Definitions for **"Heating Degree Days"**

(abbrev. HDD) A form of degree day used to estimate energy requirements for heating. Typically, heating degree days are calculated as how much colder the mean temperature at a location is than 65°F on a given day. For example, if a location experiences a mean temperature of 55°F on a certain day, there were 10 HDD (Heating Degree Days) that day because 65 - 55 = 10.

A heating degree day is a measure of how much the average of the daily high temperature and low temperature is less than a reference temperature. In other words how much heating is required. More details.

The number of degrees below 65o F for a day summed over the entire heating season.