Free Heating Degree Days calculator with clear step-by-step results.
Heating degree days accumulate the shortfall of outdoor temperature below a base temperature, which sits below room temperature because occupants, appliances and solar gain provide free heat. Multiplying by the building's heat loss coefficient gives delivered heat, and dividing by system efficiency gives the fuel actually burned.
Heating degree days
HDD = max(0, base temp - mean outdoor temp) x days
Fuel consumed
kWh fuel = UA x HDD x 24 / 1000 / system efficiency
It is the long-standing convention for UK-style degree day series: internal gains typically supply the last few degrees, so heating is only needed once outdoors falls below roughly 15.5 C. Well-insulated buildings justify a lower base.
For a heat pump, yes - it moves heat rather than generating it, so enter its seasonal COP as a percentage, such as 340.