Mark Dobson, Senior Energy Analyst

Your energy consumption can be directly affected by factors such as whether your building is occupied, the occupant’s behaviour, the temperature outside or production levels. Unless these factors are taken into consideration, your evaluation of energy performance could be misleading.

Possibly the most prevalent driving factor, and certainly the one that I get asked most about, is temperature. Unsurprising really – we love to talk about the weather. When analysing energy consumption that is affected by the outside temperature, we need a measure of the time when heating or cooling was required (and how much heating/cooling). Degree days can provide us with this information.

 

So why can’t we just use temperature?

To illustrate why temperature figures can sometimes be unhelpful, let’s ask the simple question; what was the temperature yesterday? We could use the average temperature yesterday or perhaps the temperature at a certain time of the day. However, this figure won’t portray any variations in the temperature throughout the day. An average temperature could be the same for two days with very different heating or cooling requirements. Day one has a cold morning (requiring heating) with a warm afternoon (requiring cooling) but these are averaged out showing the same average temperature as day two which is a consistently mild day needing no heating or cooling-related consumption.

This is why an understanding of degree days is vital when analysing the effect of external air temperature on your building’s energy consumption.

What is a Degree Day?

Degree days are a measure of the duration and magnitude of the difference between the external temperature and a base temperature. The base temperature is the external temperature at which additional heating or cooling is required to maintain a comfortable environment in your building.

The greater the difference between internal and external temperature, the quicker heat will be lost from your building. The longer the colder period lasts, the more heat will be lost. The colder your building gets, the more energy is required to keep it warm. A heating degree day is a measure of both the difference and duration of the cold outside.

For cooling, the opposite is the case. The longer and higher the temperature is above the cooling base level, the more cooling degree days and the greater the energy required by the cooling system.

Let’s look at a typical day’s external temperature. The graph below shows external temperature (black line) and the heating base temperature (blue line).

To calculate the degree days on this day, we sum the temperature difference for each 30-minute interval when the external temperature is below the heating base temperature. When the external temperature is equal to, or above the base temperature, the differences are treated as zero (there is no heating requirement). As there are 48 half hour periods in a day, this value is divided by 48 to convert our summed degree half-hours into the number of degree days for that day.

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Base Temperatures

As degree days are calculated using the difference between a base temperature and the outside temperature, an incorrect base temperature for your building could present a misleading picture.

There are lots of factors that affect the base temperature of your building and different buildings will have different heating and cooling base temperatures. Your base temperature will be affected by the temperature required inside, heat gains from equipment or the sun and the heat loss properties of the building itself. Whilst there are default base temperatures for heating and cooling, these are calculated using non-specific situations assuming internal temperatures, average heat gains and standard building fabric.

Of course, the other component in your degree day calculation is the external temperature; making it just as important to have accurate and local external temperature figures.

SavenergyOnline

We bring together and validate half hourly temperature data from over 90 sensors across the UK ensuring that you are provided with the most accurate information specific to your location.

This temperature data is the foundation of our degree day reporting toolkit designed to help you to analyse, monitor and target the efficiency of your sites.

SavenergyOnline our energy analytics platform, automatically selects the weather station nearest your site’s postcode to save you the task of setting your location across your portfolio. You can set your own base temperature – and if you are not sure what temperature to select then SavenergyOnline can calculate appropriate base temperatures for your sites based on your energy consumption data.

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Now we know what a degree day is and some of the important information to note, we have the tools to begin analysing our temperature-related consumption and making the most of the reports on SavenergyOnline.