Estimates annual space-heating energy, running cost and carbon from a building heat-loss coefficient and UK regional heating degree days (CIBSE TM41).
Enter a heat-loss coefficient directly, or derive one from a design heat loss ÷ ΔT, then pick a UK region preset (or your own degree-day figure) and a boiler or heat-pump system.
Indicative estimate only. Always verify with a qualified engineer.
About this degree day energy calculator
This free degree day energy calculator estimates a building's annual space-heating energy, running cost and operational carbon from its heat-loss coefficient and the local heating degree days. It is aimed at UK M&E and energy engineers, retrofit assessors and building-services designers comparing a boiler against a heat pump, or benchmarking a building's likely fuel use, at feasibility or tender stage. Enter a heat-loss coefficient directly or derive one from a design heat loss, pick a UK region preset, and choose a boiler efficiency or heat-pump SCOP. Everything runs in your browser. Nothing is uploaded.
How degree day energy is calculated
Annual space-heating demand is the CIBSE TM41 §1.2.1 relation demand = heat-loss coefficient × degree days × 24 ÷ 1000 (the engine works in W/K, so it carries the extra ÷1000 to land in kWh). You supply the heat-loss coefficient in W/K, or derive it from a design heat loss ÷ design ΔT. The degree-day presets are 20-year UK regional averages at base 15.5 °C (CIBSE TM41 / Degree Days Direct), and you can override with a local figure. Fuel input is demand ÷ boiler efficiency or demand ÷ heat-pump SCOP; annual cost is fuel × unit price and carbon is fuel × the DESNZ/Defra emission factor.
Frequently asked questions
What base temperature do the degree days use?
The traditional UK base of 15.5 °C (CIBSE TM41). Note the presets are 20-year averages and the UK long-term mean has since fallen (DESNZ DUKES puts 1991–2020 at roughly 2,061 K·day), so enter a local or more recent, base-temperature-corrected figure where accuracy matters.
Boiler efficiency or heat-pump SCOP?
Enter a boiler seasonal efficiency in per cent (an ErP A-rated condensing boiler is about 90%) or a heat-pump SCOP (MCS minimum around 2.8, typically 3.0–3.5). Switching system type quick-fills the gas or electricity unit price and emission factor.
Where do I get the heat-loss coefficient?
Enter it directly in W/K, or use the derive option: design heat loss ÷ design ΔT. For example, a total design heat loss from a heat-loss calculation divided by its internal−external temperature difference gives the whole-building coefficient.