Based on H_{sun} and the surface area of the sun, you can calculate the total power of the sun. In a distance of 1 AU (=the distance earthsun), this power is distributed over a sphere with a radius of 1 AU, which allows to calculate the intensity.
Simplified: ##H_{earth}R_{earth}^2=H_{sun}r_{sun}^2## where R is the orbital radius of earth and r is the radius of the sun.
