If the outside temperature is 30 degrees F, and your thermostat inside is set to 68 F, than how much work did you heating system do and how much did you just pay for that days heat?
A “Degree Day” (DD) is the term used in energy performance measurements to describe the number of degrees your heating or cooling system had to work against in order to warm or cool your house.
In the example above, the outside temperature is 30F (on average) for the day, and the inside is 68F, so the DD for that day is simply 68-30=38. When the outside temperature is 60F and the inside is 68F, the DD for that day is only 8 and much less energy would being used by your heating system.
Sounds simple, right?
Well, as I read more and more into the pitfalls, assumptions, discrepancies, and calculations, it quickly moves into the engineering level of ‘complex’. To get some of the details follow this link: DegreeDays, Handle with Care!
But the concept is important — because if you have the heating bills in front of you from last year and this year and try to compare the costs, you realize that you can’t make a direct comparison because last year it might have been colder over all than this year.
But, if you made a significant change in your heating system or you plugged up the holes or added insulation, you really want to know if it made a difference. You need to be able to justify paying for improvements and the best way to do that is to run some numbers.
That’s why the ‘Degree Day’ was invented. If you know that there were 6232 degree days last year and only 6002 this year (a warmer year), then you can ‘normalize’ the data, which then lets you compare costs or gallons of oil over two different time periods. You would be able to say “if I had exactly the same winter as last year, I will save $435 on my heating bill”.
I am working on a case study right now where the homeowner has provided me with his data for gallons of oil purchased each month over the last few years and wants to know if the improvements he added in insulating the basement have made a difference.
I will post the data and details over the next few days in a Case Study.
In the meantime, if you have a set of data associated with your heating bill and would like some help comparing costs from year to year, please send it to me and I’ll give it a try. Posting some case studies (anonymously, if you like) might help inspire others to plug up some holes or change out that ancient oil burner for something much more cost efficient!
Also, the easiest way to get the Degree Day data for your area is from this website: DegreeDay.net
Your local gas or oil company might have Degree Day information as well.