Solar Constant
Radiant power of the sun per square metre at the top of Earth's atmosphere -- approximately 1,361 W/m².
Also known as: Solar Irradiance Constant
What Is the Solar Constant?
The solar constant indicates the radiant power of the sun hitting a surface of one square metre oriented perpendicular to the radiation at the top of Earth’s atmosphere. Its value is approximately 1,361 W/m².
Physical Background
The solar constant is not exactly constant but fluctuates slightly:
| Factor | Variation |
|---|---|
| Elliptical Earth orbit | +/- 3.4% (perihelion vs. aphelion) |
| Sunspot cycle (11 years) | +/- 0.1% |
| Long-term fluctuations | Very minor |
Relationship to the Earth’s Surface
Of the 1,361 W/m² at the edge of the atmosphere, a maximum of approximately 1,000 W/m² reaches the Earth’s surface under clear skies. The difference is absorbed and reflected by the atmosphere. This value of 1,000 W/m² is also used as the basis for the Standard Test Conditions (STC) of solar modules.
Significance for Photovoltaics
The solar constant defines the theoretically maximum available solar energy. In practice, it is used for the following calculations:
- STC conditions are based on 1,000 W/m² (ground level)
- Yield estimates account for the actual irradiance at the location
- Comparison of solar resources at different locations
Practical Tip
The Standard Test Conditions (1,000 W/m²) correspond to a clear summer day around noon. The annual average irradiance in Germany is only about 125 W/m² — this shows how much weather and time of day affect the yield.
Related Terms
Global Irradiance
Total solar radiation hitting a horizontal surface -- the sum of direct and diffuse radiation.
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.
kWp (Kilowatt Peak)
Unit for the maximum power output of a solar module or PV system under Standard Test Conditions (STC).