Elsewhere, I have seen regular discussion of the relative power outputs of different amps.
The issue is that some makers use RMS power, some use intermittent RMS power, some use RMS before the onset of distortion, some use RMS with various degrees of distortion, some use the theoretical power if outputting a square wave and some use theoretical transient power measured in various ways...
This is particularly true of class D amps where some astonishing levels of output power get claimed.
However, what we are really interested in is how much power can an amplifier consistently put out over an extended period (e.g. a whole song).
One way is to consider the amps power consumption, as this should be based on real world figures, not hand waving. Power out cannot exceed power in for any extended period. Input power may be given in Volt Amps (VA) in which case a power correction factor of 0.65 to around 0.8 depending on the quality of the power supply will need to be applied to derive watts.
I will compare two amplifiers:
Bugera Veyron. Claimed output power: 2000 Watts @ 4 Ohms. Rated input power: 110 Watts.
Orange Terror. Claimed output power: 500 Watts @ 4 ohms. Rated input power: 690 VA. (Volt-amps, multiply by a power factor of between 0.65 and 0.8 to convert to watts, so 480 to 550 watts).
Assuming both class D amps have similar efficiencies, the Orange is consuming 4-5 times the power of the Bugera, which is rated as being four times as powerful as the Orange.
Even allowing a dozen watts for the Orange's valve preamp, I smell a rat...