I remember back in the 80s a M8 of mine had an Amstrad Tower System that he proudly told me was 500W... It even said it on the box.
But that was the only quoted figure, and it said in tiny letters, PMPO. So things haven't changed!
By contrast, my hifi power amplifier comes with an individual test certificate for that particular amp, signed by the tester, which quoted a whole raft of figures for outputs at various loads and frequencies, slew rates and who knows what else+ the most exciting of which was 962W/ch @4ohms for 1KHz with THD <0.05%. There was also a 20 year guarantee, which has just run out!