Ha! Thanks for running through the numbers, very interesting. The air in that 15 mile ball is almost transparent to infrared, though, so I'd probably assume that the surface area of the sphere would receive the lion's share, rather than the volume within it. So the energy delivered should fall off roughly with the square of the radius rather than the cube. Might be enough to give a sunburn, if nothing else.

Edit: On my phone I'm getting ~5.8 joules per square meter. Google seems to be mangling the units, though, so I can't be sure it's not dropping or adding in a factor of 1,000, though.

Edit 2: assuming that figure is correct, unless it's released in <1/200 of a second, it's not competitive with the sun at the equator (>1,000 Watts/Joules per m^2 per second)