How much Power do speakers really use ???????

At modest levels, you will only average a few watts. However, those peaks can get up there. If you crank the volume a little and average maybe 10-15 watts(or are using inefficient speakers), the peaks are in the 200+ range. If the amp can't deliver that, then it clips, which compresses the music. This is the primary reason for a big amp, to reproduce high level peaks cleanly, not so much for the steady state power.
As for the big power supplies, etc., it takes a large current supply to allow sustained low bass notes to be reproduced cleanly. Larger caps provide storage for the power supply to draw upon during heavy demands. If you listen at low levels, then this would be a moot point.
Since transformers are rated in VA (voltamps) which is actually watts (volts x amps) it is easy to see how a larger VA transformer can benefit heavy bass loads(since that is where most of the power is used.) The filters (power supply caps) help eliminate ripple (AC that slips through the diode bridge) but also store electrical energy, Try discharging one of those large 40,000uf caps by shorting the terminal to ground after the power has been off a while!
Unless your amp has some sort of mute circuit, the larger the power supply, the longer the amp will play after the power is off. This will attest to a big power supply.
This is a simplistic explanation but I hope you get the picture.
Power supplies have regulation which can be loose or tight. Tight power supplies have low headroom values where loose power supplies can supply substantial peaks. Most amps use loosely regulated power supplies. What this means is, how much current the supply can sustain before it runs out of juice or can it supply demand on a instant basis only.
This translates into amps that can supply higher wattage for a brief moment(peak) but can't sustain steady power. A good example of this is NAD which might rate an amp at 40 watts continuous but can deliver momentary power output of maybe 3 times that. It makes a cheaper amp sound more powerful than it really is.

Pbb, my statement was applicable to any system of any power level, as it did not state any defined number of watts. Even high power amps are not played beyond a few watts nominal at moderate listening levels. At peaks, they use more. At higher listening levels with medium efficiency speakers, even high power amps can clip during dynamics, due to the nature of the db scale. If the speaker is around 90db efficient, and you listen around 90db, then you are only using one or two watts nominal, whether your amp is rated at ten watts or 200 watts. The rest depends on the ability of the amp to handle the dynamic peaks into the speaker load provided. In a situation where the dynamic peaks are 15db higher than that 90db average listening level, you require 32 watts above the average power required for that. That is for the 90db efficient speaker that I described above, and listening at the 1 watt level. If you have that 90db speaker and are averaging 8 watts listening level(~99db) then to handle a 15db peak, you would have to supply 256 watts to handle it. If you had a 18db peak, you'd need 512 watts.

Regarding my personal system performance values, I specified that it was my system, not theirs. My system can handle a 90db average listening level with less than 1/10 watt, and can achieve 15db peaks with about 3.2 watts.

Though the following is oversimplified, it's what I use when thinking about this issue.

I use a speaker with an 89dB/w sensitivity (and ignore how it was measured). I assume that peaks on the order of 15dB must be accommodated (a figure I remember for the human voice, one of the most demanding instruments). One watt will provide a listening level of 89dB and the peaks will require 32w (with a doubling of power as 3dB). Therefore these speakers require the following:

dB---w------peak w
80---.125---4
89---1------32
98---8------256

Using this as a ballpark, my 8w SET into 89dB/w speakers should be capable of about 83dB (.25w with 8w peaks) at one meter. My listening position is twice that, so somewhere around 80dB would be right (neglecting room effects - and my system is in a really small room). Since I listen exclusively to classical music and nearly always to small ensembles and solo instruments, this level suites my listening habits just fine. YMMV

For most applications...a quality 40 or 60w amp will suffice...and be able to drive even difficult loads to surprising levels of volume...I heard Maggies on a 40w Rega at my local dealer...and it sounded good....granted...if you want to "rock the house" or get evicted...you might need more juice...Im no expert...but even doubling your power is a marginal improvement...to really hear a gain...a factor of 3 or 4 is necessary...others will provide more technical answers...but unless you have a very large room with very inefficient speakers...I wouldnt worry about...

All of it!

I have compared the Cello Grand Masters (96db sensitivity) driven by one of their Performance amps which puts out about 1500 watts into this speaker, vs. a bridged pair that puts out about 6000 watts (per Cello). The difference was quite impressive.

You can NEVER have too much power!

Richard