Heating degree days (HDD) are a formula that actually represents how hard you may expect your furnace to work that day. The greater the number of heating degree days, the more the furnace must work in order to keep the inside environment comfortable. The formula is very simple both for heating degree days and cooling degree days. However, with cooling degree days, the formula is reversed.
The way to find out the number of heating degree days is to take the high temperature for the day and add it to the low temperature for the day. Once that is done, then divide by two. For example, if the high one day is 50 degrees Fahrenheit (10 Celsius), and the low is 32 Fahrenheit (0 Celsius), the average temperature would be 41 Fahrenheit (5 Celsius). This average Fahrenheit temperature would then be subtracted from 65 Fahrenheit (18 Celsius). This temperature was chosen as a comfortable temperature at which additional heating is no longer needed. The result would be 24 heating degree days for that particular day.
Of course, it is impossible to say how this would react for any one individual furnace. All furnaces operate at different levels of efficiency and some may be able to work less and consume less energy when dealing with the same heating degree days as another furnace. After a while, most people will learn what to expect from their furnace at different temperature levels.
However, in addition to the heating degree day number, other things may also effect how much energy a furnace uses. This may include what the thermostat is set at and how long it is set at a certain temperature. For example, if the home is vacated a significant portion of the day, the thermostat may be turned down. The amount and quality of the home's insulation is another factor that will make a big difference.
In general, while heating degree days may not help anyone mass predict what should be expected from most furnaces, it will help individuals predict what to expect from their own furnaces. Of course, individuals should also be able to do this based on looking at both the high and low temperatures. In that respect, measurement of heating degree days may not truly be much of a help. However, it does provide one additional aid that people can turn to in order to understand why their bill is what it is. Further, for those who want to predict an energy bill for a month, it can be an easier measure to use rather than trying to figure it out based on actual temperature.