A very close rule of thumb is that for every 25A of power generated requires 1Hp from the engine.
The reason I say rule of thumb is becuase a certain amount of energy is lost due to friction of the pullies/belt/s.
So based on that rule of thumb, a 100A ALT output is going to require 4Hp from the engine.
In saying the above, ALT installations of 85-100A is about the max when driven with a single belt. Above that current rating, it should be a twin belt system. but twin V belts have a lot of friction. This is due to that the V belt is driven by contact with side of belt and side of pull