"Output power" shows how loud your amp can drive your loudspeakers. If you own a small room then you don't need much more than a few watts. If you wish to install speakers outside or in a live concert then you will need several hundred watts of power. For best music quality, you might want to go with an amp that has higher output power than you require as most amps are going to show increasing distortion once the music power increases.
Several specifications will provide the power in "Watts peak" while some others will display "Watts rms". "Peak" means that the amplifier is able to deliver the output power for a short period of time only whilst "Watts rms" means that the amp will constantly provide that level of power. In the past, vendors have customarily favored displaying the "peak power". This figure is bigger than the average or "rms" power. However, "peak power" can frequently be confusing because there is no standard showing the period of time that the amp has to be able to output this amount of power. Nowadays most amplifiers are going to specify rms power that provides a better indication of the amplifier's real performance. However, please make certain that your amplifier has adequate headroom to stay away from clipping of the audio. The most important reason is that music signals will have brief peaks of high power which the amp has to deal with. Rms power is measured with a uniform sine signal which hardly compares with a music signal regarding the power envelope. Please note that often the peak output power of the amplifier is going to depend on the impedance of your loudspeakers which is commonly between 4 and 8 Ohms. Amps have a limited output voltage swing due to the fixed internal supply voltage. As such the largest output power will vary depending on the speaker impedance. The lower the loudspeaker impedance the bigger the highest power the amplifier may output. If the maximum output power is not referenced to a speaker impedance, you ought to call the manufacturer. Typically a 4-Ohm loudspeaker is utilized as a reference.
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