Evaluating performance of the PV panels can be somewhat subjective. Its not quite an exact science because there are so many variables and the insolation can't be controlled.

Photovoltaic manufacturers specify and verify the output power of their products by a repeatable production test method called Standard Test Conditions (STC). A more realistic approach called PVUSA Test Conditions (PTC) evaluates the modules in a way that models their actual operating environment.

The parameters of the complicated PTC process are output power, irradiance, plane of the array, ambient temperature, wind speed, and a handful of regression coefficients carefully selected to make the answer come out right.

I believe PTC power rating closely represents the actual performance of my new panels in cool-weather unobstructed southern California sunlight. I used this parameter in the system sizing exercise and it seems appropriate to use it to calculate the conversion efficiency.

The PTC output rating for a KC-120-1 panel is 105.7 watts with an  irradiance of 92.9 watts per sq. ft. (1,000 watts per square meter). The active area is 8.41 sq. ft. This implies a conversion efficiency of 13.53%, which would be expected for new modern PV modules.