That's true, but 33megajoules is almost exactly the amount of energy in 1L or 1/4Gal of gasoline. I don't know how much explosive a typical ship mounted canon requires for propellant and payload, but even given all the inefficiencies turning gasoline into electricity Id be quite surprised if there's quite a bit less energy on board to supply the rail gun.
200 pound of tnt is 30.2 mJ in the M107 round you have 14.6 lbs of tnt in that shell. So basically 14 shells would equal about that many mJ. I'm sure that the ship has more than 14 shells.
