In article , Bill Evans wrote:
Back in the late 1970’s and early 1980’s when I was purchasing quality
stereo amplification equipment for various projects, I became quite familiar
with the techniques used by manufacturers to overstate the power output of
their amplifiers – specifically statements presenting total instantaneous
power, peak power, or other such number-inflating techniques. I had thought
that the practice of requiring power output in RMS to be specified at a
specified distortion level had put an end to that type of power inflation
technique and provided serious buyers with an appropriate yardstick for
comparison.

RMS values of the current and voltage are used to calculate the mean (or
average) value of the power, because these values of voltage and current are
equivalent to the DC values that would supply energy at the same rate, so "mean
power level" is the quantity that should really be specified.

The power output, like any varying quantity, will have an RMS value, but it
doesn't relate to any significant physical quantity so it isn't particularly
meaningful or useful to know it.

I suspect that the common avoidance of the correct terms "mean power" or
"average power" is simply down to the fact that advertisers don't like the
sound of them, whereas "RMS power" sounds much more technical and obscure, even
though it's wrong.

Even "maximum continuous mean power" wouldn't always tell the whole story, as
it doesn't always represent the output level that puts the greatest strain on
the output devices of the amplifier itself.

Rod.