Degree Day

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City of Ames

Heating and Cooling Degree Days

Degree days is a measure of how much heating or cooling is required in a given season. It's how much (in degrees) and for how long (in days) heating or cooling was required to maintain a certain temperature. The standard is to use 65 degrees F as the neutral temperature that does not require heating nor cooling.

This is important if you are comparing your energy consumption in two different time periods to determine the efficiency of your heating or cooling equipment, insulation, etc. Let's say you add insulation to your attic. You want to compare your energy bills from last winter (with the extra insulation) to the previous winter (before you added insulation) to see if the insulation has saved you money on heating. If one winter was significantly colder than the other, that won't be a fair comparison. You won't know if the difference in heating costs was due to the insulation or the weather. The same idea applies to cooling in the summer.

 

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 summed to compare months and seasons. The degree day chart below shows monthly totals since 2009, along with the monthly and yearly averages.  Averages have been recorded by Electric Services for over 30 years.   

How does this winter compare to previous years, in terms of heating requirements?

(We update this information at least once a season (quarterly). You can go to degreedays.net for up-to-date information at any time.)

Heating 19Heating Chart 19

 How did last summer compare to previous years in terms of cooling needs?

Cooling 19Cooling Chart 19

 

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.

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