[Annixter]

Generic calculators like that one are very inaccurate. What, exactly, is included in a "normal" vehicle or a "custom" vehicle, and what are the amp ratings for those components? The generic calculators use averages. They may assume an 8amp heater blower motor when yours draws 12, or they assume four turn signals when you have two. Add up these deviations and it's the difference between a 63amp and an 80amp alternator.



While it takes time and energy, to properly choose an alternator's amperage one needs to measure the system by adding up the actual amp ratings for each bulb, motor, etc.--preferably from the actual parts' tags or spec sheet--and then do some load factor additions for a buffer.



The electrical system of a car is just as important and potentially dangerous as the drivetrain, and I wouldn't guess if my driveshaft pinion angle is within tolerance. I'd get out an angle finder, paper and pen, and get under the car.