National Weather Service United States Department of Commerce
Heating and Cooling Degree Days

Degree days 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 subtract 65 from the mean and the result is
Cooling Degree Days. If the temperature mean is below 65°F, we subtract the mean from 65 and 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.

The calculations shown in the two examples above are performed for each day of the year and the daily degree days are accumulated so that we can compare months and seasons.
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 kWh's per degree day or therms of natural gas per degree day.

When comparing energy use, you should also consider other energy uses that are not impacted by weather such as lights, appliances, etc. You can estimate the energy used for these purposes 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. 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 over a longer period, such as an entire heating season or several months.

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