Degree Days – accounting for the weather

The weather – or rather, the external temperature – has a significant effect on energy use, particularly in buildings or for heating energy use.  It is important to consider these factors when assessing energy use or looking for areas of excess consumption.

Degree days help us to understand how hot or cold it’s been.  They aren’t technically “days” at all. They’re a unit of measurement that’s been adopted as the industry standard for weather models.

The base temperature used to calculate degree days in the UK is 15.5ºC, because at this temperature most UK buildings do not need supplementary heating.  Degree days is a measure of the difference between the baseline and the actual outdoor temperature multiplied by the number of days. For example – 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 (days) = 16 Degree Days

Note – it is not possible to have a negative degree day value as when the outdoor temperature exceeds 15.5ºC no heating is needed. When the outdoor temperature does exceed the 15. 5ºC baseline, then the degree days are set to zero.

Because the outside temperature varies according to which part of the country you’re in, the UK is split up into 18 degree day regions

UK degree day regions map

1. Thames Valley                 10. North Eastern

2. South Eastern                  11. East Pennines

3. Southern                          12. East Anglia

4. South Western                 13. West Scotland

5. Severn Valley                   14. East Scotland

6. Midland                            15. North East Scotland

7. West Pennines                 16. Wales

8. North Western                  17. Northern Ireland

9. Borders                            18. North West Scotland


To help understand energy consumption across our estate, we have developed a quick reference tool which shows the historical variance from average temperatures in each region.  For any given month of each year since 2000, you can use the tool to check whether or not the temperature was warmer or colder than average in each region.


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