PTC is an acronym standing for PV-USA Test Conditions and is US-American solar panel test condition standard.
PTC - condition details and testing setup
PTC has been developed at the University of Davis, California. As compared to Standard Test Conditions (STC), PTC is a more real-world approach to measure the actual performance of a solar cell.
It specifies an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum and an ambient temperature of 20°C at 10 meters above ground level with wind speed of 1 m/s.
Performance differences between STC and PTC-rated modules
The differences of 20°C ambient temperature at 10 meters above ground level with 1 m/s wind speed applied under PTC, results in a cell temperature of about 50°C, instead of the 25°C under STC conditions and is therefore much closer to the real-world daily operational conditions of solar cells.
Since cell voltage drops about 0.08V/°C in environments exceeding ambient temperatures of 25°C, STC voltage ratings may actually equal to lower voltage ratings under PTC conditions, which in the end - using Ohm's Law - results in reduced watts.