Energy Savings Calculator
How much will you actually save on heating bills after replacing your old windows? This calculator uses U-value ratings, current UK energy prices, and your heating type to give you a realistic annual figure — not marketing fluff.
- Heat lost through old windows
- — kWh/yr
- Heat lost through new windows
- — kWh/yr
- CO₂ reduction
- — kg/yr
- 10-year total saved
- —
Assumes UK heating degree-days (2,200 HDD baseline, adjusted by region), 2026 retail energy prices (gas ~6.5p/kWh, electric ~27p/kWh, heat-pump effective 8p/kWh at COP 3.5), and a heating season of roughly 200 days. Real savings vary with occupancy patterns, insulation in walls/loft, and boiler condition.
What affects real savings
- Wall insulation first. If your walls are uninsulated solid brick, upgrading windows before walls captures only ~30% of the available savings. See our grants section for whole-fabric schemes.
- Heat pump owners see the biggest proportional gain. Every kWh saved is expensive heat — a triple-glazing upgrade typically pays back 2–3 years sooner with a heat pump than with gas.
- Triple glazing isn't always worth it. If you're on gas with a new boiler and well-insulated walls, high-spec double typically pays back faster than triple. See our double vs triple guide.
Next step
Payback Period Calculator — plug in your install cost and
this annual saving to see your break-even year.
Grant Eligibility Checker — ECO4 / HUG2 / GBIS can cut
your out-of-pocket install cost to £0 if you qualify.
Cost Calculator — get the install-cost side of the equation.