How many amps your system pulls at full power, whether your alternator keeps up, and the battery and capacitor rules of thumb.
Total amp RMS power (W)
Amp class
Class
D
A/B
Class D
Class A/B
Class D
Class A/B
Alternator output (A)
Vehicle’s own load (A)
Lights, fan, engine computer: 40–60 A is common.
Alternator left over at full power
9.4 A spare
Amps draw at full power
91 A
Total with the vehicle
141 A
RMS your alternator can feed
1,104 W
Capacitor rule of thumb
1 F (1 F per 1,000 W RMS)
Guidance only, never a safety rating. Follow your amp and vehicle manuals. The fuse protects the wire: put it within 18 in of the battery. Disconnect the battery before you wire.
How it works
Enter the total RMS power of your amps, the class, your alternator’s output and the vehicle’s own load. You see how many amps the system pulls at full power and whether the alternator keeps up.
Example: 1,000 W RMS of class D draws 90.6 A. With a 150 A alternator and 50 A for the vehicle, 9.4 A is left over. That alternator can feed about 1,104 W RMS of class D.
The capacitor rule of thumb for 1,000 W is 1 farad. It smooths peaks; it does not make up for a small alternator.
Frequently asked questions
How many amps does a 1,000 watt amp draw?
About 90 A at full power for class D (1,000 ÷ 0.8 ÷ 13.8), and about 145 A for class A/B. With music it averages much less.
When do I need a bigger alternator?
When the voltage at the amp drops under about 12.5 V on bass notes after the Big 3. The headroom here shows how close you are.
Does a capacitor add power?
No. It stores a little energy for short peaks. It can’t replace alternator output or a battery.