Window Energy Savings Calculator
Estimate the heating-season energy you save by dropping your windows from an old U-factor to a new one, using your own heating degree days and energy rate. The result is conduction heat loss only — a clear, honest planning number, not a whole-house energy audit.
Calculator
Dropping 100 sq ft of glazing from U 1.00 to 0.30 over 5,000 HDD saves about 2,462 kWh ≈ $369/season (heating conduction only; cooling not included). Look up your HDD at degreedays.net or NOAA.
A window’s U-factor measures how fast heat flows through it: the lower the number, the better the window keeps warmth inside. When you swap leaky single-pane glass (U around 1.0) for a modern low-E double-pane unit (U around 0.30), you cut the conduction heat loss through that glass roughly in proportion to the drop in U. This tool turns that drop into a heating-season estimate in BTU, kilowatt-hours and dollars, using the classic degree-day heat-loss formula.
Two of the inputs are yours to look up, because they are local, not universal: heating degree days (HDD) describe how cold your winters are, and your energy rate is what you actually pay per unit of heat. Find your HDD at degreedays.net or NOAA and read your rate off your utility bill. Because both come from you, the estimate is correct for your climate and your prices, and it never goes out of date. The figure counts heating conduction only — it does not include air leakage, solar gain or summer cooling, so treat it as a conservative floor on the savings.
Formula
BTU = (u_old − u_new) × area × HDD × 24
kWh = BTU ÷ 3,412
cost = kWh × rate
- u_old, u_new — the old and new window U-factors; their difference is the improvement in heat flow.
- area — the glazing area you are upgrading, in square feet.
- HDD — heating degree days for your location (the 24 turns degree-days into degree-hours).
- 3,412 — BTU in one kilowatt-hour, converting heat to electricity units.
For a fuel-heated home, divide the BTU by the fuel’s energy content and its efficiency instead of using kWh — the kWh line shown here is the electric-resistance case, which is the simplest to read.
Worked example
You replace 100 sq ft of single-pane glass (U 1.0) with low-E double-pane units (U 0.30) in a climate of 5,000 HDD, heating with electricity at $0.15/kWh:
(1.0 − 0.30) × 100 × 5,000 × 24 = 8,400,000 BTU
8,400,000 ÷ 3,412 = 2,462 kWh
2,462 × $0.15 = $369 per heating season
About $369 a year in avoided conduction loss through that 100 square feet of glass. Change the U-factors, the area, your HDD or your rate and the number tracks your real situation — a milder climate or a smaller U drop saves proportionally less.
Conduction only — what the estimate leaves out
This is deliberately a single-mechanism estimate: heat conducted through the glazing during the heating season. It leaves out the things that vary too much to pin down here — air leakage around old sashes (often a big part of why old windows feel drafty), solar heat gain that low-E coatings also change, and cooling-season effects in summer. Real total savings from new windows are usually somewhat higher than the conduction figure because sealing up the air leaks helps too, but conduction is the part you can compute cleanly from published U-factors, so that is what this tool reports.
Use manufacturer U-factors from the NFRC label for the new window, and a realistic reference value for the old one — roughly 1.0 for clear single-pane, 0.50 for an older uncoated double-pane, 0.30 or lower for a modern low-E unit. If you heat with gas, convert the BTU to therms (100,000 BTU per therm) and divide by your furnace efficiency before applying your per-therm rate, since a furnace is not 100% efficient. Whatever the fuel, this is a planning estimate to help you weigh an upgrade, not a guaranteed bill reduction — comfort, condensation and noise are real benefits it does not try to price.
Frequently asked questions
How much can I save with energy-efficient windows?
It depends on how much glass you upgrade, how big the U-factor drop is, how cold your winters are and what you pay for heat. In the worked example — 100 sq ft from U 1.0 to U 0.30, 5,000 HDD, $0.15/kWh — the conduction saving is about 2,462 kWh, or roughly $369 a heating season. Enter your own numbers for a figure that fits your home.
What is a window U-factor?
U-factor is the rate at which heat flows through the window per degree of temperature difference — the lower the number, the better the insulation. Clear single-pane runs around 1.0, older double-pane around 0.50, and modern low-E double-pane around 0.30 or below. It is the inverse idea of R-value.
Where do I find my heating degree days?
Look up annual heating degree days for your location at degreedays.net or from NOAA climate data. HDD measures how cold your heating season is; a higher number means more heating demand and larger savings from a window upgrade.
Does this include cooling and air leakage savings?
No. The estimate covers heating-season conduction through the glass only. Air leakage around old sashes, solar heat gain and summer cooling are left out, so real total savings are often a bit higher. Treat the result as a conservative planning floor.
I heat with gas — how do I adapt the numbers?
Take the BTU figure, divide by 100,000 to get therms, then divide by your furnace efficiency (for example 0.90) before multiplying by your per-therm rate. The kWh line shown here is the simplest electric-resistance case; the BTU value is the same for any fuel.