Early in the 20th century, engineers developed the concept of heating degree-days as a useful index of heating fuel requirements. They found that when the outdoor daily mean temperature falls below a certain baseline, most buildings require heating to maintain a comfortable indoor temperature level.
Daily mean temperature can be calculated in several ways. At its simplest, it is the sum of the daily maximum and minimum temperatures, which is then divided by 2. However, when data is available, it may instead be determined as the mean of many temperature readings taken at regular intervals throughout the day, such as every hour.
In the United States, the baseline outdoor temperature usually is set at 65 degrees, with the goal of maintaining an indoor temperature of 70 degrees. Each degree of mean temperature below that baseline is counted as 1 heating degree-day. The greater the number of heating degree-days, the more fuel is needed to maintain the desired indoor temperature. For example, using a base temperature of 65 degrees, a day with a daily mean temperature of 35 degrees would be rated as 30 heating degree-days and would require twice as much fuel to maintain a 70-degree indoor temperature as a day with a mean temperature of 50 degrees (15 heating degree-days). Conversely, “cooling degree-days” are used to determine air-conditioning requirements.