Look on the back of the amplifier for the impedance rating. The ohm rating tells you how much resistance the internal circuits provide. Standard impedance ratings are 4, 8 or 16 ohms. The lower the number, the greater the power output will be.
Divide the input voltage by the ohm rating to determine the current passing through the amp. In a house, your amp will likely be connected to a standard 120-volt AC outlet. In your car, the amp will run on a 12-volt DC battery. Therefore, if you have a 4-ohm amplifier running from a 12-volt battery, it has a current of 3 amperes.
Multiply the current by the input voltage to get the power rating in watts. Multiplying 3 amperes times 12 volts is 36 watts of peak output power. However, trying to continuously deliver sound output at peak power will damage the amp. You need to know the average, or root mean square, of the peak power output.
Divide the peak out by the square root of two to get the root mean square, commonly used for basic AC and sound waveforms. A 36-watt peak output converts to an RMS measurement of 25.45 watts. This is the measurement of the maximum continuous output the amp can sustain without risk of damage.