Degree Days

One November could be twice as cold as another, and fuel demand for space heating can vary by a factor of two between corresponding months. The number of degree days is a measure of how cold it has been: the larger the number, the colder.

Degree days are generally calculated on a 24-hour basis, but a cumulation of, say, a week's measurements would indicate the heat demand of that week. Figure 1 illustrates that the number of degree days depends on the season and the location.

Fig. 1. Degree day recordings (degreedays.net). Every week of the year, from the start of the heating season in October 2008, Mytilini (Greece) had a lower number of degree days than Tirstrup (Denmark). The summer period with low numbers is longer in Mytilini, and it starts about two months earlier.


The number of Heating Degree Days (HDD) indicates the amount of energy needed to heat a building, and the number of Cooling Degree Days (CDD) indicates the amount of energy needed to cool a building. They are defined relative to an outdoor base temperature, beyond which the building does not need heating / cooling.

The Danish Meteorological Institute uses 17 C (62.6 F) as the base temperature for the HDD. They reason that electrical equipment and radiation from human beings raise the indoor temperature by 3 C, and also no space heating is required beyond 17 + 3 = 20 C. The textbook definition is the following,

HDD = 17 - m {for m < 17, otherwise 0}

The number m is the mean value (average) of temperature measurements in degrees celsius over a 24-hour period.

Example. Heating degree days

1) The mean value of 24 hour-by-hour measurements is m = 15, thus

      HDD = 17 – m = 17 – 15 = 2

That is, two degree days.

2) The mean value of 24 hour-by-hour measurements is m = 18, thus

      HDD = 0

That is, zero degree days.

The number of degree days indicates the demand for space heating. For example, a supplier of fuel oil for domestic furnaces can estimate when a customer needs a refill, by watching the number of degree days.

Some years are colder than others, and Figure 2 shows a variation of almost +/- 20 percent in the number of degree days per year.

Given the number of degree days, one can estimate the fuel consumption and discover if the actual fuel consumption is unusually large. A standard year in Denmark is 2 906 degree days, and it is thus a matter of scaling to find the standard heat demand of the house.

The Danish Meteorological Institute publishes every month degree day measurements from about 35 different stations in the country. There is none at Samsø, but Røsnæs fyr is probably the one with weather conditions most similar to Samsø.

Fig. 2. Degree days per year at Rosnas, Denmark (dmi.dk). The heating period 2006-2007 was warmer than the surrounding years in Rosnas (Denmark) and households saved energy accordingly. A standard year in Denmark (average of many years) is 2906 degree days.

Created by jj. Last Modification: Saturday 23 November 2013 12:49:58 CET by jj.