Standard Test Conditions (STC)

Standard Test Conditions (STC) are the laboratory conditions under which solar panels are tested and rated for their power output. STC provides a universal benchmark that allows fair comparison of panels from different manufacturers.

The three parameters that define STC are: cell temperature of 25°C (77°F), solar irradiance of 1,000 W/m² (equivalent to bright noon sunlight), and an air mass of 1.5 (representing the spectrum of sunlight passing through 1.5 atmospheres). When a panel is rated at 400W STC, it means it produces 400 watts under these exact conditions. However, real-world conditions rarely match STC — rooftop panels typically operate at 40–65°C (not 25°C), and irradiance fluctuates throughout the day. This is why actual production is lower than nameplate ratings. To provide a more realistic performance estimate, panels are also rated under Nominal Operating Cell Temperature (NOCT) conditions, which better represent typical field conditions. When comparing solar panels, STC ratings are useful for apples-to-apples comparisons, but NOCT ratings give a more realistic picture of expected daily output.