What Size Alternator Do You Need for a Car Audio System?

When it comes to car audio, more power means more sound and vice versa. If you want your music to be big and loud with a huge bass response, you need a big amplifier with a big subwoofer—and a lot of power to run it.

The Alternator's Role

The burden of producing all that energy falls on the alternator. The alternator is the part of the engine that charges the battery and runs the electrical systems. Most factory alternators aren't designed to handle loud, high-fidelity sound.

If you install a big, powerful amp without making the necessary power adjustments, the car compensates by restricting power to some of the car's electronics. This usually presents as flickering dash lights, dimming headlights, and a weak bass response from the subwoofer.

Do You Need a Bigger Alternator?

The easiest workaround is to keep the volume at a level that the alternator can handle. If you want a powerful sound to go along with your powerful amp, though, you have a couple of options.

Option 1: Install a Stiffening Capacitor

This is like a battery in that it stores electrical charges through a series of conductive plates. Amplifiers can't draw more power from a charging system than what the system is designed to make.

Subwoofers are especially problematic because low frequencies require more energy to produce. A stiffening cap allows a car's amplifier to run at higher output levels by providing a reserve tank of emergency current.

However, a capacitor is a viable solution only if you experience flickering lights or diminished bass responses at high volumes. If you're still drawing too much power at low or mid volumes, the stiffening capacitor isn't going to cut it.

Option 2: Install a High-Output Alternator

In most cases, the solution is to install a high-output alternator. This is an expensive option, but it's the only way to ensure that your high-performance audio system gets the current it needs without compromising your car's electronics.

Capacity Considerations

To figure out how much capacity or amperage the alternator must deliver, determine how much extra power the audio system demands on top of the current system.

The easiest way to ballpark that figure is to use this formula:

amps x volts = watts

For example, if you installed a 2,000-watt amp, assuming a nominal voltage of 13.5V, you'd add roughly 150 amps of demand to the electrical system. Also, calculate how much amperage is drawn by the rest of the car, then add that to the sound system.

The total amperage you arrive at will be a ballpark estimate of the maximum power draw of the car and indicates how big of an alternator you need (measured in amps). It's best to have a professional figure this out; every car and sound system is different. These figures also don't account for the difference in output produced at different engine RPMs (revolutions per minute).

Idle vs. Rated Output

For high-output alternators and car audio systems, the rated output of an alternator typically refers to the amount of current it produces at high RPMs—the kind of engine activity found at highway speeds.

When your engine is idling—that is, running at low RPM—it will be capable of providing only a fraction of that amperage. In some cases, an alternator may be able to produce only half of its rated output.

This is why most people notice problems like flickering lights when idling in traffic or at a stoplight. In this situation, the demand is at its highest while the productive capacity of the alternator is at its lowest. In some cases, turning the volume down while the car is running at low RPMs fixes the problem. Or, choose a higher-amperage alternator.