Each degree that the mean temperature is below 65 degrees is one heating degree-day. So, if today’s average temperature is 55 degrees, that is 10 heating degree-days.
Engineers determined that when the mean outdoor temperature drops below 65 degrees, most buildings require heating to maintain an indoor temperature of 70 degrees. In an effort to make weather data easy to use in planning, they developed this definition to estimate fuel consumption needs. Fuel distributors often use this index to schedule home deliveries. Since energy consumption in homes derives primarily from natural gas and electricity, these utilities predict power demands by adding up this index over time.
The amount of heat required to maintain a building’s temperature is proportional to the accumulated heating degree-days. So, heating degree-day totals are usually reported each day, as well as a total for the season. This allows us to quickly judge whether the season is above, below or near normal.
Over a year, northern Wisconsin has about 9,000 heating degree-days, while southern Wisconsin has about 7,000 heating degree-days. Over a year the indoor heating requirements for southern Wisconsin are about 7/9ths that of northern Wisconsin.
This past January most of southern Wisconsin had temperatures that were 4 degrees above normal while December’s temperatures were more than 5 degrees above the mean. With those warm temperatures, Madison’s cumulative heating degree-days this winter is a few hundred below normal.