is the average rate at which radiations from the sun are received by the outer layer of the earth atmosphere’s that has a mean value of 1370 Watts per square meter.
As said above in the definition ‘’average rate’’, the solar constant is not a physical constant, it varies. It is not like the Planck constant or the speed of light which are absolutely constant in physics.
The actual direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year due to the Earth’s varying distance from the Sun, and typically by much less than 0.1% from day to day. Thus, for the whole Earth, the power is 1.740×10^17 W, plus or minus 3.5%.