heating degree day

Heating degree days (HDD) is the number of degrees per day that the daily average temperature (the mean of the maximum and minimum recorded temperatures) is below a base temperature, usually 65°F (18°C), unless otherwise specified. Annual patterns of heating degree days can be used to determine indoor space heating requirements and heating system sizing. Total HDD is the cumulative total for the year/heating season. The higher the HDD for a location, the colder the daily average temperature(s).