Rubbish! Mostly the parameter that tells us more is usually well down the lists of characteristics and that is the db rating of speakers. I often read that. "My speakers are 100 Watts so I need a 100 Watt amplifier" more Rubbish if anyone did actually put 100 Watts into them the voice coil would instantly burn out.
I'll take as an example an 85db rated loudspeaker, at 1 metre and with a 1 Watt input the output will be 85db. For every 3 db increase in volume twice the power is needed, sounds awful, but where is the starting point?
Then there is the fall off rubbish for every metre you are away from your speakers the sound drops by 3db and you will need double the power, e.g. 2 Watts to achieve the 85db. These measurements are made in an Anechoic room the techie equivalent of a large flat field and no wind.
Since we do not listen in the above but in rooms there will be reflections from the walls and the fall off will be very little. There is an Android app that will sample db down to 0.4 seconds there is also an iPad/iPhone one that is not quite as good.
Try it in your room at your normal volume level and walk slowly from your listening position to your speakers.
Here is a reading sampled at 0.4 sec and for a duration of 30 secs in my little room at the listening position. The volume from an electronic music CD at it's sustained high level section was very loud.
The converse of the 3db increase is also to be considered with every 3db in volume reduction using half the power.
85db at 1 volt and 1 Watt. My CHP70 speakers in Frugel Horn Mk3 cabinets.
82db @ 0.5 Watts
79db @ 0.25
76db @ 0.125
73db @ 0.0625
70db @ 0.03125
67db @ 0.01562 or 1/64th Watts This is nearest the average db reading above.
Find the db rating of your loudspeakers and please do the simple maths above. If you have speakers at or above 90 db all you would need is a one Watt amplifier.
You will realise now that you are listening to 1/64 to 1/32 Watts most of the time, occasionally a loud combination of notes interacting with each other will produce an instantaneous volume spike or transient and the volume might jump by 15 or even 18 db. So worst case, 85db minus 67db = 18db so an 18db spike will need 1 Watt!
You can clearly see now that amplifier Watts are Rubbish of the worst kind, and often an excuse is made for valve or tube amplifiers giving 5 or so Watts which are 'different' !!! Another load of rubbish, output Watts are the same no matter how the amplifier is made.
Warning 85db is on the threshold of ear damage do not listen at this level for anymore than 20 minutes and then allow 24 hours for your ears to recover.
Anything above this and the damage will be permanent.