How to Calculate for Degree Days
A "degree day" is a unit of measure based upon measuring temperature difference and time, which is used to estimate fuel consumption, and to specify the nominal heating and cooling loads of a building. Below, we will detail how to calculate both Heating degree days and Cooling degree days.
Keep these rules in mind when calculating Heating degree days and Cooling degree days:
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The baseline temperate is always 65 degrees Fahrenheit.
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When the mean temperature is less than 65 degrees Fahrenheit, you will calculate the Heating degree days. These are equal to the total number of hours that the temperature is less than 65 degrees Fahrenheit for an entire year.
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If the mean temperature is more than 65 degrees Fahrenheit, you will calculate the Cooling degree days. These are equal to the total number of hours that temperature is more than 65 degrees Fahrenheit for an entire year.
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If the temperature is always 65 degrees, then zero energy is expended to heat or cool an environment.
The formulas you will use:
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Heating Degree Days = [the baseline temp (65)] - [the day's average temp ((high + low) / 2)]
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Cooling Degree Days = [the day's average temp ((high + low) / 2)] - [the baseline temp (65)]
To calculate the degree days, based on a given day:
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Find the day's high temperature (ex. 60).
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Find the day's low temperatures (ex. 40).
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Add the day's high and low temperature (ex. 100).
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Divide the value from step 3 by two to get the day's average temperature (ex. 50).
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If the average is less than 65 degrees, subtract the average (ex. 50) from the baseline temperature (65) to find the number of Heating degree days. Using our example numbers, this would equal 15.
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If the average is greater than 65 degrees, subtract 65 from the average to find the number of Cooling degree days.
