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 ?.