### What is the 90.1-2013 Appendix C "envelope performance factor"?

ASHRAE Std. 90.1-2013 Appendix C states that you have to compute an "envelope performance factor" for both your proposed and 'base' buildings. This seems to be intended to be done using energy simulation software.

But it seems to **leave out a precise definition of how this factor is to be **~~computed.~~computed.

I get the impression that the factor for each model may just be the total annual energy cost for the building. It does say in one spot (C3.3 Purchased Energy Rates) that:

"... rates for purchased energy shall
be used to determine the proposed
envelope performance factor and the
base envelope performance factor"

Is my interpretation correct? (That the total energy use is what is required?)

Are there any materials which clearly define these terms? Thanks