If you have a loudspeaker rated at 85dB sensitivity (1w@1m) and you sit 10 feet away from the plane of the loudspeakers than you max SPL at the listening position is 110dB if you amplifier provides 1500 watts output. A 500 watt amp could only produce 105dB. Both levels are very loud, but if you wanted 20dB headroom above a loud listening level, say 90dB, then 1500 watts begins to make sense. If all you wanted was 10dB of headroom above the 90dB listening level, then 150 watts would suffice.
I use this site for the calculations.
I use this site for the calculations.