We all know when it’s hot or cold, objectively speaking. And it doesn’t take a rocket scientist to figure out that a scorcher of a day requires air conditioning and a deep freeze means you’ll turn on the heat. While you might know you’re using heat or AC on a particular day, it can be difficult to determine how much energy you’ll need to use to power those systems. Enter heating degree days and cooling degree days.
Degree days aren’t technically “days” at all. They're a unit of measurement that's been adopted by national organizations such as ASHRAE as the industry standard for weather models.
So what exactly is a heating degree day? Heating and cooling degree days (HDD and CDD) essentially indicate how hot (or cold) it is outside for a given day and for how long it was at that temperature. This can be more helpful than knowing the temperature alone for estimating how much energy you used on heating and air conditioning. National Grid notes that studying degree-day patterns can help energy managers and others evaluate the increases or decreases in heating bills from year to year.
How to Calculate Heating and Cooling Degree Days
1. Determine the base temperature—the outside temperature at which your building needs no air conditioning or heating—for your building.
2. Obtain the average outdoor temperature for a day by taking the day's high and low temperatures.
3a. If the average temperature is exactly the base temperature for your building (usually 65 degrees Farenheit, but varies building to building), there are neither heating nor cooling degree days for that day.
3b. If it's above 65 (base temperature), subtract 65 from it to find the number of cooling degree days.
3c. If it's below 65, subtract it from 65 to find the number of heating degree days.
For example, if a day's high temperature was 60 degrees Fahrenheit and its low was 40 degrees, then the day's average temperature was 50 degrees. Subtracting 50 from 65 equals 15, which indicates that there were 15 heating degree days for that day.
The National Weather Service Forecast Office provides an interesting snapshot of average heating and cooling degree days in various U.S. locations (how’s that for fun cocktail party trivia?). Here are two of a longer list:
Bismarck, North Dakota
Heating degree days 8,932
Cooling degree days 499
Heating degree days 0
Cooling degree days 3,134