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December 2009 – Heating degree days, or HDDs, are a unit of measurement that relates to the heating of a building. The product of measuring temperature (in degrees) over time (in days), HDDs are measured relative to a baseline outdoor temperature that theoretically represents an "ideal" comfort temperature, below which a building will need to be heated to feel comfortable. In the U.S. the baseline temperature for HDDs is 65 degrees Fahrenheit.
A heating degree day is actually not a day at all, but a unit that can measure temperature over any given period of time. If you use 65 degrees as a baseline, then a day with an average temperature of 60 will accrue 5 HDDs. If there were 30 days like that in a row, that month would accrue 150 HDDs.
In terms of energy efficiency, HDDs are useful for comparing the energy efficiency of a building over time, or the energy efficiency of several buildings in different climates.
Comparing Heating Energy Consumption
Because the outdoor temperature affects a building's heating energy consumption, it can be hard to determine the relative efficiency of two buildings in different climates, or of the same building over time.
For example, the same building will need to use more heating energy during a cold winter than it did during the previous, milder winter, given the change in outdoor temperature. Likewise, at any given time a building in Vermont is likely to need more heating energy than a building in Florida - even if the buildings are equally energy efficient. In both cases, energy consumption fluctuates due to temperature, which makes it difficult to assess the efficiency of one building over another, or of the same building over time.
This is where HDDs are useful. By isolating the variable of outside temperature, it becomes much clearer which buildings are operating efficiently (or inefficiently), regardless of the outside temperature.
For instance, say a family installs several efficiency measures one summer, but their subsequent winter heating bills stay the same as those of the previous year. By isolating the variable of average winter temperatures, which often fluctuate from year to year, HDDs can help assess the efficiency of the family's home before and after the efficiency measures were installed. So if the winter after their summer installations is colder than the previous one, their heating needs will be greater - yet the new efficiency measures will help the family maintain a comfortable home while paying much less than they otherwise would have.
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