Is that a good question some years later? Is the Bifacial Performance Ratio still calculated as the monofocial one?
Yes, until know the biracial performance ratio is calculated as the mono-facial.
"PR for bifacial systems
The definition of the performance ratio should be something like a standard, defined by an official instance, and accepted by everybody.
Now I have not yet seen any reference which would define a performance ratio for bi-facial systems. Therefore PVsyst cannot propose any specific value in the present time.
The value provided presently with the PVsyst results uses the definition of the Monofacial systems, so that the bi-facial gain comes as an increase of this ratio.
NB: The main objective of the PR is to find an indicator for comparing real and simulated data, therefore which may easily be evaluated using simple (and "primary") measured data.
However neither the rear side irradiance, nor the part of the bifacial gain is available in usual measurements."