Alternator Size Calculator
Add up what your electrical system draws and find out whether the alternator can keep up, or how big a replacement needs to be.
Electrical load
- Alternator ratings are at cruising speed. Output at idle is typically around 40% of rated.
- Upgrading the alternator means upgrading the charge wire between it and the battery to match.
- Continuous operation near rated output is what kills alternators. Size for headroom.
Rated output is not idle output
Alternator ratings are measured at high rotor speed, roughly what you get at highway cruise. At engine idle most alternators make around 40% of that number. A 130 amp unit is producing something like 50 amps while you sit at a red light with the lights, blower and stereo running.
That is why battery drain shows up in stop-start traffic rather than on the highway, and why the fix is often a smaller pulley or a higher idle rather than a bigger alternator.
Leave room for charging
The alternator has to run everything on the vehicle and put back what starting the engine took out of the battery. If the electrical load consumes everything the alternator makes, the battery never recovers and slowly dies over weeks, which looks like a bad battery rather than a charging problem.
Winches are a battery problem
A 9,500 pound winch under load pulls 300 amps or more. No practical alternator makes that. Winches run out of the battery and the alternator recovers afterwards, which is why serious recovery setups use a second battery and heavy cable rather than a huge alternator.
Frequently asked questions
How many amps does my alternator need to be?
Add up everything that runs continuously, add around 20 amps for battery charging, then add 25% headroom. A modern vehicle with no additions usually lands between 100 and 150 amps, which is why that is what manufacturers fit.
Why does my battery drain at idle but not on the highway?
Alternator output falls with engine speed. At idle you may only have 40% of the rated amps available, and if the load is higher than that the battery makes up the difference. Drive at speed and output rises above the load, so it charges again.
Do I need a bigger alternator for a car audio system?
If the amplifier's real world draw pushes total load past what the alternator makes at idle, yes. A rough estimate is amplifier RMS watts divided by ten for average draw. Many systems are fine on the stock alternator with better wiring and a healthy battery.
Does upgrading the alternator require new wiring?
Yes. The factory charge wire is sized for the factory alternator, and pushing more current through it drops voltage and heats the wire. Upgrade the alternator to battery cable and its fuse to match the new output.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.