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.
x 100 = 13.53%
(92.9 watts/sf x
This efficiency value is used in the reality
check exercise. The results are well-aligned with the
theoretical incident energy that was independently predicted by my solar
equations. I therefore conclude that the PV array is performing to