This reference of the AC loss with respect to Pmpp(STC) is a default
value, it is a first guess proposed by the software for a quick estimation of a reasonable value.
The real relevant value is based on the Rwiring value
, that you should ideally calculate for your actual wiring installation.
The resulting ohmic wiring loss is indeed expressed as a percentage of the Energy remaining at this point of the simulation.
You can have a look on our FAQ "How to determine the Wiring loss Parameter"
and also How to determine the parameters for external transfo loss ?