Q. Can you define Solar Constant?
The luminosity of the Sun L is 3.827×1026 Watts. When the Sun is straight overhead on a clear day how many watts would we expect to illuminate a square meter of the Earth's surface?
This value is initiated by spreading the luminous flux over an imaginary sphere with its centre at the centre of the Sun and its radius equal to the Earth's distance from the Sun, 1.496 × 1011 meters.
A sphere has a surface area equal to
A = 4πr2= 4π(1.496 × 1011)2
A = 2.81 × 1023m2.
Subsequently the value we seek is L/A or 1361 W/m2. This is called the Solar Constant and is mainly useful in analyzing solar powered equipment and evaluating energy conservation methods.