If you "assume" that motor is say, 70% efficient, that means it actually consumes about 10hp electrically. The electrical equiv of 1hp is about 800 watts, so 800w X 10HP is 8000 watts. At 14V, that's 8000/14 which is about 572 amps.
LOLOL so my initial guess about a 300A alternator was a little "small"
