Window Energy Savings Calculator: Heating Savings and Payback

How the calculation works

A window’s U-factor is the rate at which heat passes through it, in BTU per hour, per square foot, per °F of temperature difference. The calculator multiplies the drop in U-factor by your window area and by how cold your winters are, then converts the heat saved into fuel and dollars.

Heat saved per season  = window area × (U old − U new) × heating degree days × 24
Fuel saved             = heat saved ÷ (BTU per unit of fuel × system efficiency)
Yearly savings         = fuel saved × fuel price
Simple payback (years) = window project cost ÷ yearly savings

Heating degree days (HDD) add up how far each day’s average temperature falls below 65°F over a year, as EIA explains. Multiplying by 24 converts them to degree hours to match the hourly U-factor.

U-factors come from the NFRC label on new windows. NFRC lists window U-factors from about 0.20 to 1.20. ENERGY STAR’s limits are 0.22 or lower in the Northern zone, 0.25 North-Central, 0.28 South-Central and 0.32 Southern.

Fuel energy content is from EIA: 100,000 BTU per therm, 91,452 per gallon of propane, 3,412 per kWh; the tool uses 137,381 BTU per gallon of heating oil. Default prices are EIA averages: gas about $1.35 a therm (January 2026), propane $2.638 and oil $6.138 a gallon (week of October 12, 2026), and electricity 18.31¢ per kWh (July 2026).

The formula counts heat conducted through the glass and frame. It does not include air leaks, solar heat gain, or summer cooling.

Worked example

The default case replaces 200 square feet of older double-pane windows in a 5,000-degree-day climate. The home heats with a 90% gas furnace at $1.35 a therm.

1. The old windows are rated about U-0.50; the new ones are U-0.22, which meets ENERGY STAR’s Northern zone.
2. Heat saved over a winter works out to about 75 therms of gas.
3. That is about $101 a year in heating.
4. At $15,000 for the project, simple payback is about 148.8 years.

That long payback is not a typo. Replacing decent double-pane windows on energy savings alone rarely pays.

A second example

Now take a colder, oil-heated home with 6,000 degree days. It replaces 150 square feet of single-pane windows, about U-1.0, with U-0.25 units for $12,000. Oil is $6.14 a gallon in an 83% efficient boiler.

• Fuel saved: about 142 gallons of oil a year
• Yearly heating savings: about $872
• Simple payback: about 13.8 years

Single-pane glass, an expensive fuel and a cold climate together make new windows far more worthwhile.

What your result means

The yearly savings figure is the heating cost you could avoid by lowering U-factor alone. If payback is longer than the window’s expected life, energy savings will not cover the cost. That can still be fine, but buy for the right reasons: comfort near the glass, less condensation, easier operation, noise, or failing frames.

If cutting your bill is the main goal, look at cheaper fixes first. Weatherstripping and caulking stop drafts that this calculator does not count. ENERGY STAR low-e storm windows can deliver much of the benefit at a fraction of the cost. Our energy-efficient windows guide compares replace, repair and storm windows, and the air sealing guide covers drafty frames.

Climate, cooling and incentives that change the answer

Climate zone. In the North, U-factor matters most. In the South, solar heat gain coefficient (SHGC), which measures how much of the sun’s heat the glass lets in, can matter more, because blocking summer sun cuts cooling. This calculator covers only heating, so southern homes should weigh SHGC and cooling separately.

Air leakage. Old windows often leak air around the sash and frame. Those savings are real but vary so much from house to house that EPA leaves them out of its storm window estimates, and so does this tool.

Incentives. The federal 25C credit no longer applies to windows placed in service after December 31, 2025. There is no 2026 federal tax credit for windows. Some utilities still offer window or storm window rebates; check the ENERGY STAR rebate finder or your utility. Our guide to energy tax credits in 2026 explains what changed.

Common mistakes

• Counting the whole wall opening instead of the window unit area.
• Guessing an old U-factor that is far too high, which inflates savings.
• Using the full project price when part of it is for repairs or trim you would do anyway.
• Ignoring cheaper options such as storm windows, weatherstripping and caulk.
• Expecting new windows to fix drafts that come from walls, attics or doors.

Limits of this calculator

The tool counts conducted heat loss through the window during the heating season only. It leaves out air leakage, solar heat gain in winter, summer cooling, and comfort. It treats all windows as one area with one U-factor. Real savings can be higher or lower. This is an educational estimate, not a quote or advice. Get at least three written quotes, and ask each for the NFRC-rated U-factor and SHGC of the exact product.