The average temperature can be somewhat helpful in estimating the energy you’ll use to heat or cool your home during a month. But has this ever happened to you: a billing cycle arrives where the average temperature is about the same as before, but your electric bill is noticeably higher? If all other energy use remains average for your home, this situation might arise because the second cycle has more heating or cooling degree days. Read more to find out what degree days are, how to calculate them, and what they can mean for your energy bill.

What is a Degree Day?

Degree days measure how much heating or cooling is needed to keep a comfortable indoor temperature. They help measure how much energy gets used to maintain the temperature of a building. Degree days are not a perfect measurement. How well a building is insulated or how it is used and occupied impacts the energy needed to heat or cool it, too. As you’ll learn, degree days are still better for estimating heating and cooling use than average temperature.

How are Degree Days Calculated?

In the U.S., degree days are calculated by taking the difference between 65F and the mean daily temperature (average of the high and low temperatures for the day). On days where the mean is above 65F, the difference between the two is measured in cooling-degree days. When the mean is below 65F, it’s measured in heating-degree days. If the mean temperature is 75F, there are 10 cooling-degree days. A daily mean of 50F gives you 15 heating degree days.

How This Impacts Your Energy Usage

For example, a mean daily temperature can be 65F, but the highest was 80F, and the lowest was 50F. These temperature changes reflect a higher need for heating and cooling to keep indoor temperatures stable than if the high for the day was 70F and the low was 60F. Both days have a mean temperature of 65F, but the first day has 15 cooling-degree days and 10 heating-degree days. That’s a total of 25-degree days. The second example day only has a total of 10 degree days. Degree days better reflect the level of temperature variance, and that more accurately shows how much energy will be needed to keep your home the same temperature no matter what’s going on outdoors. 

Like the average daily temperature, the average monthly temperature can be the same whether there are some days with extreme temperatures or just many days where the temperature stays close to the average. However, degree days for a month give a more accurate idea of the energy used to heat or cool your home.

The number of degree days in a month is calculated by measuring the degree days for each day and then adding them all together. We’ve seen how it is possible to have two days with the same average temperature but a very different number of degree days. The same is true for a month. If you are interested in finding out the degree days in a month, there are online tools you can use to skip all the calculations and find the information. It will help you better understand the energy you use to heat or cool your home.

Energy-saving advice and good home insulation can help minimize the impact of degree days on your heating and cooling costs. Regardless, even the best-prepared home will have fluctuations in heating and cooling costs for some months. If the average temperature was the same during two billing cycles, but one of them results in a higher bill, take a look at the degree days. You might find the culprit.