When stating the amplifier output power of their amplifiers, manufacturers rate their amplifiers in using completely different testing methods, often causing confusion to consumers.
You need clear information when purchasing an amplifier.
Something you should know is that your car battery and alternator provide both the voltage and the current that your amplifier requires to make its power. Voltage can vary from one vehicle to another. Voltage can also vary in relationship to the engine revolutions (RPM).
Usually, a higher voltage output from the vehicle equals to a higher power output from the amplifier (except for those amplifiers which use strongly regulated power supplies which then have drawbacks from other point of views). The supply tension used to state power output of an amplifier is very important since most of the manufacturers measure power at 14.4 V, 13.8 V or 12.5 V.
A 50 W x 2 amplifier supplied with 14.4 V (those with non-regulated or loosely regulated power supplies) could show a decrease in output power up to a 15% when supplied with 12.5V. While, in the opposite case, a 50 W x 2 amplifier tested at 12.5 V could show an increase in power up to a 15% when supplied with 14.4 V! This means a 15 W difference between two amplifiers both with rated power output of 50 W x 2.
It is important to know that we are comparing two or more amplifiers with power rates tested using the same method, since in the car audio industry there isn't any authority which checks and certifies the product specifications. Some manufacturers state power in “RMS” Watts, while other use the “Continuous Average” power method, two measurement methods which yield almost the same power output rating. A 50 W RMS x 2 on 4 Ω amplifier (we will discuss ohms later on) can actually deliver 50 W x 2 with an output of the amplifier “clipped”. “Clipping” occurs when the sound coming from the amplifier is distorted. Clipping particularly occurs when the sinusoidal wave (music) presented at the amplifier input from the head unit, is output from the amplifier as a square wave (distortion). Some manufacturers state power using the “Maximum” or “Peak” ratings. This is a marketing “ploy” that generates numbers that are double that of an amplifier rated using the RMS method. Generally speaking “Maximum” or “Peak” ratings are not the right measures to use to compare amplifiers power output. A 50 W x 2 “Maximum” power at 4 Ω is equivalent to a 25 W RMS x 2 at 4 Ω.
Now let us get back to the “ohm” matter. Amplifier power output is usually stated at 4 Ω. “Ohms” express the speaker voice coil nominal impedance or “load” presented to the amplifier. Each speaker has nominal impedance according to the voice coil design. Most car audio speakers have a 4 Ω nominal impedance. This is why speaker manufacturer's usually state the nominal power at 4 Ω.
There are some speakers with 2 Ω nominal impedance, some with 1 Ω, some with even a 0.5 Ω nominal impedance, but if the amplifier is stated as “stable” on such impedances, you can wire the speakers all together to produce, most of the times, more power than with 4 Ω speakers connected to amplifiers stable only at 4 Ω. Besides, you can find power data and power measurement specifications expressed in many different ways, which can sometimes be deceitful. So protect your investment: make sure you compare the stated power of two or more amplifiers using the same unit of measurement.