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Example data
| Windows | Area each | HDD | CDD | U (base → Low‑E) | Rates (elec / heat) | Net savings | Payback |
|---|---|---|---|---|---|---|---|
| 10 | 15 ft² | 2000 | 800 | 0.50 → 0.30 | $0.18 / $0.12 | $65.77 | 63.86 yrs |
Formula used
The calculator estimates conductive heat transfer through windows using annual degree days:
- Thermal energy (kWh) = U × Area × DegreeDays × 24 ÷ 3412
- Heating purchased energy ≈ Thermal kWh ÷ HeatingEfficiency
- Cooling electricity (kWh) ≈ Thermal kWh ÷ COP
- Annual cost = PurchasedEnergy × EnergyRate
- Net annual savings = (BaselineCost − Low‑ECost) − MaintenanceChange
- Payback = NetUpfrontCost ÷ NetAnnualSavings
- NPV = −NetUpfrontCost + Σ(NetAnnualSavings ÷ (1 + DiscountRate)t)
Notes: U‑factor uses common window label units (Btu/hr·ft²·°F). Degree days approximate seasonal exposure and are not a full building energy model.
How to use this calculator
- Count the windows you will upgrade and estimate area per window.
- Enter your local HDD and CDD values for a typical year.
- Use window label values for baseline and Low‑E U‑factors.
- Set energy prices and system performance (efficiency and COP).
- Add install cost and any rebates to estimate payback and value.
- Click Calculate Savings, then download CSV or PDF if needed.
Energy transfer baseline
The estimate focuses on conductive heat transfer through windows. It uses kWh = U × Area × DegreeDays × 24 ÷ 3412. With 10 windows at 15 ft² each, total area is 150 ft². At baseline U 0.50 and 2,000 HDD, heating thermal load is about 1,055 kWh per year. If your heater runs at 90% efficiency, purchased heating is about 1,172 kWh‑equivalent.
Low-E upgrade impact
Low‑E glazing lowers U-factor, reducing heat loss in winter and heat gain in summer. Using U 0.30, heating thermal load drops to about 633 kWh and purchased heating to about 703 kWh‑equivalent. Cooling is modeled with CDD and COP. With 800 CDD and COP 3.2, baseline cooling electricity is about 422 kWh, while Low‑E is about 253 kWh.
Annual cash savings
Energy becomes money by multiplying by your rates. At $0.12 per kWh‑eq for heating, costs are about $141 baseline versus $84 with Low‑E. At $0.18 per kWh for cooling, costs are about $76 baseline versus $46 with Low‑E. That is roughly $87 gross annual savings before any maintenance change you enter.
Investment metrics
Net upfront cost equals install cost minus rebates. Simple payback is upfront divided by net annual savings. NPV discounts each year’s savings at your discount rate over the analysis horizon; positive NPV supports the upgrade economically. IRR is an approximate rate that makes discounted savings equal the upfront cost, helping compare to other uses of cash. If net annual savings are negative, payback is shown as N/A, signaling that assumptions favor the baseline case. Adjust U‑factors, degree days, or incentives to reflect actual labels, local climate normals, and utility bills for your home and usage patterns.
Scenario testing
The sensitivity view varies electricity price by ±20% so you can see how tariff changes shift cooling savings and payback. For higher CDD locations or expensive electricity, cooling savings can dominate. In colder regions, HDD and heating efficiency matter more, so testing both rates and system performance improves realism.
FAQs
What U-factor values should I use?
Use label U‑factors for your existing windows and the proposed Low‑E product. If you do not know the baseline, start with 0.45–0.70 for older double pane and refine using manufacturer specs.
Where do HDD and CDD come from?
Use annual degree days from a trusted climate source for your city. Pick a recent multi‑year average for planning, or last year’s values if you want results that track a specific utility bill.
Why are there separate heating and cooling rates?
Heating may use gas, oil, or a heat pump. Enter the cost per kWh‑equivalent for your heating fuel, and the electricity price for cooling. This keeps the financial comparison consistent across fuels.
How does heating efficiency affect savings?
Higher efficiency means you buy less fuel for the same thermal load. Because savings are based on purchased energy, a very efficient system can reduce dollar savings from a window upgrade, while inefficient systems amplify them.
What does NPV tell me here?
NPV converts future net savings into today’s dollars using your discount rate. Positive NPV suggests the upgrade returns more value than your required return over the selected horizon, after accounting for the upfront cost.
Why can payback show N/A?
If net annual savings are zero or negative, dividing upfront cost by savings is not meaningful. Recheck U‑factors, degree days, prices, and rebates, and consider whether comfort or condensation benefits justify the project.