STC stands for Standard Test Conditions and is the major solar panel output performance testing condition used by most manufacturers and testing bodies.
What is STC?
The electric output performance of crystalline silicon and thin film PV modules are generally measured under standard test conditions (STC), ensuring a relatively independent comparison and output evaluation of different solar PV modules.
STC is an industry-wide standard to indicate the performance of PV modules and specifies a cell temperature of 25°C and an irradiance of 1000 W/m2 with an air mass 1.5 (AM1.5) spectrum. These correspond to the irradiance and spectrum of sunlight incident on a clear day upon a sun-facing 37°-tilted surface with the sun at an angle of 41.81° above the horizon.
This condition approximately represents solar noon near the spring and autumn equinoxes in the continental United States with surface of the cell aimed directly at the sun. However, these conditions are rarely encountered in the real-world.
STC-based performance measurements are applied in the flash tests of many manufacturers.
Although an industry-wide accepted standard of comparability between modules, it often happens that in the manufacturing process STC requirements are not diligently met by the manufacturers' internal quality control, with frequent deviations in lamp spectrum, cell temperature, environment temperature, irradiation etc.
Purchasers of solar modules do therefore often request the inspection services of 3rd party solar PV module quality inspectors to ensure the manufacturer's stated performance claims.