Heating and cooling degree days measure the amount of heating or cooling necessary in your building(s). They are calculated based on the difference between outdoor temperatures and the base temperature (typically 65°F). If your indoor temperature is 65°F, it is assumed your building will need cooling, and if it's below 65°F, it is assume that your building will need heating.
Note: The Department of Energy (DOE) may recommend different base temperatures depending on regional climate conditions and building standards.
Heating degree days (HDD) is the number of days you'd have to heat your building by 1° to accommodate the heating requirement. For example, if a day's average outdoor temperature is 55°F, this day is worth 10 HDD because it's 10° below 65°F.
Cooling degree days (CDD) is the number of days you'd have to cool your building by 1° to accommodate the cooling requirement. For example, if a day's average outdoor temperature is 80°F, this day is worth 15 CDD because it's 15° above 65°F.
HDDs and CCDs care calculated each day of the year to determine the total annual HDDs and CCDs.