Heating degree days are indicators of household energy consumption for space heating. One heating degree-day unit is given for each degree that the mean daily temperature departs below the base of 18 °C. One cooling degree-day unit is given for each degree that the mean daily temperature departs above the base of 18 °C.

How heating and cooling degree days are computed?

Take the highest (Tmax) and lowest temperature (Tmin) for the day, and average them. If this number is greater than 18 °C, then we have (Average temperature - 18) cooling degree days. If the average temperature is less than 18 °C, then we have (18 - Average temperature) heating degree days. Running totals are kept for these units over a time period of a year so fuel distributors and power companies can assess average demands.


Heating Degree Days in equation form:
HDD = Tbase - Ta if Ta is less than Tbase
HDD = 0 if Ta is greater or equal to Tbase
Where: Tbase = temperature base
Ta = average temperature, Ta = (Tmax + Tmin) / 2

Cooling Degree Days in equation form:
CDD = Ta - Tbase if Ta is greater than Tbase
CDD = 0 if Ta is less than or equal to Tbase
Where: Tbase = temperature base
Ta = average temperature, Ta = (Tmax + Tmin) / 2