: Project and energy settings
Project details
Total area of windows/glazing being compared. 300 ft² is typical for a mid-size home or small commercial unit.
Energy rates & efficiency
Central AC: 10–14. High-efficiency: 16–20.
Defaults are US national averages (2024): gas $1.20/therm · electricity $0.16/kWh (EIA). Canadian users: typical rates, BC Hydro ~$0.10/kWh · Ontario ~$0.13/kWh · Alberta ~$0.15/kWh · natural gas ~$0.60–$0.80/therm (Cdn$). All outputs are in USD, multiply by ~1.35 to estimate CAD.
Annual energy cost comparison
USD, US Dollars
$,
estimated annual savings with Spec B over Spec A
Annual cost breakdown
Heating cost, → ,
Cooling cost, → ,
Spec ASpec B
Component
Spec A
Spec B
Savings (B vs A)
Heating, U-factor loss
,
,
,
Heating, solar credit (SHGC passive gain)
,
,
,
Cooling, U-factor conduction gain
,
,
,
Cooling, SHGC solar gain
,
,
,
Total annual energy cost (glazing portion)
,
,
,
,
estimated payback period
Methodology & assumptions
Calculation method: Simplified ASHRAE degree-day method, the standard approach for preliminary fenestration energy analysis. Heating loss: U × Area × HDD × 24 hr/day = annual BTU loss through glass. Converted to fuel cost via furnace AFUE or heat pump COP. Heating solar credit: SHGC × Area × annual solar insolation × heating-season fraction. High-SHGC glass passively reduces heating load in winter, this is reflected as a credit (benefit) in heating cost. Cooling conduction: U × Area × CDD × 24 hr/day = annual BTU conducted in through glass in summer. Converted to kWh via cooling EER. Cooling solar: SHGC × Area × annual solar insolation × cooling-season fraction. Solar heat gain through glass that the AC must remove. Climate data used: Representative HDD/CDD (base 65°F) and annual vertical-surface solar insolation (BTU/ft²/year) for each ASHRAE zone group. Values are representative averages, actual city-specific data will differ. Solar season split: Annual solar insolation divided between heating and cooling seasons proportionally to HDD/(HDD+CDD) and CDD/(HDD+CDD) respectively. Not modeled: Building orientation, window shading/overhangs, thermal mass, infiltration, internal loads, occupancy schedules, part-load HVAC operation, or climate change trends. These factors can significantly affect actual savings. Inputs required: NFRC whole-window U-factor and SHGC values (not center-of-glass). Use product data sheets or NFRC certified ratings.
Disclaimer: All monetary outputs are in US Dollars (USD). Canadian users: multiply by the current CAD/USD exchange rate (approximately 1.35 as of 2025) for Canadian dollar estimates, and adjust energy rates to match your province and utility. Energy cost estimates are for the glazing portion of your building's energy load only, they do not represent total building energy costs. Actual savings will vary based on building geometry, orientation, HVAC system, occupancy, local utility rates, and climate variation year to year. This tool is intended for comparative and preliminary planning purposes only. It does not constitute an energy audit, engineering analysis, or guarantee of actual savings. For detailed energy modeling, use EnergyPlus, eQUEST, or engage a certified energy auditor.