There's an additional flaw, or two, I believe, since SWST decided to use the mass from Robert Anderson's website, which generously presumes that each Death Star has the same high density per meter cubed as a Star Trek ship does.
The maximum I get, assuming 500 trillion metric tons and a generous 100 kps velocity, is somewhere around 5.0E+28 J, though this energy was applied over some time, not instantly and represents the
maximum orbital velocity of the first Death Star. If as you assume that 99% of the mass is taken off, then we're down to 4.95e26 J, which is much more in-line with the energy release of a chain-reaction explosion of the reactor yeilding a "small sun" in output.
Now, the Death Star first comes out of hyperspace a 5:54 as seen
here. We cut to the Rebel briefing and the "Maximum velocity" statement does not come until 7:47. About 2 minutes later, and we assume that is highly generous given it will take 30 minutes to orbit Yavin, whereas earlier the battlestation is shown a considerable distance away from the big gas giant, perhaps 2-3 planetary diameters away, thus 2 minutes to expend 4.95e26 J is very generous. So divide by 115 seconds gives us a peak reactor propulsion output of 430,434,782,608,695,652,173,913,043.47826 watts.
-Mike