Horsepower(true power)
Watts(true power)
Volt amps(apparent power)
To get 1 horsepower output from a motor you will require more than 746 watts input, because there's no such thing as a 100% efficient motor.
So let's say you got a motor that's 90% efficient.
You'll need 746 watts input multiplied by the inverse of 90%.
(746x(1/.9))=829 watts.
Then there's the relation ship called power factor that all motors posses, which requires more current input to get a true power output.
Onna motor like you'd have on a grinder figure a powerfactor of about 0.8.
So 829 watts multiplied by the inverse of 0.8 equals (829x(1/.8))=1036 volt amps.
Volt amps divided by the supply voltage(single phase) equals the input current required.
1036/120=8.6amps.
All this math goes out the window if there's gears and the like involved, then there's the pitch of the grinder screw, and the sharpness of the grinder cutter blades, size of the grinder plate openings, and the amount of sinue and silverskin you feed the grinder.
FWIW, I get by with a 1/2HP Omcam grinder, which is one of the better models of semi proffesional design.
http://www.omcan.com/meatgrinders/smg50.html