Degree-Days Simplified

Degree-Days are funny fellows. Firstly there are two types Heating & Cooling Degree-Days. So to keep it simple let's just look at Heating Degree-Days...

As we saw previously, there is an outside temperature called the "Balance" when you don't quite need any heating. Think an early day in autumn (fall for our US brethren). It makes sense that it should be about the same as the temperature when you don't need any cooling either - so heating and cooling are in "balance".

As temperatures fall below this balance temperature heating is needed. The colder it gets the more heat you need .

Power "is related to"  How Cold it is.

Obviously the "is related to" is not an equals sign because on the right we seem to have just things to do with weather, and obviously two different buildings in the same weather may need quite different amounts of heating.

People who like spreadsheets would rather be more specific and this is possible.  First of all lets define how cold it is a bit better....

What does twice as cold as 5 C or 41 F mean.  Maybe we could relate it to how much heat we need.

Let's suppose a particular building uses heat twice as fast at 1C (33.8 F) as it does at 5 C (41 F)

If at 1C it uses  100kW
then at 5C it uses 50kW

because 100 is twice 50kW

We could guess that it should use no power are around 9 C

Why? -  because relative to 9C,  1C is twice as far away as 5 C  ( 8C= 2* 4C)

So for a building with a balance temperature of 9C twice as cold means twice as many degrees below 9C.

Degree-Day Calculation Example

We can define a heat demand as a power related to a  number of degrees below balance temperature. We can draw this on a chart relating degrees of heating demand to the resultant energy consumption....

Chart courtesy kWIQly

The line is approximately straight.  For each "degree-day" to the right the energy consumption moves up an equivalent amount.

Note that there are some "bad-fits" on the left (corresponding to summer or low demand) and on the right (corresponding to the plant working near its limit.

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