Which of the following is the value of solar constant ?
1.4 kW/m2
The solar constant is about 1·4 kW per square metre — option 2.
What it means. The solar constant is the amount of solar energy falling per second on one square metre of a surface held perpendicular to the Sun’s rays, at the top of the Earth’s atmosphere, at the mean Earth-Sun distance of one astronomical unit. The accepted value is
\(S\approx1361\ \text{W/m}^{2}\approx1.4\ \text{kW/m}^{2}\)
| Quantity | Value |
|---|---|
| Solar constant | About 1,361 W/m2, taken as 1·4 kW/m2 |
| Measured at | Top of the atmosphere, perpendicular surface |
| At the Earth’s surface, clear noon | Roughly 1 kW/m2 after absorption and scattering |
| Averaged over the whole globe | About 340 W/m2, that is S divided by 4 |
Why the global average is a quarter of it. The Earth intercepts sunlight over its cross-sectional area, a disc of area \(\pi R^{2}\), but that energy is spread over the whole rotating sphere, of area \(4\pi R^{2}\). The ratio is exactly 4, which is why the figure quoted in climate discussions is about 340 W/m2 rather than 1,361.
Why it is not quite constant. The Earth’s orbit is slightly elliptical, so the value varies by about 3·3 per cent over the year, being highest at perihelion in early January. The Sun’s own output also varies by roughly 0·1 per cent over the eleven-year sunspot cycle. The word “constant” is therefore a convention.
Why it matters. It fixes the equilibrium temperature of the Earth and so underlies the whole of climate science, and it sets the theoretical upper limit on how much power a solar panel of a given area can ever produce — which is why a panel producing about 200 W per square metre is already converting a substantial fraction of what reaches the ground.
Hence, the answer is 1.4 kW/m2.
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