Comparing DD usage over two different years.

Jock and Dan

Jock is the number cruncher and expects to save money.  Dan is the facilities manager who arranged for insulation to be installed.  Unfortunately, the number of kWh didn't go down by much after the installation was installed.  Dan needs to prove that the insulation is useful, so he uses DD...

Pre insulation

2013   452976kWh used       3320 DD

Post insulation

2014   445241kWh used       4092 DD

452976 : 3320 = 136 kWh per DD

445241 : 4092 = 109 kWh per DD

(136-109):136 x 100 = 19.85%

This means that post insulation, there is 19.85% more heating efficiency.

How many types of degree days are there?

There are three main types of degree days: heating degree days, cooling degree days, and growing degree days.  If you can understand exactly how one type of degree days work, it's very easy to understand the others.

Why are heating degree days useful?

The energy consumption of building heating systems is more complicated than the energy consumption of TVs, kettles, or computers. 

Essentially, the colder the outside air temperature, the more energy it takes to heat a building.

Some places are colder than other places (Caribbean vs North Pole), and some times are colder than other times (day vs night/summer vs winter).

The amount of energy needed to maintain a building at a given temperature will probably fluctuate.

Heating degree days are a simple way to quantify all of this!!!

The idea is that the amount of energy needed to heat a building in any day/week/month/year is directly proportional to the number of heating degree days in that day/week/month/year.

It is important to note that a heating degree day is not a physical day such as Monday, Tuesday etc.  It is simply a unit used for this measurement.

No matter which method you use, you must always begin with base temperature.

The base temperature is the temperature below which the building needs heating.

Example of calculation if base temperature is 17C:

               tao                                      DD

1st July    16      1 deg x 1 day              1

2nd July   15      2 deg x 1 day              2

3rd July    17     0 deg x 1 day              0

4th July    19     0 deg x 1 day              0

5th July 

00.00 - 12.00 

                15     2 deg x 0.5 day           1

12.00 - 24.00

                16     1 deg x 0.5 day         0.5

DD are actually calculated half-hourly. If the temp was 15C for half an hour, the calculation would be 2 x 1/48th day and the other sets of half an hour would be added.

Dan tries to keep the office building heated to around 20C - after many years on the job he has determined that this is the temperature at which he gets the least number of people complaining that it's too hot or too cold.

On a summer day, when the outside temperature is 20C or above, as you can probably guess, Dan switches the heating off - there's no point in heating a building when it's already warmer than the temperature you want it.

In fact, Dan has figured out that he can switch the heating off when the outside temperature reaches 17C - a few degrees below the desired inside temperature.  The office has a lot of warm people in it and a lot of warm office equipment too - this essentially provides a few degrees of free heating.  In technical terms, this would be described as an average internal heat gain of 3C.

So, when the outside temperature is below 17C, the heating needs to be on, and when the outside temperature is above 17C, Dan can switch the heating off without incurring any more complaints than usual about it being too cold.  Of course more people might complain that it's too hot, but that's a different story.

What this means is that the base temperature of Dan's building is 17C.

All buildings have a base temperature - it varies from building to building, but you can think of it as depending on two things:

1. What temperature is the building heated to?  (e.g. Dan's building is heated to 20C).
2. How much free heating comes from the people and equipment inside the building?  In other words, what's the average internal heat gain?

The base temperature of your building will determine the base temperature of the heating degree days that you should use to do your calculations.