Amplifier output is measured as electrical energy at the load (speaker). The amount of acoustic energy you get depends on the efficiency of the speakers (converting electrical power to sound), not the amp.
Amp efficiency comes to play on the INPUT side. A typical power amplifier running class AB1 is only 50% efficient. That means a 500 watt output amp draws 1000 watts from the battery, and loses 500 watts of heat to the heatsink.
Newer technologies are more efficient, but I'm not sure by how much.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's