The solar constant is the average intensity of the Sun’s radiation at a distance of 1 astronomical unit (the average distance of the Earth from the Sun).

1,361 watts per square metre (W/m²).

In fact, the Sun can be directly overhead only in the tropics. As shown below, when the Sun is lower in the sky, the intensity of the solar radiation is reduced because its rays are spread out over a larger area and because they must pass through more atmosphere before they hit the ground. At night when the Sun is below the horizon the solar intensity is clearly zero.

If we average out over an entire 24 hour cycle the amount of solar radiation hitting the Earth’s surface (known as the solar irradiance) on a clear day at the equator on the equinox is approximately 340 W/m².

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