I installed 2 alternators last year, they were 90 amps and 70 amps. That's at and under 1 hp.
As for always generating, I was in the belief that one under full load would be heavier to spin. You see, this is a powerful charging, and hence the "cutout" element on the alternator, it cuts out the charge to the battery, otherwise you will boil it/blow it.
So something here is not right. And as for the alternator not generating anything without the battery, well, that is correct, it neds a current to get things going, but ....there are also dynamos that don't need a battery....the alternator is just another approach.
It needs the volts to work, but bear in mind that the battery itself doesn't generate anything....
In short, I am not seeing any special benefit that makes this better for a car than a powerplant.