What is a degree-day?

Freezing winter weather or a long, sweltering summer--either one can increase your utility bills. But how much of the rise in the cost is a result of the weather? You can find out by using a unit of measure called the "degree-day." A degree-day compares the outdoor temperature to a standard of 65 degrees Fahrenheit (F); the more extreme the temperature, the higher the degree-day number and the more energy needed for space heating or cooling.

Hot days, which may require the use of energy for cooling, are measured in cooling degree-days. On a day with a mean temperature of 80^o F, for example, 15 cooling degree-days would be recorded. Cold days are measured in heating degree-days. For a day with a mean temperature of 40^o F, 25 heating degree-days would be recorded. Two such cold days would result in a total of 50 heating degree-days for the 2-day period.

How are degree-day data used?

By studying degree-day patterns in your area, you can evaluate the increases or decreases in your heating or air-conditioning bills from year to year. In some areas, degree-day information is published in the local newspapers, usually in the weather section. Information may also be available from your local utility. Its public relations department may be able to tell you the number of degree-days in the last billing period and how they compare to the number of degree-days in previous billing periods. You can obtain U.S. national heating degree-day totals for longer periods, and cooling degree-day totals, too (see Sources¹ below).

The Energy Information Administration (EIA) provides information about heating degree-days and cooling degree-days in the Monthly Energy Review. The heating degree-day table below lists the population-weighted degree-days that occur in each Census Division of the United States. It compares monthly and cumulative totals to similar totals for previous periods.

Is this year warmer or colder than last year?

For example, the degree-day table below shows that, in the East North Central States, January 2008 was colder than January 2007 (with both years warmer than the normal of 1,302 heating degree-days). In January 2008, 1,216 heating degree-days were recorded, up from 1,112 degree-days in January 2007.

On the other hand, the Mountain States were slightly warmer in January 2008 (1,011 heating degree-days) than in January 2007 (1,050 heating degree-days). The data show that, on average, the United States was colder in January 2008 than in January 2007, but warmer than normal in both 2007 and 2008.

Heating Degree-Days by Census Division
	January 1 through January 31					Cumulative July 1 through January 31
				Percent Change					Percent Change
Census Division	Normala	2007	2008	Normal to 2008	2007 to 2008	Normala	2007	2008	Normal to 2008	2007 to 2008
New England	1,246	1,106	1,130	-9	2	3,708	3,227	3,503	-6	9
Middle Atlantic	1,158	979	1,023	-12	4	3,349	2,778	2,963	-12	7
East North Central	1,302	1,112	1,216	-7	9	3,774	3,378	3,477	-8	3
West North Central	1,390	1,277	1,367	-2	7	4,085	3,740	3,900	-5	4
South Atlantic	643	509	609	-5	20	1,726	1,477	1,503	-13	2
East South Central	820	706	825	1	17	2,230	2,047	1,991	-11	-3
West South Central	593	630	582	-2	-8	1,498	1,451	1,365	-9	-6
Mountain	951	1,050	1,011	6	-4	3,098	3,084	2,940	-5	-5
Pacific (b)	564	626	627	11	(s)	1,817	1,749	1,849	2	6
U.S. (b) Average	917	835	883	-4	6	2,656	2,390	2,455	-8	3
(a) Normal is based on calculations of data from 1971 through 2000. (b) Excludes Alaska and Hawaii. (s)=Less than 0.5 percent and greater than -0.5 percent.

 

Census Divisions

(Census Divisions Map)

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New England
Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont

Middle Atlantic
New Jersey, New York, Pennsylvania

East North Central
Illinois, Indiana, Michigan, Ohio, Wisconsin

West North Central
Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota

South Atlantic
Delaware, District of Columbia, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, West Virginia

East South Central
Alabama, Kentucky, Mississippi, Tennessee

West South Central
Arkansas, Louisiana, Oklahoma, Texas

Mountain
Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming

Pacific
California, Oregon, Washington

1. Sources: There are several degree-day databases maintained by the National Oceanic and Atmospheric Administration (NOAA). The information published here is developed by the National Weather Service Climate Prediction Center, Camp Springs, MD. The data are available weekly with monthly summaries and are based on mean daily temperatures recorded at about 200 major weather stations around the country. NOAA has other sources of information about degree-days for the country as a whole. The data provided here are available sooner than the Historical Climatology Series 5-1 (heating degree-days) and 5-2 (cooling degree-days) developed by the National Climatic Data Center, Federal Building, Asheville, NC, 28801, which compiles data from some 8,000 weather stations.

More information on this subject can be found in the following EIA publications:
      
       Monthly Energy Review
       Annual Energy Review