Heating Degree Day
A heating degree day (HDD) is a measure of how cold a location is over a year, expressed as the cumulative degrees below a 65°F base temperature each day, summed over 365 days. HDD is the standard input for sizing heating equipment and predicting annual heating energy use. A location with 5,000 HDD (e.g. Boston) needs roughly 2.5x the annual heating energy of a location with 2,000 HDD (e.g. Atlanta) for an identical home.