STC (Standard Test Conditions) is a standardized environment in which the performance of solar panels is measured to ensure comparability between products from different manufacturers. STC defines the reference conditions under which the maximum power output (Wp – Watt peak) of a solar panel is measured.
STC conditions
25°C module temperature: The temperature of the solar panels is maintained at 25°C during the measurement, which is achieved in a controlled laboratory environment. This is not the same as the outside air temperature, but refers to the actual temperature of the solar cell.
- Practical significance: The performance of solar panels decreases as the temperature of the cells increases. For example, on hot summer days, the cell temperature of solar panels can be as high as 50-70 °C, which can result in lower performance.
Air Mass 1.5 (AM 1.5): Air Mass indicates the distance that the sun's rays have to travel through the Earth's atmosphere.
- AM 1.5 means: Sunlight travels one and a half times longer through the atmosphere than when the sun is directly overhead. This is the average value of atmospheric scattering and absorption at temperate latitudes.
- Why it matters: AM 1.5 is a standardized value that represents the amount of sunlight most often encountered in real-world conditions.
1000 W/m² irradiance: 1000 watts/square meter represents the intensity of sunlight that can be achieved at noon on a clear, sunny day, when the sun is at its highest point.
- What does this mean? This amount of energy is the maximum amount of solar radiation reaching the Earth's surface under ideal conditions.
- Practical variations: In real-world conditions, the amount of radiation varies depending on the time of day, weather, and season, which affects the performance of solar panels.
Purpose of using STC
The purpose of the STC is to provide a standardized measurement basis for comparing the performance of solar panels. Since real-world conditions differ from the STC, the standardized test environment allows manufacturers to:
Provide objective performance data: Customers can see exactly what performance the solar panels are capable of under ideal conditions.
Ensure comparability of different solar technologies: Based on the STC, the products of all manufacturers are tested under the same conditions.
STC vs. Real-world conditions (NOCT)
Real-world conditions often differ from STC conditions. For this reason, manufacturers use other measurement standards, such as NOCT (Nominal Operating Cell Temperature) , which better reflects the actual operation of solar cells.
| Parameter | STC | NOCT |
|---|---|---|
| Module temperature | 25 °C | 45-50 °C |
| Irradiation | 1000 W/m² | 800W/m² |
| Outside temperature | Not specified | 20 °C |
| Wind speed | Not specified | 1 m/s |
Practical advice for buyers
Don't just look at STC: While STC data is useful, it's also worth considering performance under NOCT conditions to get a realistic picture of your solar panel's efficiency.
Installation location and conditions: The performance of solar panels depends largely on the number of hours of sunshine, the angle of inclination and the temperature of the given geographical location.
Summary
STC (Standard Test Conditions) are standard reference conditions for measuring and comparing the performance of solar panels. These conditions ensure that manufacturers provide objective data on the maximum performance of their products. While STC is important for comparison purposes, real-world operating conditions can vary significantly, so buyers should consider NOCT data and local conditions.