Power ratings for amplifiers measure how well an A/V receiver amplifies (increases) outgoing audio signals. The power rating for an amplifier is measured in watts per channel. This is usually measured as RMS, or root mean square, instead of peak, which means that it is a measure of sustained power, not the highest possible instantaneous peak.
You can’t take the amplifier power ratings at face value when comparing A/V receivers, because manufacturers play a lot of interesting tricks when they give these watt ratings. The result is that a receiver can be more or less powerful than another receiver with the same rating.
To get a feel for a receiver’s power, examine the following four specifications closely:
Distortion: Power is measured at a certain number of watts at a certain level of distortion (noise created by the amplifier). You want low distortion (of course). The tricky part comes in when you examine how the amplifier’s power output is measured — specifically at what distortion level it’s measured.
An amp that is measured, for example, at 100 watts per channel at 0.02 percent THD (total harmonic distortion, the standard measurement) is quieter and is probably more powerful than one that is measured at 100 watts per channel at 0.2 percent THD. You can do a direct comparison only if both are measured at the same THD percentage.
Impedance: Almost all amplifiers are rated at 8 ohms impedance (a measure of electrical resistance), so you can compare ratings this way, but a few are also measured at 6 or even 4 ohms. These lower resistances can give an artificially high power rating — be suspicious.
Not all amplifiers built into receivers (or even in separate power amplifiers) can power 4-ohm speakers without overheating, popping a circuit breaker, or just plain breaking down. Check to see whether a receiver can support these lower impedances if you choose speakers that require it. This is rare unless you are buying high-end gear — most home theater speaker systems are rated at 8 ohms of impedance (though the actual number varies as the speakers reproduce different frequencies).
Frequency range: Lower frequencies (the bass frequencies) require more amplifier power than higher frequencies. Because of this, some receiver manufacturers test their systems not at the full 20 to 20,000 Hz range, but with a limited range (such as 40 to 20,000 Hz). This can also create an artificially high power rating.
Number of channels driven: Home theater receivers should be capable of driving at least five speakers (some have amplifiers for six, seven, or more speakers for extra surround channels). Power ratings should state how many speakers are being driven when the system is tested. Preferably, all channels are driven simultaneously at the stated power. Some systems give power ratings in stereo mode (with only the front left and right speakers driven), which means that the power with all speakers being driven is less than the stated amount.
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Source:http://www.dummies.com/how-to/content/how-to-decipher-amplifier-power-ratings.html
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