GMK’s Alternator Sizing Calculator
Enter RMS watts to estimate alternator size. AGM/LTO + daily/demo included.
Basic estimate using RMS watts and simple assumptions (daily vs demo). This is a starting point: wiring + batteries still matter.
60A is a decent default. Big trucks / lots of accessories can be higher.
Battery bank = how long you can play. Alternator = how fast you recharge.
Assume Class D amps (more efficient)
Upgraded charge wire + proper fusing (recommended)
Main issue is dimming at idle / stoplights
Notes: Alternator rating is not idle output, and it does not mean your battery “charges at that many amps.”
Batteries accept current based on chemistry, size, state of charge, temperature, and wiring.
For a free quote on your custom Apex high output alternator,