Degree Days

Recently I was asked to describe the concept of Heating Degree Days (HDDs).  The idea of Heating Degree Days was develop principally to acquire an estimation of energy usage potential in a given area.   A base temperature is used to determine at what point heating will be required to maintain human comfort.  The temperature most often used in the United States is 65 degrees, yet other temperatures are and can be used meaning one should always verify the base number used in any HDD calculation.  Fortunately, 65 degrees can be assumed in the United States.

Using 65 degrees as a base, the determination of the number of HDDs calculated on any given day is determined by the following forumula:

Heating DD = 65 °F – ((Daily Max Temp + Daily Min Temp) / 2) 

In other words, take the average temperature for the day (High plus Low divided by 2) and subtract that from our base temperature of 65 degrees.  For an example.  If the high was 10° and the low was -10°, the average temperature that day was 0°.  Our base of 65° minus 0° is 65 HDDs accumulated that day.  With an average high around 15° this time of year and an average low near 0°, our average temperature would be near 8°, meaning we tend to accumulated approximately 55 to 60 HDDs a day in the core of winter.

Although most of our furnaces were used in October, below is a map of the accumulated HDDs since November 1, 2014 through yesterday, January 20, 2015 to give an estimate of the number of calculated HDDs for our current heating season as November is usually the permanent start of our furnaces needing to be used on a daily basis.  During the past 2.5 months most of North Dakota into northwestern Minnesota have accumulated around 4000 HDDs.  This is fairly close to average for that period, meaning considering how cold last year was, most homes, all things being equal, should have had lower heating bills to this point in the season.  As a quick example, last winter, from November 1 through January 20, Fargo Moorhead recorded about 4200 HDDs, this year (graphic below), we have accumulated  3755 HDDs.  Therefore your heating bill through that period would be approximately 12% less than it was one year ago, all things being equal (fuel costs, solar into house, wind).

HDDs across the North Dakota Agricultural Weather (NDAWN) stations since November 1, 2014
HDDs (Base 65°) across the North Dakota Agricultural Weather (NDAWN) stations since November 1, 2014



On an annual basis,  North Dakota and northern Minnesota average between 8000 and 10000  HDDs per season.  Outside of high elevations in the Rocky Mountains, this area records more HDDs than any other area in the lower 48 states.

Yearly Mean HDDs in the northern Plains
Yearly Mean HDDs in the northern Plains

How can these data be useful?  For example, Chicago averages around 6500 HDDs per season.  If we use an average of this area of 9000 HDDs, an exact same home in Chicago would about 30% less energy to heat than locally.  There are of course other factors that would need to be taken into account, like wind and solar, that would alter these figures but it does give an approximation of differences in energy use potential.  These data in turn can be used to estimate energy usage over a given area based on number of homes, historical usage of energy to give energy suppliers an idea of how much electricity, oil and gas that will be needed annually.  Plus, energy consumption can be estimated based on projected temperatures for a given period of time associated with both short and long-term forecasting.

A similar methodology is used for potential energy usage in cooling structures in the summer, Cooling Degree Days (CDDs),  by using this formula:

Cooling DD = ((Daily Max Temp + Daily Min Temp) / 2) – 65 °F

Cooling Degree Days tend not to be quite as accurate for energy consumption as HDDs are, but it is still useful.

Both HDDs and CDDs can be calculated for any of the North Dakota Agricultural Weather Stations, or mapped by heading to this website:


Daryl Ritchison