How much power do I need? Find out using this method...

I've been hanging out on diyaudio and ran across this simple technique to measure the power used by your speakers.  So many discussions here revolve around this question that I thought I'd post the method...

If you have a multi meter, you can use the test tone & technique provided here to calculate how many watts your speakers actually use:

It’s super informative & useful: all you do is set your volume at the max listening level you use, then play the -12db 120hz test tone while measuring voltage at the speaker terminals. Square that value and you’ve got the maximum watts needed. Plenty to read at the link...
And as I just posted in another thread: if you don't have a DMM, they're worth having around... I bought this one for $25 and it does just fine.
Thanx cal3713. Great link! Every audiophile should have a meter if just to test fuses and continuity in other devices and as you found out relatively 
decent ones can be had for little cash. Another nice tool to have is a dB meter to check volume levels. This one cost a whopping $15.00
If you want to test the effectiveness of your tube traps just put on a 30 Hz test tone and walk around your room with the meter. If your variation is less than 3 dB away from the walls you are doing a great job.
Not really. If you calculate the power consumed at that particular volume knob setting you can double that power value and there will be only a 3 dB increase in volume. Even though it’s twice as loud it is almost imperceptible to our ears -- at least 6 dB increase in SPL will make it noticeably louder, which would be quadrupling that calculated power.

A slightly more accurate way of determining how much power is enough is to use the speaker’s sensitivity vs listening distance with an SPL meter. For example, if you sit 4 meters away and you tolerate a maximum of 90 dB peaks, that calculates to 102 dB at a distance of 1 meter from the speaker (halving the distance is a 6dB gain). If the speaker has a sensitivity of 90 dB at one meter, then the power required to raise it 12 dB at one meter is 16 watts (90 dB/1w, 93 dB/2w, 96 dB/4w... ).

However, that would be in a theoretical room. Real rooms have things in it with absorbing and reflecting surfaces that throws a wrench in those calculations. Best go by the speaker mfr’s power rating limits.
Most stereo systems are played too loud in an attempt to approximate live levels.This only leads to listening fatigue. I like average levels around 70 - 80 db, with peak levels about 90 + Beyond that my ears start to shut down!
A peak level 10 db above average requires 10 x the power in watts, approximately.
For a speaker rated at 90db:
1 watt should produce 90db
2 watts 93 db
4 watts 96 db
8 watts 99 db and so on...
Keep doubling the power to gain 3db.
Every 10 db should double the "perceived" volume.
roberjerman, your ears are shutting down because of distortion not volume. I routinely subject innocents to 95 dB after appropriate warm up.
They start trying to talk to you not realizing how loud it is. Now most rock concerts are up at 110 dB but that volume is unnecessary to get the feeling you are there without damaging your ears. 95 dB is perfect. But, very few systems can do that effortlessly.
gs5556, I can easily hear one dB. 3 dB is painfully obvious and 6 dB is an earthquake. You need to live with a system that accurately measures volume in dB or a dB meter. Volume at distance is more complicated because you are listening to a stereo pair. As you move closer to one speaker you move away from the other. Volume one meter out from one speaker is almost the same at the listening position. Just use the meter at the listening position. That is where it counts. Measuring power of the speaker terminals with a high impedance meter is perfectly appropriate. It won't be a perfect number but close enough to give you an idea what is happening.   
I'm confused with the 6db being an earthquake?  Are you referring to 95 as the base line and +6db as the earthquake?  My system isn't so unusual but 95 db is a joke.  My Coincident speakers are 94db at 1 watt.  My 325 watt Mac is overkill to attain 95db.  My other amps are 100 watt tube monoblocks which obviously have different distortion than the Mac.
Would I call 95 loud?  Sure.  95 for a movie is more likely.  Dynamic headroom is a big difference.  No one listens at an exact sound pressure level the whole time.  I'm a bit confused.
Hi elevick. You have solved the problem by having very efficient loudspeakers and with your Mac you should have no problem making your ears bleed as long as the speakers can perform at those volume levels. Many if not most can't. But I have no experience with your speakers. The problem for most people running those levels is that either the amp is clipping transients or the speaker is distorting. 
I was responding to gs5556's comment about differences in perceived volume levels. To any listener a 1 dB increase in sound pressure is obvious, 3 dB is a rather large increase and 6 dB is a huge increase. If I were to increase the volume 6 dB on one speaker only the other one would literally disappear. Try it. There is another issue with point source speakers. They way the cast an image makes them sound surreal at loud volumes. It is like having the volume of the 6th row back in the 20th row.
Line source or linear arrays solve this problem by casting a much larger image so the volume and the row match up. 
In no way do I listen to all music at 95 dB. Also remember that the rating of your speaker is at 1 meter. I listen at the volume the music at hand sound right. There is a rare record that I will listen to at 100 dB like the Dead's Infra Red Roses. What fun to blow peoples brains with that one. If you want to know how your system and house deal with very low frequency transients get that disc. It is a selection of drum solos. Every recording has a "right" volume level depending on the type of music and the way it was mastered. I miss spoke. Some recordings have no right volume level usually because they are mastered poorly. As an example I was listening to Paul Simon's Graceland last night. The instruments are way out in front of his voice like he is standing in the next room over. So either the band is too loud or his voice is too far away. I love the music but it could have been mastered better or at least more realistically. Most people think this is a great recording. Waiting For Columbus is an excellent job all around. Turn it up and the band is right there everything in perspective. It was probably mastered off the sound board. For studio recordings check out The Smiths, The Queen is Dead. Great job! Waltz for Debbie I listen to at 90 dB. These are peaks by the way, peak volume measured with a dB meter. My preamp measures volume in dB but that has nothing to do with absolute sound pressure levels. dB is a relative term.