For what its worth, I just spent the past 2 evenings parting out a couple of alternator based battery chargers for a large commercial UPS system, they were designed by engineers, but I'm not sure they were good engineers
. Power in was 230v 3 phase, this was through a Variable Frequency Drive to power a 15hp 1725rpm electric motor which drove a 220amp 12v truck alternator. The key pads on the VFD's had been taped over, so I'm guessing they were set to run at 1 speed, the pulley on the motor was 6" diameter, alternator used a 2" pulley. Voltage regulator appears to be a commercially produced unit, but data tag was removed. There was another box of electronics with a Condor current regulator, and lots of little "shop made" boards with some circuitry and resistors rats-nested together
So 7.5hp for a gasoline powered unit driving a 100amp alternator sounds about right