[EnergyPlus_Support] Proper use of R-squared evaluation for energy use

[Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Dear Forum,

I don’t recall if this has been a topic in the past � my apologgies if it has! (Just point me to a past post)

When creating an energy model for an existing building that has historical (metered) consumption data, I typically calculate an R² value for the goodness of fit between actual and predicted energy.  I do this for electricity use and also for natural gas / fossil fuel use.

That is fairly straightforward (or this statistics-challenged engineer wouldn’t calculate it at all!)   

Here’s the question:

·         There are normally 12 data pairs for electricity and 12 for gas

·         Assuming that I have made the dates coincident for electricity and gas,

·         and also use consistent units (e.g., kWh for both)

·         If I want the overall R² value (composite of all energy), should I use 24 data pairs in the calculation or 12?

 

This question arises because I am looking at a model which, when using 24 data pairs, shows:

·         R² = 84% for electricity,

·         R² = 61% for natural gas,

·         and R² = 57% overall. 

 

Note that the overall value is less than either of its components.  If I use 12 data pairs, the overall R² = 69%.  Intuitively, 69% makes more sense because it falls between the other two values, so I am inclined toward use of 12 data pairs.  I’m even more inclined, however, to make the calculation correctly!  So I’m reaching out for input, hoping that some of you are more knowledgeable in the area of statistics than I.  Which is more correct � 12 or 24 data pairs?  Thankks in advance.

 

 

 

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

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