Ames Electric Department

Heating and Cooling Degree Days

Degree days in Ames are based on the assumption that when the outside temperature is 65°F, we don't need heating or cooling to be comfortable. Degree days are the difference between the daily temperature mean, (high temperature plus low temperature divided by two) and 65°F. If the temperature mean is above 65°F, we cool it to 65°F by subtracting 65 from the mean. The result is Cooling Degree Days. If the temperature mean is below 65°F, we heat it adding degrees back to reach 65. The result is Heating Degree Days.

Example 1: The high temperature for a particular day was 90°F and the low temperature was 66°F. The temperature mean for that day was:

( 90°F + 66°F ) / 2 = 78°F

Because the result is above 65°F:

78°F - 65°F = 13 Cooling Degree Days

Example 2: The high temperature for a particular day was 33°F and the low temperature was 25°F. The temperature mean for that day was:

( 33°F + 25°F ) / 2 = 29°F

Because the result is below 65°F:

65°F - 29°F = 36 Heating Degree Days

Calculations like those shown in the two examples are performed for each day of the year and the daily degree days are accumulated so we can compare months and seasons. The degree day chart below shows monthly totals since 1994, along with the monthly and yearly average. Averages have been recorded by Electric Services for over 30 years.

Heating Degree Day Totals For Ames, Iowa. (65°F Base)

Updated: 5/5/08

	1996	1997	1998	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	Average
January	1508	1516	1228	1458	1328	1320	1030	1342	1402	1349	948	1298	1421	1352
February	1231	1088	824	874	899	1309	934	1185	1162	883	1061	1322	1322	1100
March	1048	850	995	781	671	1060	976	868	677	803	816	655	937	859
April	535	577	418	448	433	342	449	385	309	271	265	497	498	453
May	257	302	59	165	106	160	199	145	110	167	138	55		150
June	22	0	87	38	32	37	4	17	12	0	11	0		21
July	0	3	0	0	6	0	0	0	0	0	0	0		2
August	0	3	0	2	10	2	4	0	17	0	1	0		8
September	132	50	40	147	107	112	57	95	19	35	108	68		106
October	350	362	359	411	293	431	537	333	313	363	486	260		394
November	1049	896	701	598	973	451	842	817	644	705	705	780		822
December	1396	1098	1090	1151	1730	1012	1042	1083	1077	1344	950	1311		1241
Total:	7528	6745	5801	6073	6588	6236	6074	6270	5742	5920	5489	6246		6503

Cooling Degree Day Totals For Ames, Iowa. (65°F Base)

Updated: 5/5/08

	1996	1997	1998	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	Average
January	0	0	0	0	0	0	0	0	0	0	0	0	0	0
February	0	0	0	0	0	0	0	0	0	0	0	0	0	0
March	0	0	0	0	0	0	0	0	2	0	0	9	0	0
April	2	0	0	0	0	19	35	27	26	22	19	22	2	13
May	59	19	84	30	71	60	63	30	116	43	96	115		66
June	241	250	156	161	126	215	309	191	204	308	269	251		222
July	275	361	303	407	237	372	437	333	306	392	412	374		338
August	250	218	274	178	255	285	258	350	183	282	292	374		267
September	89	108	154	50	124	45	148	122	184	178	43	146		107
October	10	79	1	4	12	10	8	28	6	41	28	43		15
November	0	0	0	0	0	0	0	0	1	0	1	0		0
December	0	0	0	0	0	0	0	0	0	0	0	0		0
Total:	926	1035	972	830	825	1006	1258	1081	1028	1266	1160	1334		1028

HOW TO USE DEGREE DAYS: The most common use of degree days is for tracking energy use. Without degree days, comparing the energy used over two periods would be analogous to calculating the miles per gallon rating for your car without knowing how far you had driven. If you wanted to know if the attic insulation you added over the summer was saving energy, you would use your energy bills to determine how much "fuel" was used before and after the retrofit. Then, using the degree days, you could determine "how far you went" during those periods. Instead of calculating miles per gallon, you would determine kilowatt hours (kWh's) per degree day or therms of natural gas per degree day. This type of analysis can give you a rough idea of the impact weather had on your energy bills.

OTHER FACTORS: When comparing energy use, you may get a better idea of actual heating and cooling costs if you account for other energy sources in your home. Isolating heating and cooling energy can be accomplished by examining the energy used during temperate months, such as May and October, when little heating or cooling energy is used. The energy used during these periods reflect your base monthly consumption. Subtracting the base use from the total consumption during a winter month will yield an estimate of the energy used just for heating. Subtracting the base use from a summer month will provide an estimate of cooling energy. It is also important to consider the usage period reflected in your energy bill. Your meter is probably not read on the first day of each month and therefore will not be for the same time period as the degree day totals. You can allow for this by comparing your energy bill over a longer period, such as an entire heating season or several months.