Standard Test Conditions (STC)
Standard Test Conditions (STC) are the laboratory conditions under which solar PV panels are tested and rated for performance. All solar panel power ratings (watts), efficiency ratings, and electrical specifications are stated at STC to enable consistent comparison between different manufacturers and products.
STC parameters:
- Irradiance: 1,000 W/m² (peak clear-sky sunlight)
- Cell temperature: 25°C (77°F)
- Solar spectrum: Air Mass 1.5 (AM1.5) — representing sunlight filtered through 1.5 times the atmospheric thickness at sea level
STC vs. Real-World Conditions
STC represents idealized peak-performance conditions that are rarely achieved simultaneously in real-world installations:
Irradiance: 1,000 W/m² is achieved at solar noon on a clear day with the panel perpendicular to the sun. Most of the day, irradiance is lower.
Temperature: Panels installed on rooftops or the ground typically operate at 40–70°C in sunlight, far above the 25°C STC reference. At 50°C, a panel loses approximately (50-25) × 0.4%/°C = 10% of its STC rated power.
Net result: A panel rated at 400W STC might produce 340–360W under typical operating conditions at mid-day in a warm climate.
PTC Rating (PVUSA Test Conditions)
PTC (PVUSA Test Conditions) is an alternative rating standard used in some US incentive programs and permitting jurisdictions:
- Irradiance: 1,000 W/m²
- Ambient temperature: 20°C
- Wind speed: 1 m/s (provides some panel cooling)
PTC ratings are typically 5–15% lower than STC ratings, making them a more realistic estimate of real-world peak output.