Q: I'm working on a project where we're looking at an Option C for electricity at a manufacturing site where the ECMs are all focussed around HVAC / lighting. There are so many changing product lines (different lines in baseline and reporting period!) that it's difficult find a consistent and reliable production variable, but, we do have sub-meter data for the production. Is it theoretically possible to use the production sub meter data as an independent variable for the whole site data? The thought is that the activity of the production lines has an impact on the HVAC / lighting loads (as well as other variables - degree days etc.) and so we're just using energy consumption of the production lines as a proxy for production activity. However, it doesn't feel comfortable to us as the sub-meter data is not independent of the dependent variable (i.e. the whole site data)!! Does anyone have any views / experience with this? Is this a regression analysis faux pas? Very grateful for any thoughts.

A:  I agree that the method you describe is not quite typical. It is more common to use a production metric as the independent variable. That said, there are definitely times when no clean production variable is available. It may not be recorded or product lines may change so quickly that there is no common element. In those cases, I think you have hit upon a clever and acceptable approach. It works in this case because you are not attempting to estimate savings on the production lines.

You say that “the sub-meter data is not independent of the dependent variable (i.e. the whole site data)”. That would be true if the dependent variable was the whole facility use. However, in your case I think you can isolate on the HVAC and Lighting loads. I recommend subtracting the submetered use from the whole facility use. That will give you the Lighting and HVAC use, plus some noise. Try regressing that net load against your production variable (kWh) and whatever other variables you are considering.

One uncertainty to consider is how much (if any) non-production load is actually served by the "production" submeter? If any of your lighting or HVAC is on that meter, you will miss those savings.