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. Thus, degree-day measurements can be used to describe the effect of outdoor temperature on the amount of 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 degrees 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 degrees 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.

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.

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. For example, the degree-day table below shows that, in the East North Central States, January 2007 was colder than January 2006 (with both years warmer than the normal of 1,302 heating degree-days). In January 2007, 1,112 heating degree-days were recorded, up from 901 degree-days in January 2006. On the other hand, the Mountain States were colder in January 2007 (1,049 heating degree-days) than in January 2006 (801 heating degree-days). The data show that, on average, the United States was warmer in January 2006 than in January 2007 (and warmer than normal in both 2006 and 2007).

Heating Degree-Days by Census Division
	January 1 through January 31					Cumulative July 1 through January 31
				Percent Change					Percent Change
Census Division	Normala	2006	2007	Normal to 2007	2006 to 2007	Normala	2006	2007	Normal to 2007	2006 to 2007
New England	1,246	993	1,106	-11	11	3,708	3,290	3,227	-13	-2
Middle Atlantic	1,158	874	980	-15	12	3,349	2,904	2,779	-17	-4
East North Central	1,302	901	1,112	(s)	23	3,774	3,291	3,378	-10	3
West North Central	1,390	945	1,277	-8	35	4,085	3,441	3,739	-8	9
South Atlantic	643	464	509	-21	10	1,726	1,541	1,477	-14	-4
East South Central	820	551	705	-14	28	2,230	1,977	2,047	-8	4
West South Central	593	348	630	6	81	1,498	1,217	1,451	-3	19
Mountain	951	801	1,049	10	31	3,098	2,667	3,084	(s)	16
Pacific (b)	564	507	626	11	23	1,817	1,557	1,749	-4	12
U.S. (b) Average	917	674	835	-9	24	2,656	2,301	2,390	-10	4
(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

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 EIA Monthly Energy Review.