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Window Upgrade Energy Savings Calculator
See how many kilowatt-hours — and how much money — an old single-glazed window upgrade actually saves each heating season.
Why U-value is the number that matters
A window's U-value (in W/m²K) measures how much heat passes through it — the opposite of R-value, so a lower U-value means a better-insulating window. Old single-glazed aluminium-framed windows commonly sit around U = 5.0–5.8 W/m²K, while modern double or triple-glazed low-e windows can reach U = 1.0–1.4 W/m²K, a four- to five-fold improvement.
Turning watts into an annual saving
The calculator multiplies the U-value difference by your window area and the indoor-outdoor temperature difference to get an instantaneous wattage saving, then multiplies that by the number of hours your heating system runs each year to get the total kilowatt-hours saved — and, with a local energy price, the annual money saved.
Frequently asked questions
What heating-season hours figure should I use?
A reasonable default for a temperate climate with several cold months is 3,000–4,000 heating hours per year, but colder climates with long winters can exceed 5,000 hours while mild climates may see under 2,000 — check local heating-degree-day data or your own thermostat run-time history for a more precise number.
Does this account for solar heat gain through the glass?
No — this is a heat-loss (conduction) comparison only. New windows also typically change solar heat gain and daylight performance, which affects cooling loads and comfort but isn't part of this specific calculation.
Why is the temperature difference the same for both windows?
Because the comparison is meant to isolate the effect of the U-value change alone — using the same area and the same indoor-outdoor temperature difference for the old and new window keeps every other variable constant so the savings figure reflects the glazing upgrade only.
Should I include the frame area or just the glass?
Use the total window area including the frame, since the U-value you enter should be the whole-window U-value (glass plus frame) as published on the manufacturer's energy label — this is the figure that's actually representative of real heat loss through the installed window.