How Big Is Big Enough?
An alternator's output is dependent on speed, but this can be deceiving because this output is not linear. Instead, it follows a curve and at idle small changes in the alternator's speed can make a big difference in its output capacity. An 80-amp alternator is capable of producing 80A maximum. It does not produce power all the time but rather only when it is needed and only in the amount needed. Therefore, if a truck used 52 amps continuously a 60-amp alternator could be used. However, an 80-amp alternator would work better. Why? Because the 80-amp alternator is working at 65 percent of its capacity whereas the 60-amp is working at 87 percent of its capacity. This percentage, or duty cycle, affects the voltage that the alternator is able to maintain. The 60-amp unit will typically produce the 52 amps at a lower voltage than the 80-amp unit in the same situation. Therefore it is good to get the alternator with the highest amp rating available for that particular mounting situation.

How To Determine Amp Load
The most accurate means of determining amp load is with an ammeter. However, a pretty accurate guesstimate can be made by using specs from the manufacturers of the electrical components in use and the following chart. If a particular item has a range, it's always advisable to go higher than lower when figuring amp draw.