Window Energy Savings Calculator — Glazing Upgrade Savings

Replacing single glazing (U-value 5.0) with A+ rated double glazing (U-value 1.2) can save a typical UK home £100-£300 per year on heating bills, depending on the number and size of windows. The Energy Saving Trust estimates a fully single-glazed gas-heated semi-detached house could save around £140 per year, rising to up to around £285 per year for a detached house. Homes with many large windows or in colder regions will see the biggest savings.

Triple glazing (U-value ~0.8) provides about 33% better thermal performance than A+ double glazing (U-value ~1.2). However, the additional cost saving is relatively small — typically £30-£80 per year more than good double glazing. Triple glazing is most worthwhile in very exposed locations, north-facing windows, or for noise reduction. The payback period for the extra cost over double glazing is often 20+ years. Use our U-Value Calculator to compare wall and window U-values.

Under Approved Document L (2021), replacement windows in England must achieve a U-value of 1.4 W/m²K or better, or alternatively the whole window must have a Window Energy Rating of band B or better. For new-build properties, the target U-value is 1.2 W/m²K. In Scotland, under Section 6 of the 2022 Technical Handbook (in force since 1 December 2022), replacement windows in existing dwellings must achieve 1.4 W/m²K or a Window Energy Rating of band B or better.

The instantaneous heat-loss rate is Q = U × A × ΔT, where U is the glazing U-value (W/m²K), A is the total window area (m²) and ΔT is the inside–outside temperature difference (°C). To turn that into annual energy this calculator uses the established UK degree-day method: annual energy (kWh) = U × A × degree days × 24 ÷ 1000, with heating degree days measured to a 15.5°C base temperature. The UK standard degree-day year is about 2,463 (base 15.5°C); regional values range from roughly 2,033 in London to over 2,900 in northern Scotland, so the figure can be adjusted for your area.

Yes, window glazing type significantly affects your Energy Performance Certificate (EPC) rating. Upgrading from single to double glazing can improve your EPC by one or two bands. This matters because rental properties in England and Wales must currently achieve at least an EPC band E, and under government plans confirmed on 21 January 2026 the minimum standard rises to band C for all private rented tenancies by 1 October 2030. Better windows also increase property value. See our EPC Rating Estimator and Draught-Proofing Calculator for more ways to improve your home's energy efficiency.