Window Energy Savings Calculator — Glazing Upgrade Savings

This calculator estimates the energy and cost savings from upgrading your windows by comparing the heat loss through your current glazing with the heat loss through the upgraded glazing.

The formula

Heat-loss coefficient (W/K) = U-value × Window area

Annual energy (kWh) = U-value × Window area × Heating degree days × 24 ÷ 1000

This is the established UK degree-day method (CIBSE / Open University) for turning a U×A heat-loss figure into annual energy. Heating degree days are measured to a 15.5°C base temperature and account for the whole heating season.

Annual saving = (Current kWh − New kWh) × Energy cost per kWh

U-values used

• Single glazing: 5.0 W/m²K
• Old double glazing (pre-2002): 3.0 W/m²K
• Modern double glazing: 1.4 W/m²K
• A+ rated double glazing: 1.2 W/m²K
• Triple glazing: 0.8 W/m²K

UK assumptions

Annual energy uses the UK standard degree-day year of 2,463 heating degree days (base 15.5°C); you can adjust this for your region (London ≈ 2,033, northern Scotland > 2,900) in Advanced options. CO₂ savings are based on the gas heating emission factor of 0.183 kg CO₂e/kWh (DESNZ 2024, Gross CV), as most UK homes use gas central heating; for electric heating the DESNZ 2024 grid factor is about 0.207 kg CO₂e/kWh. The default energy cost of 7.33p/kWh reflects the current UK gas price cap rate (Ofgem Q3 2026). Electricity-heated homes should use 26.11p/kWh.

These results are estimates: real savings depend on your home's exposure, how warm you keep it, draught-proofing and frame condition. The Energy Saving Trust estimates a fully single-glazed semi-detached home saves roughly £140 and 380 kg CO₂ a year by fitting A-rated double glazing.