What are degree days?
A degree day is a single number that quantifies how hot or cold the weather has been for a certain region over a month (or week). The colder the weather in a given month the higher the degree day value. This number helps us account properly for the effect of weather on energy consumption (for example – it allows you to take account of the weather when you compare your energy usage from one year to the next).
Using degree days for energy management
Four valuable uses of degree days in energy management are:
- Constructing control charts that compare benchmarked and actual energy usage (the closer they are, the greater your energy efficiency)
- Evaluating success of savings measures
- Detecting faults in heating systems and their control
- Preparing annual budgets
For more information see our publication:
Degree days for energy management (CTG004).
This publication provides an introduction to degree days and shows how simple analysis can be a powerful tool in understanding weather-related energy consumption.
Degree days: technical details
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 = 16
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.
Download degree day data
The Carbon Trust no longer supplies this data, but it is freely available from:
University of Oxford Environmental Change Institute (ECI): Degree day downloads (opens in new window).
Download historical degree day data:
Degree days historical data to June 2011 (CTD012)
For further information
Call our advice line on 0800 085 2005 or read more about energy metering and monitoring.