See what a lower-U-factor window saves over a heating season. Enter the glass size, the old and new U-factors, your heating degree days, and fuel cost to compare annual energy and cost.
Annual Cost Savings
— per year
Per window, from lower conductive loss.Usage Tip
Compare whole-window U-factors from the NFRC label, not center-of-glass. A lower U-factor helps most in cold climates and on the windiest, least-sunny walls.
THE MATH
Annual heat loss = U × Area × HDD × 24
Savings = (U_old − U_new) × Area × HDD × 24
Fuel cost = heat ÷ efficiency × price
Savings = (U_old − U_new) × Area × HDD × 24
Fuel cost = heat ÷ efficiency × price
HDD (heating degree days) sums how far each day falls below 65°F over the year — higher HDD means a colder, longer heating season.
A therm is 100,000 BTU; cost uses your fuel price and system efficiency.
A therm is 100,000 BTU; cost uses your fuel price and system efficiency.
This compares steady conductive loss through the glass only; it excludes solar gain, air leakage, and the frame, so it understates the full comfort benefit.
Find your local HDD from a weather service; US values run from roughly 2,000 in the south to 9,000 or more in the far north.
Per-window savings are small — the real case for better glass is the whole-house total plus comfort, fewer drafts, and less condensation. Weigh annual savings against the upgrade cost for payback.
Find your local HDD from a weather service; US values run from roughly 2,000 in the south to 9,000 or more in the far north.
Per-window savings are small — the real case for better glass is the whole-house total plus comfort, fewer drafts, and less condensation. Weigh annual savings against the upgrade cost for payback.