Window energy-savings estimator
Estimate the annual heating-season saving from replacing old glass with a lower-U window — a fenestration-only conduction delta from the numbers you enter.
Swapping 100 ft² of glass from U 0.75 to U 0.30 in a 5,000-HDD climate saves about $90.00 a year in heating. This is a fenestration-only heating-season delta — the window’s own conduction saving, NOT a whole-building load calc; Manual J / HVAC sizing is an HVAC pro’s job, and the price and HDD are yours to enter.
Calculator inputs
Replacing old windows saves heating energy because a lower U-factor conducts less heat out of the house. This estimator computes that saving as a steady-state conduction delta over the heating season: it multiplies the glass area by the drop in U-factor, by the heating-degree-days (how cold and how long your winter is), by 24 hours, then converts to delivered fuel through your heating-system efficiency and prices it with the number from your utility bill.
It is deliberately narrow: this is the window’s own conduction saving only, not a whole-building load calculation and not a cooling-season number. Air leakage, solar gain, wall and attic losses, and equipment sizing all matter for the full picture — that is Manual J territory for an HVAC pro. Use this to weigh energy against comfort and cost, not to size a furnace.
Formula
annual saving = area × (Uold − Unew) × HDD × 24 ÷ 1000 ÷ furnace_eff × energy_price
Area in ft², U-factors in BTU/hr·ft²·°F, HDD in °F-days, energy price in $ per 1,000 BTU of delivered fuel. The ÷ 1000 converts BTU to the thousand-BTU pricing unit.
Worked example
Replacing 100 ft² of single-pane glass (U 0.75) with double-pane low-E (U 0.30) — a ΔU of 0.45 — in a 5,000-HDD climate, behind a 90% furnace, at $0.015 per 1,000 BTU:
100 × 0.45 × 5,000 × 24 ÷ 1000 ÷ 0.90 × 0.015
= 5,400 kBTU saved ÷ 0.90 = 6,000 kBTU fuel × $0.015 = $90.00 per year
Feed that saving into the payback calculator against your project cost to see the break-even horizon.
Getting the inputs right (and reading the result honestly)
Where to get the inputs. Total glass area = the sum of width × height ÷ 144 for every window you are replacing (the window-area calculator does one at a time). HDD for your city is published by NOAA and many utilities. Energy price per 1,000 BTU comes from your bill — divide your $/therm by 100 for natural gas, or convert $/kWh (1 kWh ≈ 3.412 kBTU) for electric heat. Expect modest numbers: window energy payback is usually long, and comfort, noise and looks are often the real drivers.
Reference table
| Glazing | Typical U-factor |
|---|---|
| Single pane (old) | 0.90–1.20 |
| Clear double pane | ~0.48 |
| Double pane, low-E + argon | ~0.30 |
| Fiberglass double low-E + argon | ~0.28 |
| Triple pane, low-E | ~0.18 |
Labeled typical planning U-factors for the ΔU — use the NFRC label for the exact numbers. See the U/SHGC by frame & glazing table.
Frequently asked questions
How much do new windows save on energy?
For a typical whole-house replacement the heating-season conduction saving is often a few hundred dollars a year at most, and frequently less — in the example above, 100 ft² of glass saves about $90/yr. The exact figure scales with your glass area, the drop in U-factor, how cold your winters are, and your fuel price.
Does this include cooling / air-conditioning savings?
No. This is a heating-season conduction delta only. Cooling savings depend mostly on SHGC and solar orientation — see the SHGC reference — and are not added here.
Is this a whole-building energy audit?
No. It is a fenestration-only estimate of the window’s own conduction saving. A full building load (Manual J) accounts for air leakage, walls, attic, ducts and equipment — that is an HVAC pro’s job.
What is a heating-degree-day (HDD)?
HDD measures how cold a year is: each day contributes the degrees its average temperature falls below 65°F. Higher annual HDD means a longer, colder heating season and larger window savings. NOAA and many utilities publish HDD by city.