Alternator Output Calculator
Check whether your alternator keeps up with the electrical load at cruise and at idle, and see the amperage headroom or shortfall.
How it works
Idle output = rated × idle% · Headroom = output − total load
An alternator only makes its rated amps near cruising rpm; at idle it often produces half that or less, which is where electrical systems get caught out. Subtracting the total accessory load from the output at each rpm shows the headroom — positive means the battery is charging, negative means it is being drained to make up the difference. A big audio install, winch or added lighting can push idle demand past supply, so plan for a margin, not a break-even.
Worked example
A 130 A alternator making 50% at idle produces 65 A. Against an 80 A load it has +50 A of headroom at cruise but −15 A at idle — so sitting in traffic with everything on slowly drains the battery.
Frequently asked questions
How do I know if my alternator is big enough?
Add up the current draw of everything that can run at once and compare it to the alternator’s output — especially the reduced output at idle. If the load exceeds idle output, the battery makes up the difference and will eventually go flat in slow traffic.
Why does output drop so much at idle?
Alternator output scales with rotor speed. At idle the pulley spins slowly, so the unit may deliver only 40–60% of its rated amperage. That is why heavy loads reveal themselves as dimming lights when you stop.
What happens if the load exceeds output?
The battery supplies the shortfall and steadily discharges. Short-term you may see dimming lights and slow blowers; sustained, it leads to a dead battery and a no-start, and chronically undercharging shortens battery life.
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This calculator is for educational purposes. Double-check important results before acting on them.