and therefore 
you can see how a quick figure for power can be calculated.
A "normal" amplifier's output stage will not be able swing completely between the supply rails. By "normal" I mean a conventional class AB device.
By ignoring the limitations of output voltage swing we can easily arrive at a figure for power which is, probably for most people, good enough.
We assume, then, that the amplifier's output voltage can swing to both rails.
Given that and therefore you can see how a quick figure for power can be calculated.
For example, an amplifier runs from a 9V battery. Assume an output swing of 9Vpp and a 'speaker impedance of 8Ω, which gives us 1.26W.
Another amplifier has supply rails of ±15V. A 30Vpp output swing would develop 14 watts in an 8Ω 'speaker, or 28W in a 4Ω one.
Because of the assumptions regarding the output voltage, the figures are a little optimistic, but nevertheless useful.
The two graphs below will even save you the trouble of the simple calculations.
If two amplifiers are used with a "bridged" load, then with a couple of caveats, four times the power obtained from the above graphs may be realised.
The caveats are that the power supply must be able to deliver twice the current that it would otherwise do, and so must the amplifiers.