Detecting PID on a PV system by comparing output data, is very difficult. After all, the solar irradiation depends on the place and time of measurement.
In general, the Performance Ratio (PR) of a PV park will decrease by 2.5% to 30% when PID occurs.
PID effect in individual PV module
In an individual PV module, the PID effect is stronger in cells that are located at the periphery (closer to the aluminium frame). The PID effect is less strong in cells that are located in the centre of the module.
PID effect in string PV modules
In a string of PV modules, the PID effect is stronger in panels that are closer to the negative side of the string. After all, the negative potential with respect to earth is the highest at that side.