Well, let me see if I can explain it this way using actual impedances which work "behind the scenes" with a constant voltage system.
Lets say we have a 10 watt amplifier with the 8 ohm output directly connected to an 8 ohm speaker. One would expect all of the 10 watts to be delivered to the speaker.
Now say we have that same 10 watt amplifier with a 70.7 volt output. In many cases the 70.7 volt output is created using a transformer similar to the ones on the speakers but in reverse- the 8 ohm winding is connected to the 8 ohm output of the amplifier and the high impedance winding is the 70.7 volt output.
If you do the math you will see that the 70.7 volt winding (output) impedance is 500 ohms at 10 watts. So, to deliver that 10 watts to a single speaker we would need a transformer on the speaker with a 500 ohm primary and an 8 ohm secondary.
Now suppose we had two speakers and we wanted to supply them with 5 watts each. The taps on the speaker transformers would need to be 1000 ohms.
To take this a step further now suppose we have a 100 watt amp. The 70.7 volt output will be 50 ohms. Hang that same speaker with the 1000 ohm tap on it and you will still get 5 watts from the speaker.
So the "secret" behind the constant voltage system is that as long as we are talking about the same voltage (hence constant voltage) you can simply mark or identify the taps on the speaker transformer primaries according to the wattage that will be provided to the speaker.
You don't have to worry about the impedances of the amp output or speaker transformer taps, that's all been done for you. Rule of thumb is you simply add up all your speaker wattages and provide an amplifer at least 10% larger to account for transformer losses and head room.
Is 2.5 watts kind of low? Depends. In Yankee stadium yes, in an office probably too loud especially with 5 speakers. Keep in mind that the amp level is usually turned down below maximum output so you won't even get 2.5 watts per speaker except perhaps at low (bass) frequencies if the transformers you used have a frequency response that low. But that's another story.
-Hal
