I have bought a DIY 2 way speaker kit (Seas Indunne) and cant figure out how many watts I need out of a channel to drive it.
It uses a woofer that is rated for 80 watts long term/250 watts short term and a tweeter that is rated for 55 watts long term/150 watts short term
It depends on how you intend to listen to your system, how you decide on the listening level, and where you play it. Obviously you may tend to play it louder in an open space or hall, than in a living room.
There is a school of thought that says the amplifier power capability should either be a bit more, or a lot less, than the speakers. This assumes you sometimes set the listening level by turning it up until you hear distortion, and then back off.
If the speakers limit by distorting, as the suspension runs out of linear travel, you would hope that you are still within the thermal rating of the voice coils. Turn it down then, and all is good.
If the amplifier is slightly smaller than the speakers, then you might be tempted to turn it up until the amplifier limits by hitting the rails. That generates a whole slew of high harmonics at the full power of the amplifier that pass straight through the crossover to the tweeter. By the time you've spotted the distortion it's already too late, kentucky fried tweeter. If the amplifier is a lot smaller than the speakers, then the tweeter should be able to handle this abuse.
Big 'system' amplifiers often have soft limiting, that turns the wick down automatically while still short of clipping to avoid this problem, whether a smaller domestic one would is another matter.
