The alternator powers the vehicle, all components and recharges the battery. A standard factory alternator ranges from 40 amps to 120 amps depending on vehicle. When the alternator is under high demand or the demand exceeds the alternators capability, your voltage will begin to fall and rely on the reserve of the battery. Lets use a 65 amp alternator for example in determining what aftermarket audio equipment the alternator can sufficiently power. In perfect condition, your 65 amp factory alternator is charging at 14.4 volts and is capable of supplying 936 watts. (65A x 14.4V = 936 watts) Your vehicle requires 30%-50% of this to run and operate standard vehicle components. If we use the middle, 40%, you are left with approximately 561 watts for your aftermarket audio equipment. (936W x (100% – 40%) = 561W) This 561watts is based on a 100% efficient amplifier which we know is not realistic. Lets assume you choose an 80% efficient amplifier, which is very efficient. If this amplifier uses 561watts and is 80% efficient, it will produce 448.8 watts for your speakers or subwoofers. (561W x (.8) = 448.8 watts) Going beyond this point and you begin to tap into the reserve of the battery. The result is a decrease in amplifier efficiency and an increase in heat due to decreases in voltage and increases in amperage. This is very hard on the alternator and amplifiers in the system. By now Im sure you see why an upgraded alternator is vital when upgrading your audio system.

DETERMINING POTENTIAL OF CURRENT ALTERNATOR:
Step 1:
Alternator Amperage Rating x 14.4V = Potential Wattage
A x 14.4V = W

Step 2:
Potential Wattage x (Maximum Potential Percentage Used to Run Vehicle) = Watts Available for Aftermarket Audio System
Answer from Step 1 x (100% – 40%) = Watts for Aftermarket Amplifier(s)

Step 3:
Answer from Step 2 (100% Efficient Amp) x (Actual Efficiency of Amplifier) = Watts that can be produced in current application