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There are two standards that make it easier to compare amplifier output ratings the FTC and EIA ratings standards.
The FTC standard was established by the Federal Trade Commission and requires that a manufacturer’s stated power ratings must meet the advertised frequency range (usually 20 Hz to 20 kHz) at the rated THD (Total Harmonic Distortion) with both channels driven.
The EIA rating, established by The Electronic Industries Association, is the power output of a single channel driven at typically 1 kHz with 1% THD clipping. The EIA standard will typically be 10 to 20% higher than the FTC ratings.
We go one step further than the EIA ratings and drive both channels as we feel the fairest, most honest, and most revealing rating method is the obvious one: to specify power with all channels fully driven, as this is a more real world scenario. When fewer channels are being tested, the more power the ones that aren’t being used can deliver. This can artificially increase the apparent power output. By testing with both channels driven we are decreasing the standard 10-20% difference of one channel driven.
What does a 10-20% power difference mean in reality? For the average person to hear a difference in loudness/level, you would need to double the power to the speakers. You can hear the difference between 200 watts and 400 watts, a 3 dB increase or doubling of power, but you won’t hear a difference between 200 watts and 240 watts as small difference in output power are not discernable to the human ear.
To put it in a nut shell we basically provide a more conservative power spec of the EIA standard because we test with both channels driven during our tests not just one.

EIA este standardul de masurare la 1khz

Most manufacturers, however, don’t publish a 2-ohm FTC spec even if the amplifier can easily handle normal program material into a 2-ohm load. An FTC rating requires that the amp undergo a "warm-up" test, meaning the amp’s output ratings are measured after 60 minutes at 1/3 power and five minutes at full-rated power. Since 2-ohm operation is right on the edge of where an amp’s current-limiting protection circuits will kick in, the amp will automatically go into protection mode before the five-minute threshold. Fortunately, music waveforms are less demanding than constant-level sine waves or pink noise used for testing. Because of this, most manufacturers only list the EIA power rating for 2-ohm operation.

Dupa cum vezi in pdf , diferenta dintre 2ohm EIA si 2 ohm FTC e de 500W , eu zic ca e destul de bine