Do they mention the duration if the pulse. There is a big difference between 45 Mw for a microsecond and 45 Mw for an hour. You could power the former with the alternator on your car, although cycle rate would be rather slow.
Actually, 45 Mw for a microsecond is only 45 Joules. Your car could provide that in less than a second - typical car alternator is somewhere in the 1 Kw area, so it would only take .045 seconds to generate that 45 Joules of energy.
But I’m guessing the pulse necessary for a rail gun is probably at least in the 10s of milliseconds area. Let’s say 50 ms. so 45 e06 * 50 e-03 is 2.25 MJ. That would take your car’s alternator a couple thousand seconds to produce.