The globally averaged solar insolation is given by the
ratio of the total sunlight absorbed by the Earth to the area of the Earth.
If Fs is the intensity of sunlight reaching the Earth (normal
to the path of sunlight), and the total cross section of the Earth is PI*R2
The amount of sunlight absorbed by the Earth is Fs*(1-albedo).
The total area of the Earth is 4*PI*R2.
So this gives the global average solar insolation as Fs*(1-albedo)*PI*R2
This is balanced by outgoing longwave radiation (sigmaT4) and an effective temperature of the planet can be found
(where sigma is the Stefan-Boltzmann constant)
Results from this model compare favorably for many of the solar system's planets.