Energy and running costs
Annual heat demand from design heat loss and degree days
- where
- heat loss coefficient (W/K)
- whole-house design heat loss (W)
- design inside temperature (°C)
- design outdoor temperature (°C)
- annual space heating demand (kWh/yr)
- annual degree days (K·day/yr)
- annual heat demand including hot water (kWh/yr)
- hot water demand (kWh/yr)
Explain this equation
Estimates the yearly heat a home needs from its design heat loss and the local degree days. Use it for running cost and carbon estimates.
The design loss divided by the design temperature difference gives H, the watts lost for each degree inside is above outside. Degree days add up those degrees over a heating year; times 24 makes degree-hours, and dividing by 1000 gives kWh. Hot water demand is added on top.
Energy is power per degree times degree-hours over the season.
Sense check: an 8.5 kW loss at a 24 K difference is about 352 W/K; with 1835 degree days that is about 15,500 kWh, or 18,400 with 2,900 of hot water.
Watch out: degree days use a base temperature, usually 15.5 °C, to allow for free gains. Real use depends heavily on how the household runs the heating.
Estimates a year’s heat use from the design heat loss and the local degree days. It is the reverse of the MCS 031 EPC method, and feeds the running cost and carbon cards.
Enter your values to see the result.
Good to know
Degree days and the outdoor design temperature come from MCS 031 Table 1 by postcode area. Hot water demand can come from the EPC (MCS 031 uses the EPC figure); a typical household uses roughly 2,000–3,000 kWh a year of heat for hot water.
Common mistake
Using a heat loss with a large safety margin. Any margin goes straight into the annual demand and inflates running costs.
Limits
A degree-day estimate: it assumes heat loss in proportion to the inside–outside difference and does not separately model internal and solar gains, heating patterns or a lower set-point. Real use can differ by 20% or more.
Sources
- MCS 031 Issue 4.0 Heat pump pre-sale information and calculations (Pspecific = 1000 × Q / (24 × D); Pt = Pspecific × (21 − do); W/m²; Appendix A Table 1 degree days and outdoor low temperatures by postcode; Appendix B worked example)
- CIBSE Domestic Heating Design Guide (2026 edition; design to BS EN 12831-1, room temperatures, air change rates, pipe and emitter sizing)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.