The basic unit conversion is 746 watts/horsepower.

100 amps @ 12 v = 1200 watts (actually, 15 v is more realistic, so
1500 watts)

1500 watts / 746 watts/hp = 2 hp

Allowing for real world inefficiency, you've got plenty of engine for
a bigger alternator or for the engine to be loafing while it runs a
100 amp one.

Roy

--- In Amptramp@yahoogroup s.com, "timowens3k" <timowens3k@ ...> wrote:
>
> Good day all. This is my first real post and I hope I am not being
> redundant on this question. I have an extra riding lawn mower with a
> 12hp vertical shaft motor. I am thinking about picking up a 100 amp
> alternator at the local pull-a-part. Would this be to big for the
> engine? If so, what size would you recommend?
>