Degree days are a fairly simple mathematical concept that was developed to assess the severity of the climate from one year to the next and from one region to another.
What are degree days used for?
- To forecast consumption.
- To measure the amount of heat required during a given period.
- To verify that energy-saving measures are effective.
- To check that a heating system is not operating abnormally.
- To compare buildings located in different climate zones.
- To compare heating years with each other, e.g. following investments.
In practice, this concept is particularly useful for professionals, especially those responsible for managing the heating of large buildings.
Measuring the amount of cold
Everyone knows that if winter is colder and longer, heating bills will be higher.
Everyone also knows that heating two identical buildings, one located in Uccle (Brussels) and the other in Elsenborn (Hautes Fagnes), will not cost the same.
In 2014, the average annual temperature in Uccle was 11.9°C and 8.3°C in Elsenborn.
Despite these precise figures, we still do not have an exact, quantified idea of how cold it was. We need to be able to assess both how cold it was and how long it was cold for. This is precisely what the concept of degree days does.
The 16.5/16.5 basis
In Belgium, the natural gas sector has established the basis at 16.5/16.5.
Given that the greater the difference between the outside temperature and the desired indoor temperature, the more energy the heating consumes, we begin by calculating this difference.
As soon as the average measured temperature falls below 16.5 °C, the gas sector expects us to turn on the heating.
Any temperature below 16.5 °C will in this case be recorded as degree days. So if today the average temperature measured in Uccle was 10 °C, we have 6.5 degree days. (16.5 - 10 = 6.5).
If, on the other hand, it is warmer, e.g. 20°C, it is not necessary to turn on the heating. Therefore, any average temperature above 16.5°C will have 0 degree days.
The industrial sector and large buildings sometimes take the 15/15 basis into account.
Calculation example
| Date | Uccle (average temperature) | Uccle (degree days) | Elsenborn (average temperature) | Elsenborn (degree days) |
|---|---|---|---|---|
| 1/1/2015 | 1°C | 15,5 Dd | - 3°C | 19,5 Dd |
| 1/1/2015 | 4°C | 12,5 Dd | - 2°C | 18,5 Dd |
| 1/1/2015 | 5,5°C | 11 Dd | 1 °C | 15,5 Dd |
| Total Dd | 39 Dd | 53,5 Dd |
You will notice that it was much colder in Elsemborn than in Uccle just by adding up the degree days.
If we do the same for the whole month and for the whole year, we obtain a number proportional to the building's heating requirements, the degree days for the location and the period in question.
Annual averages
Averages for these degree days have been established by the Royal Meteorological Institute over the last 30 years; these are known as “normal degree days”. They serve as a reference for defining the average severity of winter.
- The coldest year since 1980 was 1985 with 2,844 degree days.
- The warmest year was 2014 with 1,828 degree days.
What are degree days not used for?
Degree days are not useful for determining the amount of heating power required. Rather, it is the maximum possible cold in the region, combined with the degree of insulation and the size of the building, that determines the amount of heating power needed to provide sufficient heat during the coldest possible conditions.