Let's say the alternator is putting out 60 amps (not likely, especially not for long) at 14 volts. Power (in watts) = volts x amps. So actual power delivered is 840 watts. Lets add another 10% for mechanical losses (produced as heat by the alternator) and call it 924 watts. To convert watts to horsepower you divide watts by 745.7, giving 1.24 horsepower needed to drive the alternator when it is putting out max power. The belt also needs to transfer some amount of power to the water pump, but I don't know how to compute that value. I'd be surprised it it were more than another horsepower.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
