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What are degree days?

When you use fuel or electricity for space heating, demand will tend to vary according to how cold the weather is. This of course is what makes the consumption seasonally variable to such an extent; and the erratic nature of the weather, even averaged over a month, is what makes fuel consumption so hard to manage. In the UK, for example, it is possible for one November to be twice as cold as another, and hence for heating fuel demand to vary by a factor of two between corresponding months. But can we really say that one month is 'twice as cold' as another? Does it mean anything?

Yes, it does. When the outside air is above a certain temperature your building won't need heating. This is what we will call the 'base temperature'; in the UK it's common to assume a value of 15.5 Celsius, but this is just a historical convention. If the average outside air temperature on a given day is below this base temperature, you will need heat; and your heat requirement that day will be in proportion to the temperature deficit in degrees. Add up the daily temperature deficits over a month, and you get cumulative degree-days (degrees multiplied by days, do you see?). And these cumulative degree days are, by definition, proportional to cumulative heat requirements over the same period. Thus a month in which 360 degree-days are clocked up is 'twice as cold' as one with a total of 180 degree-days.

Don't get hung up on how degree days are actually calculated. It is a little more involved than I have implied, but the end result is the important thing: your space-heating demand is proportional to the regional degree-day value for the month in question.

Incidentally, you don't have to wait a month. You can accumulate degree-days over a week, (or any other interval); you can vary the base temperature to match the actual characteristics of a specific building; and if you have a cooling system you can reverse the arithmetic and look for excess temperatures above the base (no-cooling-needed) temperature.

The benefits of this are tremendous:

Other frequently-asked questions about degree days