On the datasheet of a solar panel you will find the STC rating, which stands for the Standard Test Condition.
How is the STC rating calculated for solar panels?
The STC rating is calculated under standard laboratory conditions that the majority of solar panels are tested. Under these test conditions the light source provides exactly 1,000 watts per square meter of the panels, the cell temperature is exactly 25°C (77°F), and one sunlight spectrum.
Since the STC rating is produced under controlled settings and ideal conditions, while in actual real life settings your panels will perform at a lower rate. Thus the PTC rating is good to take a look at since it’ll give you a more realistic idea on what you can expect.