You may have heard the term ‘degree day(s)’ and wondered what it means. The concept is actually pretty simple and easy to calculate.
A degree day is based on a baseline temperature of 65°F. The assumption is that 65 degrees is a comfortable temperature. Below that temperature heating is required to maintain that temperature. Above the baseline and cooling is required. By calculating heating degree days and cooling degree days, one can easily compare energy usage for heating and cooling, plan planting schedules for crops, or evaluate the energy performance of buildings.
Calculating degree days is simple. Add the day’s high and low temperatures together and divide by two to get the mean, then determine the difference between the mean and the 65° baseline.
So, if the day’s high is 86°F and the low is 50°F:
(86+50)/2=68; 68-65=3 degree days
The degree days in this example are cooling degree days, since cooling would be required to lower the mean temperature to the baseline.
If the day’s high is 33°F and the low is 15°F:
(33+15)/2=24; 65-24=41 degree days
Since heat would be needed to raise the mean temperature to the baseline, these are heating degree days.
For the month of June 2021 at my personal weather station here in Colorado, there were 116.4 heating degree days, and 167.9 cooling degree days. In other words, as one might expect there were more days above the 65° mean than below. Contrast that with February 2021 when there were 1,088 heating degree days and 0 cooling degree days.
To summarize, if the mean temperature is above 65° the results are cooling degree days; if the mean is below 65° the results are heating degree days.
While degree days are easy to calculate on your own, BizEE Software has a degree day calculator and information on how energy professionals use degree days on their website here.