How many watts do you really need?

First, decide how loud you want your system to play (in dB) at the times when you really feel like rocking out (see below).  Second, use one of the on-line system loudness calculators that allow you to input the appropriate information about your specific speakers, amp, and room.  Calculate the output level in dB predicted for your own speakers, amp, and room, and compare that with the loudness you initially decided that you want your system to be capable of delivering. You can then use a SPL meter, or one of the apps like “Decibel X”, to compare whether the loudness level your system is actually delivering aligns with the level predicted by the on-line calculator, and to help you decide whether your system is delivering an output level that satisfies your listening tastes.

The predicted loudness for my system is about 105-106 dB and I probably rarely exceed 100 dB.  Involved listening for me is in the range of 75-85 dB with occasional higher peaks.

Consider the following information when deciding on the output level you are looking for:

Your Ideal Sound Level

• Casual Listening / Background Music: 65 to 70 dB. This volume is comfortable for conversation and won't overpower the room.
• Active Listening (Music): 70 to 85 dB. This allows you to clearly hear micro-details, soundstage, and dynamic ranges.
• Home Theater / Reference Level: 85 dB average, with peaks up to 105 dB.

Protecting Your Hearing

Human ears are more sensitive to mid-range frequencies, and the louder the sound, the faster it can cause permanent damage to your hearing.

• 85 dBA: The recognized threshold where prolonged exposure requires hearing protection. At this level, hearing damage can occur after a few hours.
• 100 dBA: Hearing damage can occur in as little as 14 minutes.

I think of headroom as an amp that simply loafs along meaning the amp operating at their best sonic level and not strained often leading to increased distortion.

As always, match the amp to the speakers for best sonic performance results.

@kennyc 

"I think of headroom as an amp that simply loafs along meaning the amp operating at their best sonic level and not strained often leading to increased distortion.

As always, match the amp to the speakers for best sonic performance results."

Say you choose a speaker rated at 86 dB at 1 watt, one meter (86 dB being quite loud already) and consider there will be a slight loss from the front of the speaker to your listening position and then room gain (boundary effects) will compensate for some of that loss. Also, the bulk of the program from any given source will only require microwatts to produce a comfortable listening level and 1 watt will be quite loud for most peaks, then any power required to hit levels above 86 dB will be headroom.

10. 

As mentioned earlier, depends on the listener. I had a class set amp with 8 watts pushing 92db speakers. Sounded great but I felt it lacked ooomph when playing certain genres of music. I then picked up a class ab 20 watt amp pushing 98db speakers, fantastic. Amp “pushes” music powerfully through the speakers with all genres. For me, way more enjoyable.