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What are Standard Test Conditions (STC)?

STC stands for “Standard Test Conditions” and are the industry standard for the conditions under which a solar panel are tested. By using a fixed set of conditions, all solar panels can be more accurately compared and rated against each other. There are three standard test conditions which are:

1. Temperature of the cell – 25°C. The temperature of the solar cell itself, not the temperature of the surrounding.

2. Solar Irradiance – 1000 Watts per square meter. This number refers to the amount of light energy falling on a given area at a given time.

3. Mass of the air – 1.5. This number is somewhat misleading as it refers to the amount of light that has to pass through Earth’s atmosphere before it can hit Earth’s surface, and has to do mostly with the angle of the sun relative to a reference point on the earth. This number is minimized when the sun is directly above as the light has to travel a minimum distance straight down, and increases as the sun goes farther from the reference point and has to  go at an angle to hit the same spot.