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How will degree days help you measure energy savings from heating and insulation improvements?

Posted by Alex Barrett on 14 November 2016 at 11:01 am

Suppose you are retrofitting your house, improving its insulation or installing a more energy efficient heating system. You will want to know how much energy you are saving, but with heating costs this can be tricky to figure out. This is where "degree days" can help.

Temperature, and thus heating demand, varies substantially over time. Some seasons are colder than others, and changing weather can mean that you have to spend much more on heating one day than you do the next. So when you compare your heating costs before and after the retrofit, how can you be sure that you are comparing like to like?

To make a fair comparison you need to know how many days you had to heat your house during each period, and by how much. You can then divide your energy use in each period by the heating demand, to get comparable numbers. This is often called “weather correction”.

Unfortunately measuring a house’s heating demand directly is rarely straightforward, particularly when you want to do it retrospectively. The answer lies in published “degree day” measurements. Weather stations all around the world are measuring the temperature every day, in order to forecast the weather and study how the climate is changing. This temperature data can be used to work out your heating demand over a certain period of time.

Whether you have to heat your house or not will depend on what the outside temperature is. Most heating systems aim to keep the house at a steady, comfortable temperature, usually between 18 oC - 21oC. Generally the air in a building is a few degrees warmer than the air outside. This is due to the effect of insulation and the heat generated by appliances and people. In the UK it is usually assumed that heating is needed when the temperature drops below 15.5 oC.

This is called the “baseline temperature”, the threshold below which we need to use heating and above which we don’t. For each degree that the outside temperature drops below the baseline temperature, additional energy will be needed for space heating.

The degree day is the difference between the baseline temperature and the actual outside temperature, multiplied by the number of days you are interested in.

For example if you had an average temperature of 12 oC for a week then the calculation looks like this:

(15.5oC -12 oC) x 7 days = 24.5 degree days. 

The following week is much warmer. It gets up to 14 degrees on three out of the seven days, and 15 degrees on the rest.

((15.5 - 15) x 4) + ((15.5 – 14) x 3) = 2 + 4.5 = 6.5 degree days.

The warmer week had far fewer degree days, so our heating will have used much less energy during this time.

You generally won’t need to work this out yourself, as degree day databases are available for a large number of weather stations. One website which provides this information can be found here. You can search for a nearby weather station and find out how many degree days were measured there during a given period. Some sources will base their estimates on daily average temperatures, others may make more frequent measurements throughout the day.

Once you have your data you can divide your heating energy use by the total degree days, to find out how much energy you were using per degree day.

Hopefully this will be much lower for the period after your new measures were installed, even if the weather meant that you used more energy overall during that time.

References

Degree day data can be found at:

  1. Degreedays.net
  2. UK government average degree days for 1961-1990

More information on degree days can be found at:

  1. Degreedays.net
  2. The Carbon Trust
  3. Energy Smart
  4. Energy Lens

Image Credit: Ben+Sam via Flickr

If you have a question about anything in the above blog, please ask it in the comments section below.

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