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 1997, along with the monthly and yearly average. Averages have been recorded by Electric Services for over 30 years.
How does this winter compare to previous years, in terms of heating requirements?
How did last summer compare to previous years in terms of cooling needs?
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.