Simple answer? My opinionUmmm ... I know this is ASR and "pedantic" is kinda in the motto of the place - but the question was "How much power does one need?" And I believe that the 9/10ths answer was adequately provided... Was it not? If not, can we get another explanation/formula for us plebes to use.
If you listen at low volume < 75 dBA
Middle of the speaker manufacturers recommended range
Louder
Higher end of range.