Savings
Annual energy saving from an insulation upgrade (degree-day method)
- where
- reduction in heat loss (W/K)
- U-value before (W/m²K)
- U-value after (W/m²K)
- area insulated (m²)
- annual heat saved (kWh)
- heating degree days per year (K·day)
- annual fuel saved (kWh)
- heating system efficiency (%)
Explain this equation
Estimates the yearly heat and fuel saved by improving the U-value of a wall, roof or floor.
The fall in U-value times the area is the drop in heat loss for each degree of temperature difference. Degree days times 24 turn that into degree-hours over a year, and dividing by 1000 gives kWh of heat. The heating system has to burn more fuel than the heat it delivers, so fuel saved is heat saved divided by its efficiency.
Annual energy from the degree-day method, then corrected for heating efficiency.
Sense check: 60 m² of solid wall from 1.6 to 0.3 cuts loss by 78 W/K, about 3,700 kWh of heat, or about 4,400 kWh of gas at 85%.
Watch out: real savings are often lower, because people may heat more once the home is warmer and older walls may already perform better than the textbook U-value.
Estimates the yearly heat and fuel saved by lowering the U-value of a wall, roof or floor, using heating degree days.
Enter your values to see the result.
Good to know
Get the before and after U-values from the U-value calculator or RdSAP default tables. Heat saved = change in heat loss (W/K) × degree days × 24 hours. Dividing by heating efficiency gives the fuel bought (about 85% for a modern condensing gas boiler; enter a heat pump SCOP as a percentage, e.g. 300).
Common mistake
Using design temperature difference × 8760 hours instead of degree days. That assumes the coldest day all year and overstates savings several times.
Limits
Simple steady-state estimate. Real savings are often lower because of comfort taking, thermal bridges and heating patterns. SAP or RdSAP is the formal method.
Sources
- BRE, SAP 10.2 The Government’s Standard Assessment Procedure for Energy Rating of Dwellings (AP50 ÷ 20 infiltration estimate; shelter factor (20) = 1 − 0.075 × sheltered sides; (38) = 0.33 × n × V; (36) = 0.20 × exposed area where thermal bridging details are unknown; (39) HTC; (40) HLP = HTC ÷ floor area; MVHR effective air change n_adj + n_mech × (1 − η))
- DESNZ, Energy Trends section 7: weather (heating degree days, table ET 7.1)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.