What is the equation to find the solar flux? In watts per meter squared?

Flux(R) = (3.827*10^26 W)/(4*pi*R^2 m^2)

Express R in meters, and the flux will be in watts/m^2.

The answer given by the first responder gets solar flux by assuming that the power output of the sun is already known.

The method you wanted was, to use the known solar flux at earth's orbit,
and use that to find the power of the sun?

The total area of the sphere that is as far from the sun as the earth is
would be
4 pi (150,000,000,000 m)^2,
so if the known solar flux at earth's orbit is 1367 W/m^2,
then the total power of the sun must be

(1367 W/m^2) (4 pi) (150,000,000,000 m)^2 = 3.9 x 10^26 Watts.

The value is perhaps really closer to 3.8 x 10^26,
but the error may come from squaring the approximate 150 million km.