Heating degree days (HDD)
A measure of how much and for how long outdoor temperature falls below a base temperature, used to estimate and normalise building heating demand.
Heating degree days sum the daily shortfall of outdoor temperature below a base temperature at which a building needs no heating. The base accounts for internal and solar gains; common choices are 15.5 °C (UK), 18 °C (international), 65 °F (US, about 18.3 °C), or Eurostat's method of 18 °C base with a 15 °C heating threshold.
Annual HDD = Σ daily HDD
Heat demand (kWh) ≈ H (W/K) x HDD x 24 / 1000
Example: H = 200 W/K, 3,000 K·days: 200 x 3,000 x 24 / 1000 = 14,400 kWh
H is the building's heat loss coefficient (fabric plus ventilation). Typical annual values at 18 °C base: London about 2,700–3,000, Berlin about 3,200, Bucharest about 3,000, Helsinki about 4,400, Madrid about 1,800, Fairbanks above 7,000.
Uses include comparing energy consumption across years (weather normalisation), monitoring targets, sizing heating systems and modelling gas demand. Degree days are approximate because real buildings have temperature-dependent internal gains and variable occupancy; hourly simulation is more accurate. Global HDD has declined with warming, while cooling degree days have risen. Heat demand per degree day falls as U-values improve.
Related terms
All terms →- Cooling degree days (CDD)
A measure of how much and for how long outdoor temperature exceeds a base temperature, used to estimate air-conditioning demand.
- U-value
The thermal transmittance of a building element: heat flow in watts per square metre per kelvin of temperature difference (W/m²K). Lower is better.
- Heat pump
A device that uses a refrigeration cycle to move heat from a cooler source (air, ground, water) to a warmer sink, delivering 2–5 units of heat per unit of electricity.
- Energy Performance Certificate (EPC)
A standardised rating of a building's calculated energy performance, typically on an A–G scale, required in the EU and UK when buildings are sold or rented.
- SCOP (Seasonal COP)
A heat pump's average coefficient of performance over a standardised heating season, weighting test points by how often each outdoor temperature occurs.