# What are Degree Days?

### What are Degree Days?

Degree days are a unit of measurement used to show the fluctuation of an external air temperature against a constant, user-defined base temperature.

An external air base-temperature is the temperature at which the internal gains are sufficient to sustain the desired internal temperature. Factors, such as occupants, equipment, building material, etc. will offset the losses from ventilation etc.

For example, in a typical UK office, the ‘ideal temperature’ will normally be around 21°C, but the base temperature of the building will be around 15.5°C. This means we assume the building will be heated by 5.5°C by factors other than external air temperature and heating. So to reach our target temperature of 21°C, we only need to heat the building to 15.5°C.

There are two types of degree days that can be calculated: heating degree days and cooling degree days.

Positive heating degree days are accumulated when the external air temperature falls below the set base temperature of the building (heating is required), and cooling degree days are accumulated when the external air temperature is above the cooling base temperature (cooling is required).

An easy way to think about what a degree day represents is that for one Heating degree day to be calculated, the external air temperature would have been averaged 1°C below the base temperature. If the base temperature is 15.5°C, and for the full 24 hours of the day the external air temperature is 14.5°C. There has been one heating degree day.

Let’s have a look at some basic calculations to get an even better understanding.

This first formula is taken from the Carbon Trust

(baseTemp – outsideAirTemp) X numberOfDaysAboveTemp

As mentioned above, 15.5 degrees is the typical base temperature for heating degree day calculations in the UK, therefore, we’ll use 15.5 as the base temperature for our calculation.

Let’s say that the outside air temperature was exactly 9 degrees for 3 days straight. Obviously, this is completely unrealistic but easy to understand for our example. Substituting these values into the Carbon Trust’s formula we get:

(15.5 – 9) X 3 = 19.5 degree days.

Using a static number of days as part of the calculation will always be unreliable, sometimes degree day calculations will use an average temperature to improve accuracy, but this will obviously still result in an extremely unreliable figure, as we can’t account for dramatic temperature fluctuations above and below the base temperature throughout the day.

With the right amount of data, degree days can be calculated to any degree of accuracy, if we had the external air temperature for each second of the day we could calculate our degree days down to the second! In our experience, the best balance of performance and accuracy comes from using hours.

This means for each hour of the day, we calculate the ‘degree day hour’, then add them together to give us an accurate degree day value. This method is used by our pubic Degree Days Calculator, located at https://degreedays.io.

### Degree Days Calculator

The Allander Analytics Degree Days Calculator is easy to use, and currently a completely free tool for calculating degree days. Created by Allander Analytics Ltd, our degree days calculator can be used to calculate both heating and cooling degree days for every day over a user-defined date range for any valid UK postcode.

Our Degree Days calculator can calculate degree day values for any date range, from 01/01/2015 up until the day before yesterday. The degree-day data is easily understandable and available to view in a dynamic chart as well as in a table that can be exported to a .csv for your own use. The top-level chart displays the total heating or cooling degree day values for each day in the range: By clicking on any point in the chart, you can drill down into your degree day data, where we will display a second chart which shows the degree day values broken down even further by hour, and compared against the average heating or cooling degree day values, the hourly values of the warmest day of the date range and the hourly values coldest day of the date range. If you want to see exactly how our values have been calculated for the individual day, or just export the hourly totals, you can view a calculation summary which shows a textual explanation of the calculation as well as a chart which shows both the heating or cooling degree day values, and where they have been calculated as the external air temperature has increased or decreased around the set base temperature.  Finally, if you have any problems using our degree days calculator, there is a helper popup that contains our FAQs as well as short tutorial videos on how to use certain aspects of the calculator. To access the help popup, click the blue question mark on the Degree Days Calculator page:

We hope that you find the calculator useful. If you want to find out more about the Degree Days Calculator, you can visit https://degreedays.io or visit our new Allander Analytics YouTube channel where we’ll be uploading tutorial videos on all of our products and services. Right now, you’ll find a helpful walkthrough on how to use the degree days calculator recorded by one of our software engineers, Ross.