The most accurate and meaningful rating method for amplifier output power is the so-called RMS rating, which would read like this for a 150-watt amp:
"150 watts per channel, both channels driven, 20-20kHz, at 'x' percent distortion". (Note: for solid state amps, distortion should be below 0.5% -- distortion for tube amps will typically run somewhat higher).
This rating system specifies that the amp will produce a given output across the audible frequency range, at or below a certain distortion level, with both channels driven (driving only one channel is a much less stringent standard). Some amps do not have good power supplies, and thus will produce less power at the frequency extremes (which means that deep bass frequencies will typically not be well reproduced).
The other rating standard is "peak power", sometimes called dynamic or transient power. This rating reflects the power output capability of the amp under brief loads, such as orchestral peaks (rather than steady-state power demands). Most good solid state amps will have dynamic or peak power output that ranges from 50% to 100% higher than the RMS output. While the dynamic output is useful to know, the RMS rating is much more important.
In Europe and Asia, amplifier power ratings are more frequently quoted as dynamic or transient/peak power. The problem of amplifier power becomes a bit more complicated when rating tube amplifiers, since tube amps are essentially voltage sources, rather than current sources. Tube amps may clip somewhat more gracefully than solid state amps as they approach their maximum output. Tube amps may also produce higher total amounts of distortion than solid state amps, but the distortion occurs in different harmonic orders that are (supposedly) less objectionable to human hearing.
"150 watts per channel, both channels driven, 20-20kHz, at 'x' percent distortion". (Note: for solid state amps, distortion should be below 0.5% -- distortion for tube amps will typically run somewhat higher).
This rating system specifies that the amp will produce a given output across the audible frequency range, at or below a certain distortion level, with both channels driven (driving only one channel is a much less stringent standard). Some amps do not have good power supplies, and thus will produce less power at the frequency extremes (which means that deep bass frequencies will typically not be well reproduced).
The other rating standard is "peak power", sometimes called dynamic or transient power. This rating reflects the power output capability of the amp under brief loads, such as orchestral peaks (rather than steady-state power demands). Most good solid state amps will have dynamic or peak power output that ranges from 50% to 100% higher than the RMS output. While the dynamic output is useful to know, the RMS rating is much more important.
In Europe and Asia, amplifier power ratings are more frequently quoted as dynamic or transient/peak power. The problem of amplifier power becomes a bit more complicated when rating tube amplifiers, since tube amps are essentially voltage sources, rather than current sources. Tube amps may clip somewhat more gracefully than solid state amps as they approach their maximum output. Tube amps may also produce higher total amounts of distortion than solid state amps, but the distortion occurs in different harmonic orders that are (supposedly) less objectionable to human hearing.