What do heating degree-days tell us?
Each degree that the mean temperature is below 65 degrees F is one heating degree-day. So if today’s average temperature is 55 degrees F, the day accounts for 10 heating degree-days. Engineers determined that when the mean outdoor temperature drops below 65 degrees F, most buildings require heating to maintain 70 degrees F indoors.
The amount of heat required to maintain a building’s temperature is proportional to the accumulated heating degree-days. Heating degree-days are intended to allow weather data to be used to estimate fuel needs. Fuel distributors use this index to schedule home deliveries, and electric and natural gas utilities use it to predict power demands.
Heating degree-day totals are usually reported each day, as well as for the season, which allows us to quickly judge whether the season is above, below or near normal.
Over a year, northern Wisconsin has about 9000 heating degree-days, while southern Wisconsin has about 7000. This tells us that the annual heating requirements are about seven-ninths as large in southern Wisconsin as in northern Wisconsin.