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.