Quantitative indices designed to reflect the demand for energy needed to heat a home or business. These indices are derived from daily temperature observations, and the heating requirements for a given structure at a specific location are considered to be directly proportional to the number of HDD at that location.
HDD can be added over periods of time to provide a rough estimate of seasonal heating requirements. In the course of a heating season, for example, the number of HDD for New York City is 5,050 whereas that for Barrow, Alaska is 19,990. Thus, one can say that, for a given home of similar structure and insulation, around four times the energy would be required to heat the home in Barrow than in New York. Likewise, a similar home in Los Angeles, California, whose heating degree days for the heating season is 2,020, would require around two fifths the energy required to heat the house in New York City. [1]
Example: For a typical New York City winter day with High = 40F and Low = 30F, the Average Temperature = 35F. For that day HDD = (65 - 35) = 30. A month of thirty similar days might accumulate HDD = 900. A year (including summer average temperatures above 70F) might accumulate an annual HDD = 5000.