Calculator Inputs
Example Data Table
| Scenario | Windows | U-old | U-new | HDD | CDD | Install ($) | Year‑1 Savings ($) | Payback (yrs) |
|---|---|---|---|---|---|---|---|---|
| Baseline double‑pane upgrade | 12 | 0.60 | 0.30 | 3500 | 1200 | 9000 | ~500–900 | ~10–18 |
| Sunny climate low‑SHGC | 16 | 0.70 | 0.32 | 1500 | 2600 | 12000 | ~800–1400 | ~8–15 |
| Cold climate high‑performance | 10 | 0.85 | 0.25 | 6000 | 600 | 11000 | ~700–1200 | ~9–16 |
Ranges vary by energy prices, shading, HVAC efficiency, and installation quality.
Formula Used
- Conduction heat transfer (annual): Q = U × A × DD × 24
- Heating savings (thermal): ΔQheat = (Uold − Unew) × A × HDD × 24
- Cooling savings (thermal): ΔQcool = (Uold − Unew) × A × CDD × 24 + (SHGCold − SHGCnew) × A × S × D
- Cooling electric savings: kWh = ΔQcool ÷ (SEER × 1000)
- Gas heating savings: therms = (ΔQheat ÷ AFUE) ÷ 100,000
- Heat pump heating savings: kWh = ΔQheat ÷ (COP × 3412)
- NPV: NPV = −Cost + Σ(Savingst ÷ (1 + r)t)
Where U is the U‑factor (Btu/hr‑ft²‑°F), A is total window area (ft²), DD is degree‑days, S is average daily solar gain (Btu/ft²/day), and D is cooling-season days.
How to Use This Calculator
- Enter the number of windows and an average window area.
- Fill in old and new U‑factors and SHGC values from product labels.
- Add your local HDD and CDD; choose °F-days or °C-days.
- Set cooling-season days and solar gain based on exposure and shading.
- Select your heating system and enter efficiency and energy prices.
- Enter installation cost and expected rebates, then choose analysis years.
- Press “Calculate Savings” to see results above the form.
- Download a CSV or PDF summary using the buttons in results.
This tool provides estimates for planning. Actual savings depend on air sealing, frame quality, shading, thermostat settings, indoor comfort targets, and weather variability.
Insights
What the estimate represents
This calculator converts window performance changes into annual energy and cost impacts. It uses your window count, average area, U‑factor change, and solar gain change to estimate heating and cooling load reductions. Inputs such as degree days, solar exposure, and HVAC efficiency shape the final result. The output is planning‑grade and intended for comparing upgrade paths. For best accuracy, pull HDD and CDD from a nearby weather station and match energy prices to recent bills. Consider separate values for day and night comfort settings and realistic window area measurements.
Heating savings mechanics
Heating savings are driven by the reduction in conductive heat loss through glass and frames. Using heating degree days, the tool estimates seasonal heat transfer and adjusts for your heating system efficiency. Gas systems use AFUE, while electric options use COP or resistance conversion to electricity. Higher fuel prices or colder climates increase the value of lower U‑factors.
Cooling savings mechanics
Cooling savings combine two effects: less conductive heat entering during hot weather and less solar heat entering because of a lower SHGC. Cooling energy savings are translated to electricity using your SEER value. In sunny climates, SHGC reductions can rival insulation gains, especially on west‑facing glass. Shading, films, and exterior overhangs can also change real‑world performance.
Financial performance over time
Year‑1 savings are escalated by your energy price growth rate to model future utility costs. Discounting converts future savings into present dollars, producing NPV and a discounted payback year. Simple payback is included for quick screening, but NPV better reflects long‑lived upgrades and opportunity cost. Add maintenance savings if new units reduce repairs or condensation damage.
Interpreting the chart and outputs
The chart visualizes where savings come from and how they accumulate across the analysis period. If cooling dominates, prioritize low‑SHGC glazing, improved shading, and tight installation. If heating dominates, prioritize lower U‑factors, warm‑edge spacers, and air sealing around frames. Use the CSV or PDF export to compare bids, rebates, and assumptions consistently.
FAQs
Which U-factor values should I enter?
Use the certified U-factor from the product label or datasheet. If you only know window type, approximate: older single-pane can be 1.0+, typical double-pane around 0.45–0.60, and high-performance units 0.20–0.35.
How do I choose SHGC for my climate?
Lower SHGC reduces summer solar heat, which helps warm or sunny locations. Higher SHGC can be beneficial in cold climates if winter sun is desired. If unsure, start with the manufacturer value and test a few alternatives.
Where can I find HDD and CDD?
Look up annual heating and cooling degree days for a nearby weather station or your utility’s energy reports. Keep the same base and unit when entering values. If you use °C-days, the calculator converts them automatically.
Why can simple payback differ from discounted payback?
Simple payback ignores the time value of money and future price changes. Discounted payback applies your discount rate and escalation assumptions, so it usually takes longer. For long-lived upgrades, NPV and discounted payback give a clearer picture.
Does air leakage matter beyond U-factor and SHGC?
Yes. Poor installation or gaps around frames can dominate heat loss and drafts. This calculator focuses on glazing performance, but airtight installation and sealing often deliver additional comfort and savings that are not fully captured.
How should I use the results when getting quotes?
Run scenarios for each bid using the same energy prices and degree days. Update installed cost and rebates per quote, then compare NPV, annual savings, and payback side by side. Export CSV/PDF to keep assumptions consistent.