What is a watt anyway?  It's a unit of measure of electrical power consumption.  It is NOT a volume level.  So why does this question come up so much?  Judging a speaker's maximum sound level by it's wattage rating is akin to estimating a car's top speed by it's fuel economy.  A speaker's function is fairly simple. A speaker converts electrical signals into acoustical energy: sound. By moving back and forth, the speaker increases and decreases the air pressure in front of it thus creating sound waves. A speaker is simply a motor (cone and coil) actracted or repelled to a permanent magnet, in relation to a fluctuating electrical signal input.  In live sound, this signal starts as sound waves, converted to an electrical signal by a very similar - yet much smaller - motor (a microphone capsule). Once the sound waves are turned into electrical signals, they are amplified, equalized, manipulated in various ways in an audio mixer, and then combined into a signal that is strong enough to drives the speaker movement.  This electrical signal is not constant in amplitude, but rather is a series of short bursts of energy cruising along with the rhythm of the input music.  

As far as electrical motors go, speakers are fairly inefficient at transferring electricity to acoustic energy, with the wasted energy being turned into heat. It requires significantly more energy to recreate lower (bass) frequencies than higher (treble) frequencies of music.  For events without the need for low-frequency support (perhaps a training seminar, awards ceremony or wedding reception), the lack of need for ultra-low bass response in the system will drastically reduce the overall energy required to loudly reproduce it with faithful accuracy.  When installed in a cabinet, the efficiency of a speaker (or sensitivity) is rated as dB/1w/1m.  This means a speaker with a 99dB/1w/1m sensitivity produces 99 decibels of sound pressure, measured at 1 meter from the speaker, with 1 watt electrical input.  For every doubling of wattage, you gain 3dB of acoustic volume.  A speaker will also have a power handling rating, which is a measure of how much power (watts) can be applied to the coil before it is permanently damaged or the speaker moves too far out of the permanent magnet field (excursion).  A "peak" power handling number would be like moving your hand across a candle flame.  Yes, it's hot, but because you don't hold your hand over the flame for long you aren't burned.  A "continuous" power handling number would be similar to holding your hand over that same flame.  The same speaker might have a 300 watt continuous rating, and a 1,200 watt peak rating.  So what number do we use?

If a speaker has a continuous power rating of 500 watts, and a rated efficiency of 99dB/1w/1m, then applying 300 watts continuous of electrical signal would predictably produce a continuous sound level of 126dB at 1 meter (since 550 watts is 27dBW, added to the sensitivity).  Now what if we had a speaker that was rated at 104dB/1w/1m with a continuous power handling of 200 watts? 200 watts is 23dbW, added to the sensitivity of 104dB/1w/1m, and we would predictably produce a continuous sound level of 127dB at 1 meter.  In this example, the 200w speaker is actually louder than the 500w speaker, while consuming less electricity.  Wouldn't this be a favourable characteristic, especially if the entire system needs to run on a single 20A power circuit?  The other speaker would make a better space heater, but that's not why we brought it to the event in the first place.  Adding drivers (more speakers) increases efficiency, but only if they can be positioned close enough to each other for their sound waves to sum coherently, which is simple with low frequency cabinets (subwoofers) but increasingly difficult with the middle and high frequencies without reducing overall clarity and intelligibility.

Another important part of this equation is the loss of sound over distance.  I think I can confidently say that most people would agree that things are quieter when they are further away.  A perfect point-source of sound, outdoors with nothing for the sound to bounce off of,  loses sound pressure at a rate of 6dB for every doubling of distance.  So if the speaker can produce 140dB at 1m, then at 2m it would be 134dB; at 4m it would be 128dB; and at 8m it would be 122dB.  There are other factors at play here, such as the effects of boundaries (think the walls and ceiling of a room), and high frequency losses through air depending on temperature and humidity, but those are the general rules that we have to deal with in nature.  

To have a minimal difference in sound level between the front and back of the room, the speakers need to be high in the air.  The difference in sound level between a speaker 6' off the ground or 12' off the ground, as heard from 100' feet away, is negligible since the distance hasn't changed much in ratio.  However, the difference as heard from 6' feet away would be VERY different, since the speaker distance has doubled.  By using high trim heights, we can reduce the difference in level between the closest listening position and the furthest listening position, for a more consistent listening experience throughout the area.

If you want the simple answer, the "peak" wattage capacity of our entire sound rig is 26,257 watts including sound processors and mixers, but I still don't understand how "useful" that number is without knowing where it came from.  A much preferred unit of measure would be decibels, at a specific distance, with a specific weighting applied, and a specific meter response time.  For example, most outdoor music events with mixed age audiences prefer sound levels below 90dB(A), slow, at mixing position.  The SLOW part of that measurement does not include the fast peak bursts that make live music so dynamic and lifelike.

Something else we need to determine is the average power level. Music using shorter bursts of high energy with quieter parts in between may have a low average power level (typically referred to as RMS or root mean square).  Music with high energy from start to finish (think some genres or dance music, some pop music, heavy metal, grunge, heavy blues and punk music) have a much higher ratio of low to high volume, and thus the continuous power handling required by the loudspeaker system in relation to its peak capacity will have to be higher.  When a voice coil in a speaker motor assembly is given short bursts of energy with times or lower energy between (like a conventional kick drum hit in rock and roll) allows time for the voice coil to cool off between uses.  When this average power is increased, the voice coil temperature increases.  Increased voice coil temperatures can lead to mechanical failure, but first they will increase in resistance within the electrical circuit that is the amplifier and speaker, thereby decreasing the power applied to the voice coil.  This is known as power compression.  What this means is that as a loudspeaker reaches the upper end of its power handling capabilities, you will reach a balance point where adding more power will actually reduce the overall volume while increasing the chance of damage to the driver. The higher the average power level, the sooner this power compression will set in.  This is why careful attention to the TYPE of music intended to be played is important when selecting an adequate loudspeaker system.