If the star radiates uniformly in all directions, what is its total average power output?

imagine a sphere of radius 7x10^12m centered on the star; the surface area of this sphere is 4 pi r^2

thus, the total output of the star is distributed evenly over this sphere, so we have

15.4W/m^2 = L / (4 pi r^2) where L is the total output (luminosity)

L = 15.4 W/m^2 x 4 pi (7x10^12m) ^2 = 9.48 x10^27W