9.3 Degree Days

What is a Degree Day?

Degree days are measures of how cold or warm a location is. Degree days compare the mean (the high temperature plus the low temperature divided by 2) outdoor temperature to a standard temperature; we use 65° Fahrenheit (F) in the United States.

How do you calculate a Degree Day?

The base temperature we use is 65°F. If the mean temperature is above 65°F, we subtract the base temperature (65°F) from the mean, and the result is Cooling Degree Days (CDD). If the mean temperature is below 65°F, we subtract the base temperature from the mean, and the result is Heating Degree Days (HDD).

A cooling degree day indicates a hot day and measures how much air conditioning we need to keep a building cool. A heating degree day indicates a cold day and measures how much heating we need to keep a building warm.

What do people use Degree Day data for?

Degree days are calculated for each day of the year, and we use the daily degree days to compare months and seasons. We often count CDDs and HDDs by census regions and divisions. Generally, people study degree-day patterns to assess the climate and the heating and cooling needs for different regions of the country. For example, the West North Central Census Division generally has the most HDDs, and the West South Central Census Division generally has the most CDDs each year.

Source: U.S. Energy Information Administration - Units and Calculators Explained 

Heating Degree Days Examples

Heating Degree Day Example 2 Solution

For this problem, we need to calculate HDD for one full week. The data that is given for each day is the average outside temperature. For example, Sunday, the average outside temperature is 49°F. Monday it’s 47°F, Tuesday it’s 51°F, Wednesday it’s 60°F, Thursday it’s 65°F, Friday it’s 67°F, and on Saturday it’s 58°F.

So we need to calculate heating degree days (HDD) for each day. To calculate heating degree days (HDD) for each day, we need to enter the T_base value of 65 degrees for each day. HDD = T_base minus the average outside temperature. 

Now you can calculate the HDD for each day. The HDD for Sunday is 65 degrees (the T_base) minus 49 degrees (the outside temperature), which equals 16. Monday is 65-47=18, Tuesday is 65-51=14, Wednesday is 65-60=5, Thursday is 65-65=0, Friday is 65-67=0 (Remember when the average outside temperature exceeds 65, the heating degree days would be zero because you do not need to turn the heat on), and Saturday is 65-58=7. 

Now you can add the degree days for the week.

So the total sum for one full week is 60 degree days.