I for one have just went by the watts at 1% THD or less into whatever speaker impedance I am working with and called it close enough. Same with speaker average power ratings and whatever SPL they have.
Low amplifier wattage ratings at 1% THD and a high peak power rating at some unrealistically low speaker impedance and high THD value (1000 watts into .5 ohms @ 10% THD) tells me an amplifier is likely way over rated.
Same with how speakers that have a low SPL value but high average power rating say to me that although the speaker may handle a lot of amplifier power it does it poorly so it will not be any louder than a high SPL low wattage rated unit for what being fed to it.
There were many many amplifiers marketed for audio competition designed to output drastically more power into low impedances than into higher impedances, and the good brands were under rated. The reason is in car audio competitions, the classes were divided up based on manufacturer rated power into 4 ohms at 12v. The good amps maintained reasonable THD however.
For example, the Phoenix Gold MPS2240 was rated at 24 watts / channel stereo at 4 ohm, but 288 watts mono into 1 ohm. So with 4 of these you could enter the 0-100 watt class, but have 1152 watts to work with: