audio set-up for beginners [continued]

[red_one]

at what max power/rms of a speaker do I have to install an amp?

just like what jhnkvn said, it all depends on your listening level.

example, if you have 45W RMS speakers and head unit with 20W RMS power output, and you are already listening at full volume yet still not loud enough for you, then that is the time to add an amp.