What are Degree Days?

Degree Days is the rate of heat loss from a building, (related to the building fabric) and the temperature difference between the inside and outside of a building - the greater the temperature difference the more heat will be lost. It is not possible to directly compare your building's performance from consumption data alone as weather conditions vary from month to month and year to year.

Heating degree days are a measure of the severity and duration of cold weather, the colder the weather in a given month the higher the degree day value. If you take these into account it is possible to compare one year with another and to determine whether any initiatives have resulted in energy savings or there are problems with your building that need to be addressed.

The base temperature used to calculate degree days in the UK is 15.5^oC

At this temperature most UK buildings can heat themselves without the need for supplementary heating, due to the internal gains from occupants and equipment and the solar gains through the building fabric, i.e. the walls and windows.
Degree days is a measure of the difference between the baseline and the actual outdoor temperature multiplied by the number of days. E.g. the temperature measured hourly records a temperature of:

7.5ºC for 48 hours the degree days total would be: (15.5 - 7.5) x 2 = 16

If the temperature is above the baseline then the degree days are set to zero.

Standard Degree Day Report for December 2008
Degree days are measured in 18 locations across the country, here is the data for last month: