Power chords are just a root and a fifth, generally played on either the 6th and 5th, or 5th and 4th strings. They sound cool with distortion because the tones are far enough apart that they don't clash together when you play them, most other chords sound like crap with distortion. If you haven't done so already, I really recommend learning all the notes on the E string as these are your root notes. From here you'll always know what name the powerchord you're using.



E5 F5 F#5 G5 G#5
A 2 3 4 5 6 and so on...
E 0 1 2 3 4