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Look at the datasheet of one of the many class-AB IC power amplifiers like the LM3886:
1) When its output is almost at clipping driving 50W into 8 ohms with a plus and minus 35V supply, it dissipates 31W.
2) When the output is 30W into 8 ohms and everything else is the same as above then it dissipates 34W (its max dissipation).
3) When it is idling without a signal with a plus and minus 35v supply, its current is typically 50mA then it dissipates 2.8W.
With a 4 ohm load its output power and dissipation are more.