A lot of it comes down to what you own and the quality of it.
Most people start worrying about things like this when they have a sub $1,000 set of speakers and a lower end receiver and think that any of it somehow applies to them, when the reality is that the only time you are likely to be having power issues is when you start getting into the $1,000+ per SPEAKER price range with dedicated amplifiers or top shelf A/V receivers which can truly deliver 140+ WPC when cranked up.
The specifications for receivers are usually listed on their website or in writeups as being 'two channels driven', but it can be confusing. But, for most circumstances, the reference ends up being pretty honest about what you should expect under normal surround sound or stereo listening conditions for power output availability.
If you are thinking of getting, or already own, a set of top shelf speakers, then any number of companies, such as Emotiva, offer dedicated power amplifiers which can help to bring the most out of the speakers in use.
But, a better speaker on a weaker A/V receiver will far outperform a weaker speaker on a great amplifiier.
Then you can start getting into the REAL sea of information when it comes to room acoustics and how incredibly that impacts audio performance.
Trust me on this, you aren't in the sea yet, you're still standing in the wading pool.
