Window Energy Rating Savings Calculator UK 2026

Upgrading your windows from single glazing to A-rated double glazing can reduce heat loss through your windows by 72%, saving a typical UK semi-detached home £100 to £200 per year on heating bills. The key measurement is the U-value — the rate of heat transfer through the window, measured in watts per square metre per degree Kelvin. Single-glazed windows have a U-value of approximately 5.0 W/m²K, old double glazing sits at around 2.8, modern A-rated double glazing achieves 1.4, and triple glazing reaches 0.8. Each reduction in U-value directly translates to less heat escaping your home, which means your boiler runs less and your gas bill falls. The savings depend on your total window area, property type, and insulation quality elsewhere in the building envelope. A detached home with 30 square metres of old double glazing upgrading to A-rated units could save £150 to £250 annually at current gas prices of 7p per kWh. This calculator uses the standard heat loss formula to estimate your specific savings based on window area and current glazing type.

How to Use This Calculator

Select your current window type — single glazed, old double glazed, or A-rated double glazed.

Choose the window upgrade you are considering — A-rated double or triple glazing.

Enter your total window area in square metres (a typical 3-bed semi has 18–22 m²).

Review your estimated annual heating saving in pounds and the energy saved in kWh, plus CO&sub2; reduction.