Ohm s Law

This is where Ohm's law comes into play and will be crucial to your understanding of power raitings.

P = Power in Watts

E = Voltage - The voltage in your car will be anywhere from 12v (at idle) to upwards of 14.4v

I = Amperage - The current raiting is usually indicated by the fusing (either internal or external) on the amplifier. If there are miltiple fuses, you will need to add up all of these fuses.

R = Electrical Resistance in Ohms

SQR = Square Root

Basic Formula
This formula is the most basic form of the equation.

P = I x E

or

E = I x R

Voltage
If you know the wattage and the amperage, you can determine the voltage requirement.

E = P / I

or

E = I x R

or

E = SQR (P x R)

Current (Amperage)
If you know the wattage and the voltage, you can determine the amperage requirements.

I = P / E

or

I = E / R

or

I = SQR (P / R)

Power (wattage)
If you know the voltage and the amperage, you can determine the wattage output.

P = I x E

or

P = E2 / R

or

P = I2 x R

One thing that these formulas do not account for is the amplifier efficiency. The efficiency numbers are listed above under each amplifier type. To adapt these numbers you should use the basic equations as follows:

P = I x E x efficiency