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CarAudioCalc

Electrical calculator

Alternator Size Calculator: How Many Amps Do You Need?

Updated . Researched from published standards and manufacturer specifications.

Quick answer

A 1,500 watt system draws about 130 amps at 14.4 volts. Added to a typical 55 amp accessory load that is 185 amps, and with 20 percent headroom for actually charging the battery the alternator needs to make about 223 amps.

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Alternator output needed by audio system size (class D, 55 A vehicle load, 20 percent headroom)
Audio system RMSAmp drawPlus vehicle loadAlternator needed
500 W43 A98 A119 A
1,000 W87 A142 A171 A
1,500 W130 A185 A223 A
2,000 W174 A229 A275 A
3,000 W260 A315 A379 A

Convention Source: Computed by the calculator below; accessory load and headroom are planning conventions, amp draw uses the class D efficiency convention. Compare against idle output, not the peak rating. An alternator that makes its rated amps only at high rpm will still let voltage sag at a stoplight.

Your numbers

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Result

223
Alternator output needed 223 A Convention
Audio system draw 130.2 A Convention
Vehicle accessory load 55 A Convention
Total continuous load 185.2 A

Every alternator in the picks below is vehicle specific. Check the fitment list against your own year, engine and body before ordering, because a unit for the wrong bracket does not fit at all.

The formula

Alternator amps = (audio RMS watts / (efficiency x volts)) + accessory load, times headroom

The figure that matters on a high output alternator is output at idle, not peak output at 6,000 rpm. A 300 amp alternator that makes 90 amps at idle will still dim your lights at a stoplight. Manufacturers who publish an output curve are telling you something the ones who publish only a peak number are not.

The figures above are computed with these exact inputs: Total system RMS power 1500 W, Vehicle accessory load typical, Charging headroom 1.2.

What this calculator assumes

Some of these are published physics or manufacturer figures and some are conventions. They are not the same kind of number, so each one is labeled.

AssumptionKindWhere it comes from
Class D Efficiency: 0.8ConventionA planning efficiency for a class D mobile amplifier at typical listening levels. Real efficiency varies with load impedance, output level and the amplifier itself, and no standards body publishes a single figure

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Frequently asked questions

What is the average alternator output in amps and watts?
A typical modern passenger car or light truck alternator is rated between 100 and 220 amps, which at 14.4 volts is roughly 1,400 to 3,200 watts of electrical output. Those ratings are at highway rpm; at idle most alternators produce only 40 to 60 percent of the rated figure, which is why a 1,000 watt stereo can dim the headlights at a stoplight even on a 180 amp unit.
How many amps is a stock car alternator?
Stock alternators on most passenger cars and light trucks are commonly rated somewhere around 100 to 220 amps, and the exact figure is stamped on the alternator case or listed in the service information for your vehicle. That rating is the maximum at high rpm. At idle a stock unit often makes far less, which is why a big stereo dims the lights at a stoplight before it does on the highway.
What size alternator do I need for a 2000 watt system?
About 275 amps. A 2,000 watt RMS class D system draws roughly 174 amps at 14.4 volts; add a typical 55 amp vehicle load and 20 percent headroom to keep charging the battery and you land near 275 amps. Look for a high-output unit that publishes its idle output, and upgrade the big three charging wires at the same time.
How does the alternator output calculation work?
Alternator amps = (audio RMS watts / (efficiency x volts)) + accessory load, times headroom. The figure that matters on a high output alternator is output at idle, not peak output at 6,000 rpm. A 300 amp alternator that makes 90 amps at idle will still dim your lights at a stoplight. Manufacturers who publish an output curve are telling you something the ones who publish only a peak number are not.
Are these figures published or are they rules of thumb?
Both, and the page labels which is which. The Helmholtz port relation, the Thiele/Small alignment equations, cone area geometry, Ohm’s law, conductor resistance and ampacity, and manufacturer specifications are published and are labeled as such. Amplifier efficiency, cabin gain, the farad per thousand watts rule, port area per cubic foot and coverage percentages are conventions with no measurement behind them, and they are labeled as conventions.
Why does this calculator sometimes refuse to give an answer?
Because there genuinely is not one. A port can be too large in area for the box and tuning asked of it, a vented alignment stops describing a useful box outside a Qts range, and a sealed box cannot produce a total Q below the driver’s own. In each case the calculator returns nothing and explains why, rather than extrapolating past the end of a published relation.
Do I still need an installer if I use this?
Many owners do their own 12V work, and these are the figures that work is sized from. What matters is that a car battery delivers hundreds of amps into a short without hesitating, so the main fuse goes within 18 inches of the battery positive terminal and the battery comes off before power wire goes in. If any of that is beyond what you want to take on, a professional installer is money well spent.

Researched guidance, not professional advice. These results are planning figures from published relations and manufacturer specifications. Fit the main fuse within 18 inches of the battery positive terminal, because that fuse protects the wire and the wire is what starts a fire. Disconnect the battery before running power wire, use a grommet through every piece of sheet metal, and remember that sustained high sound pressure causes permanent hearing damage.