STC (Standard Test Conditions)
Standardised test conditions (1,000 W/m², 25 °C, AM 1.5) under which the rated power of solar modules is measured.
Also known as: Standard Test Conditions
What Are STC?
STC (Standard Test Conditions) are the internationally standardised conditions under which the electrical characteristics of solar modules are measured and compared. They form the basis for the rated power specification in Watt Peak (Wp).
STC Parameters
| Parameter | Value |
|---|---|
| Irradiance | 1,000 W/m² |
| Cell temperature | 25 °C |
| Spectrum | AM 1.5 (Air Mass 1.5) |
Meaning of AM 1.5
AM 1.5 (Air Mass 1.5) describes the path of sunlight through the atmosphere at an angle of incidence of approximately 48° — a typical value for mid-latitudes. This normalises the spectrum of the incident light.
STC in Practice
STC conditions are rarely exactly achieved in real operation:
- Cell temperature: Typically 50—70 °C in summer, not 25 °C
- Irradiance: Varies with time of day, season, and weather
- Spectrum: Changes with sun position and atmosphere
Therefore, a module typically delivers less than its rated power during operation. On cool, sunny days, however, the power can briefly exceed the STC value.
Practical Tip
STC values are for comparing modules, not for predicting yield. For realistic performance estimates, the NOCT value or simulation software such as PVGIS should be used.
Related Terms
NOCT
Nominal Operating Cell Temperature -- cell temperature under realistic operating conditions (800 W/m², 20 °C, 1 m/s wind).
kWp (Kilowatt Peak)
Unit for the maximum power output of a solar module or PV system under Standard Test Conditions (STC).
Wp (Watt Peak)
Unit for the peak power of a single solar module under Standard Test Conditions (STC).
Solar Module
Assembly of multiple solar cells in a weatherproof housing for generating electricity from sunlight.
Temperature Coefficient
Metric indicating how much a solar module's power decreases per degree Celsius of temperature rise (typical: -0.3%/°C).