U-factor is the rate of heat loss: the lower, the better. A modern double-pane low-e window has a U-factor around 0.30, an old double-pane around 0.50, and a single-pane around 1.0. The bigger the gap, the bigger the savings.
Calculator
Formula used
Annual energy (BTU) = U × area × HDD × 24 · Saved = (U_old − U_new) × area × HDD × 24
How to use the Window Energy Savings Estimator
- Measure the total area of the windows you plan to replace.
- Look up the U-factor on the NFRC label of the new windows.
- Find your local heating-degree-days at energy.gov.
- Add the project cost and your energy price for the payback period.
About this tool
The Window Energy Savings Estimator is built for speed and accuracy: every calculation runs in your browser, so your data never leaves your device and the result updates as you type. It works equally well on a phone, a tablet or a desktop, and it is free to use as many times as you like.
Common use cases
- Comparing two window products before buying.
- Deciding whether a full replacement is worth it.
- Modelling the impact of a partial upgrade.
- Estimating the CO2 reduction of an efficiency project.
Tips for the best result
- Use the latest figures you have - even a small change in rate or term can shift the outcome by hundreds.
- For annual figures, divide by 12 to get a monthly number you can compare with the result.
- Bookmark this page and revisit the calculation as the inputs change.
Frequently asked questions
What is a good U-factor?
Modern double-pane low-e windows have U-factors between 0.25 and 0.35. Triple-pane can reach 0.15. Anything over 0.50 is dated.
What are heating degree days?
HDD is a measure of how cold the climate is: the sum of (65 − average daily temperature) for every day of the year. Colder climates have higher HDDs and bigger energy savings.
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