The hum of the central heating unit during the winter months may be a soothing reassurance of domestic comfort. Sometimes, however, thoughts of the fuel bill may bring to mind the words of Robert Burns, "That sound awakes my woes, and pillows on the thorn my racked repose!" Heating is expensive, and its cost is closely related to outside temperature. In cold weather energy must be supplied to a building to replace heat lost to the outside world. But the quantity of fuel required to maintain a comfortable environment on a day with an outside temperature of, say, 7C may be only about half that needed when the temperature is close to zero. And if the temperature is even lower the boiler may work so hard to provide an amenable warmth that it scarcely stops at all. The concept of "heating degree days" has been found helpful for quantifying this relationship between temperature and fuel usage.
In this country 18C or thereabouts is reckoned to be the critical temperature for indoor comfort; if it drops below that, most people feel chilly and begin to think in terms of artificial heat. It so happens, moreover, that in average conditions the temperature inside an unheated building is some two or three degrees higher than that of the air outside. So the critical "outside" temperature is around 15C. At that figure it is probably unlikely that the central heating will be used, but the further it falls below that value, the more fuel is needed to stay comfortable, and maintain the indoor temperature at, or around, 18 degrees.
To understand what a "degree day" is, let us imagine three consecutive days with average outside air temperatures of 11, 8 and 3C respectively. The first day, with a temperature of 11 degrees (four below the critical value of 15) rates as four degree days; the temperature on the second day is eight degrees below the critical value, so day two rates as seven degree days; and an average temperature of 3C on day three gives it a rating of 12 degree days.
To get the three-day total we simply add, and get 4 + 7 + 12 = 23 degree days. In this way we can accumulate degree days for any period of days or weeks, or even for a whole heating season.
Energy companies have found that the number of heating degree days in a given period is closely related to energy demand for heating. Statistical information of this kind helps them to plan long-term gas, oil or electricity usage. And forecasts of the average temperature allow them to calculate degree days for several days ahead, and to anticipate with considerable accuracy the likely demand for the products they provide.