Looking at the specifications for a solar panel, you’re going to see two distinct ratings: STC and PTC, both of which refer to DC (direct current) Watts of the solar panel.
Standard Test Conditions, or STC ratings, are the solar panel’s name plate value. This means that if it’s a Sharp ND-250QCS, 250 Watt solar panel, the STC rating is 250 Watts. STC ratings reflect the solar panel’s production in ideal conditions, which is actually a flash of light shot at the solar panel in a lab environment.
When you’re looking to get an idea of PV output, PTC ratings are a more realistic number to look at.
PTC ratings, or PVUSA (Photovoltaics for Utility Systems Applications) Test Conditions, show the results from a test that more closely mimics real-world conditions. PTC ratings are based on 1,000 Watts per square meter of solar irradiance at 10 meters above ground level, at 20 degrees Celsius, and with a wind speed of 1 meter per second. As seen in the image above, the Sharp ND-250QCS, 250W solar panel has a PTC rating of 223.6 Watts. If you were using these solar panels for your photovoltaic system, using the PTC rating of 223.6W to estimate output would provide a more accurate, “real-world” number than the STC rating of 250W. Keep in mind that there are still other factors that will effect PV output. There will usually be some energy loss via wires, inverters, etc. There’s also normal degradation of solar panels over time and environmental factors such as soiling (dirt), heat, and shading.