Why are heating and cooling degree-days important to know?

The outside temperature affects how hard your air conditioner and furnace must work in order to maintain a comfortable interior temperature. The heating and cooling degree-day analysis is a way to understand why your electric usage fluctuates from month to month.

What are heating and cooling degree-days?

Heating and cooling degree-days are calculated by comparing the average temperature for the day to 65°, the temperature at which people are least likely to use their heaters or air conditioners.

So if the low temperature for the day was 40° and the high was 70° (40° + 70° = 110°), the average was 55° (110° / 2 = 55°). This average is 10 degrees colder than 65°, so this day had 10 heating degree-days. If the average was 75° (10 degrees warmer than 65°), there would be 10 cooling degree-days, because you would operate the air-conditioner to offset the warmer weather.

Factors such as the sun load, humidity, precipitation and wind also play a role in how much your heating and cooling system operate. Heating and cooling degree-days do not take these factors into consideration.

What is the source of the information?

The information provided from the links below comes from the National Weather Service Office in St. Louis, and it is updated monthly.