Heat loss
Annual heat loss through an element (degree-day method)
- where
- annual heat loss (kWh/year)
- U-value (W/m²K)
- net area of the element (m²)
- annual heating degree days (K·day)
Explain this equation
Estimates the heat lost through an element over a heating season, in kWh. Use it to show customers the yearly saving from insulating.
Degree days add up how many degrees the outside falls below a base temperature (usually 15.5 °C), day by day, over a year. U × A gives watts per degree; times degree days gives watt-days; times 24 gives watt-hours, and dividing by 1000 gives kWh.
Energy equals heat loss rate per degree times degree-hours.
Sense check: 20 m² at 0.3 with 2000 degree days loses about 290 kWh a year; at 2.0 it would be about 1,900 kWh.
Watch out: this is fabric loss through one element only, and actual savings depend on heating patterns and the heating system's efficiency.
Estimates the heat that leaves through one element over a heating season, using heating degree days for your area.
Enter your values to see the result.
Good to know
A degree day is one degree of temperature difference sustained for one day, so U × A × degree days × 24 hours gives watt-hours, and dividing by 1000 gives kWh. Get the annual figure for your nearest weather station from a degree-day service (base 15.5 °C is the traditional UK base, which allows for internal gains). The 2000 K·day default is an example only; UK figures range from roughly 1,800 in the south-west to over 2,800 in northern Scotland. This is heat lost, not fuel used: divide by boiler efficiency or heat-pump SCOP for delivered energy.
Common mistake
Using a base-18 °C or base-15.5 °C degree-day set inconsistently, or mixing a monthly total with an annual one.
Limits
Simple degree-day estimate for one element; ignores solar gains on the element, thermal bridging and the way a better-insulated house has a lower balance-point temperature. For savings and payback use the retrofit savings calculators.
Sources
- CIBSE TM41 Degree-days: theory and application; heating degree days to base 15.5 °C for UK weather stations from BizEE Degree Days
- BS EN 12831-1:2017 Energy performance of buildings – Method for calculation of the design heat load (transmission heat loss Φ = U A (θint − θe))
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.