Have you ever had the feeling that this winter (or summer) was milder or harsher than normal? With all the talk of climate change, global warming and extreme weather, I certainly have. Do you want to see how your energy use is doing compared to previous years, but realize that since last winter was so much milder (or harsher) that it wouldn’t be a fair compare the two? You can, but you need to account for the weather in your comparison. Fortunately there is a scientific way to take the weather into consideration and understand how your energy use really compares: Heating Degree Days (HDD) and Cooling Degree Days.
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The most understandable description of Heating Degree Days (HDD) I’ve found is from Wikipedia:
HDD provides a simple metric for quantifying the amount of heating that buildings in a particular location need over a certain period (e.g. a particular month or year).
You can read more on how to calculate HDD and CDD, but I find it much easier to look it up through the free service provided by Weather Data Depot. Input your location, the years you want to compare and change the balance point (only if you know what you are doing). Here’s an example HDD & CDD data for Medford comparing 2011 and 2012.
| Calendar Year 2011 | Calendar Year 2012 | |||||
| Month |
HDD
|
CDD
|
TDD
|
HDD
|
CDD
|
TDD
|
| January |
998
|
0
|
998
|
847
|
0
|
847
|
| February |
819
|
0
|
819
|
707
|
0
|
707
|
| March |
646
|
0
|
646
|
471
|
17
|
488
|
| April |
312
|
21
|
333
|
294
|
26
|
320
|
| May |
116
|
99
|
215
|
83
|
93
|
176
|
| June |
15
|
236
|
251
|
21
|
207
|
228
|
| July |
0
|
544
|
544
|
0
|
452
|
452
|
| August |
0
|
333
|
333
|
0
|
449
|
449
|
| September |
21
|
186
|
207
|
19
|
169
|
188
|
| October |
198
|
27
|
225
|
137
|
34
|
171
|
| November |
359
|
0
|
359
|
527
|
0
|
527
|
| December |
665
|
0
|
665
|
662
|
0
|
662
|
| Annual Total |
4149
|
1446
|
5595
|
3768
|
1447
|
5215
|
From a very quick look you can see that 2012 was a milder winter than 2011 and that the need for cooling between the two summers was pretty much the same. One caution is to make sure you are comparing the years you have in mind. I usually consider winter to be the time from October to April spanning the calendar year. Last winter was mild in New England, if you look back at the data for the winter of 2010-11 there were 4,228 HDD which is 19% higher than the 3,541 HDD for the 2011-12 winter months.
Heating and Cooling Degree Days are especially helpful when you are trying to see if your actions have made a noticeable difference in your energy consumption or if changes in consumption are actually reflecting the weather.
If you click on the Cooling Degree Days and Heating Degree Days tabs at the top of Weather Data Depot, you can get graphs showing the comparison of many years and even the option to select seasonal or annual.
Happy Greening!
Jon
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