The fact that different manufacturers, when rating amplifiers, state output power at various supply voltage, load impedance and levels of distortion, makes it impossible to compare the amplifiers according to these measurements.
Primary Output Power should, according to CEA2006, be measured with 14.4V DC supply, a 4-ohm load and with 1% or less total harmonic distortion in the output. Other specifications included in the standard involve different impedances and supply voltages. This is where the manufacturer can indicate the conditions which make their amplifier perform optimally.
Since a loud speaker is much more complex than a resistor, it is not accurate to do measurements only using the rated resistive load (typically 8 ohms). This may result in harmonic distortion and strictly reduced voltage when connecting a real-life loudspeaker to the amplifier.
An 8 ohm speaker can have dipping up to 2 ohms. Therefore a more accurate way of testing an amplifier would be to do it along with shifting impedances. One method of measuring an amplifier’s output power, in an exhaustive and equitable way, is through the use of a PowerCube