Will new windows pay for themselves?

Why this calculator is honest about payback

Window companies often quote savings in percent of your energy bill. The Department of Energy's own consumer guide says ENERGY STAR windows can save as much as 7 to 15 percent or more on annual household energy bills, depending on location and the window being replaced, and then adds that new windows can mean an extended payback period. The two statements fit together: for a hypothetical $2,000 energy bill, a 10 percent saving is $200 a year, and a $9,000 replacement pays that back in 45 years. This calculator does the second half of the sum. It takes the extra heat your old windows let through, turns it into fuel and dollars, and divides the price by the saving.

The method

Heat flows through a window at a rate of U × area × temperature difference. Added over a year, the temperature difference is the degree days times 24 hours. Heat saved (Btu a year) = (U old − U new) × area (ft²) × heating degree days × 24. The fuel saved is that heat divided by the energy in one unit of fuel (100,000 Btu per therm of natural gas, 91,452 Btu per gallon of propane, 3,412 Btu per kWh) and by the system efficiency. For cooling: kWh saved = (U old − U new) × area × cooling degree days × 24 ÷ (SEER × 1,000). Payback is the installed cost divided by the yearly saving; the net over the service life is the saving times the years minus the cost. These formulas are our own derivation from the definition of the U-factor, not a method published by DOE.

Worked example

150 square feet of single-glass windows (U 1.07) replaced with U 0.30 in Chicago (5,974 HDD, 1,003 CDD, NOAA normals), heated with a 95 percent natural gas furnace at $1.50 a therm, cooled by a SEER 14 air conditioner at 17.23¢ per kWh. Conduction falls by 72 percent. Heat saved: 0.77 × 150 × 5,974 × 24 = 16.6 million Btu, which is 174 therms and $261. Cooling saves 199 kWh, $34. Total $296 a year. At $9,000 installed the simple payback is 30 years, and over a 20-year life the net result is −$3,086. In Phoenix, with 874 HDD, the same windows save $201 a year; in Miami $163. Replacing a double-pane clear window (U 0.47) with the DOE's best available triple-pane (U 0.12) in Chicago saves only $134 a year.

Limits of this method

• Conduction only. Sun through the glass (solar gain) warms a house in winter and overheats it in summer; the SHGC on the NFRC label measures it. We do not model it, and DOE says to choose by climate and orientation.
• Air leaks. Old frames leak. DOE says sealing air leaks can save 10–20 percent on heating and cooling bills and recommends insulating and air sealing before replacing windows. The saving from tight new frames is not in our number, so a very leaky house will save more than the form shows.
• Degree days are airport averages. Your thermostat setting, the house's internal heat gains and the building's balance point all differ from the 65°F base. Use degree days as a ratio between climates, not as a precise bill.
• Frames and whole-window U. Use the whole-window U-factor (frame plus glass). The reference values on this page for old windows come from a 1997 DOE booklet quoting 1993 ASHRAE data; modern products of the same type may differ.
• Prices. We do not show window prices: they vary by size, frame, region and installer. Costs, fuel prices and life in the form are examples to replace.

Do new windows pay for themselves?

Rarely on energy savings alone. The Department of Energy's consumer guide to windows says replacing old windows is a significant investment and that new windows can have a potentially extended payback period; it also says homeowners recoup about 78 percent of the cost when the house is sold. With the form's placeholder numbers (150 square feet of single-pane windows, U 1.07 replaced by U 0.30, in Chicago on gas heat, $9,000 installed) the energy saved is $296 a year, a simple payback of 30 years, longer than the windows will last. Comfort, noise, rot and appearance are real reasons to replace windows, but they are not on this bill.

What is a U-factor?

The U-factor is the rate at which heat flows through a window, in Btu per hour per square foot per degree Fahrenheit. DOE says it generally ranges from 0.20 to 1.20 for windows, and that the lower the number, the better the window insulates. It is the reciprocal of R-value (R = 1 ÷ U). The NFRC label on every ENERGY STAR window gives the whole-window U-factor, which includes the frame, not only the glass. Use the whole-window number from the label of your old window if you have it; otherwise use the reference values below.

What U-factor does an old window have?

The U-factor of a typical old window depends on the glass, the gap and the frame. A Department of Energy booklet from 1997, quoting the 1993 ASHRAE Handbook, lists 1.07 to 1.30 for single glass, 0.48 to 0.81 for double glass with an air gap, and 0.34 to 0.64 for double glass with a low-e coating and argon, depending on the frame type (values are for 3 by 5 foot windows). A DOE purchasing guide from December 2021 uses 0.47 for a double-pane clear window and 0.12 for the best available triple-pane window as examples. Those are reference values, not measurements of your windows.

What are heating and cooling degree days?

Degree days measure how cold or hot a place is over a year. Heating degree days (HDD) add up, day by day, how far the average outdoor temperature is below 65°F; cooling degree days (CDD) add how far it is above 65°F. NOAA's 1991-2020 climate normals give, for example, 5,974 HDD and 1,003 CDD a year at Chicago O'Hare, 874 HDD and 4,765 CDD in Phoenix and 104 HDD and 4,656 CDD in Miami. Heat lost through a window is proportional to U × area × degree days, so a cold climate saves several times more than a mild one: with the same windows and prices the form shows $296 a year for Chicago, $201 for Phoenix and $163 for Miami.

What about solar heat gain and drafts?

This calculator counts only heat conducted through the window, so it can understate the saving in a house with very leaky windows and overstate or understate it where sun comes in. Air leaking around old frames can matter as much as the glass, and DOE recommends air sealing and insulating first: sealing air leaks can save 10 to 20 percent on heating and cooling bills. The solar heat gain coefficient (SHGC) sets how much of the sun's heat comes through; DOE advises low SHGC in warm climates and, in cold ones, a high SHGC on south-facing windows. The calculator does not model it.

Are storm windows or repairs cheaper than replacing windows?

Often, per dollar of energy saved. DOE says low-e exterior or interior storm windows can save 10 to 30 percent on heating and cooling costs, depending on the window already installed, and that replacing single-pane with double-pane windows "may be cost effective" (its words) but that storm windows are an option. Try the form with a lower cost: the same 150 square feet fitted with storm windows at a placeholder $2,500 and an effective U of 0.50 shows $219 a year and a payback of 11 years. Prices are yours to check with quotes.

Sources: U.S. Department of Energy, Consumer Guide to Energy-Efficient Windows (DOE/EE-2468, 2021); DOE Energy Saver Guide 2022 (Windows, Air Sealing); DOE FEMP, Purchasing Energy-Efficient Residential Windows, Doors, and Skylights (December 2021); DOE, Selecting Windows for Energy Efficiency (DOE/GO PUB-788, January 1997, data from ASHRAE Handbook, 1993); NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, annual/seasonal normals (public domain, read October 28, 2026); U.S. EIA, Electric Power Annual 2025, Table 2.4. Related tools: heat loss calculator, insulation calculator, heating cost calculator, thermostat setback calculator. How we check our calculators.