[T.B. Yoits]

For the benefit of non-musician readers / listeners, please diagram the first and fifth note making up the power chord in various keys.

The C Major chord, for example, would be the Root, 3rd, and 5th notes.
5th -   G
3rd -  E
Root - C

The C minor chord, for example, would be the Root, 3rd, and 5th notes.
5th -   G
3rd -   Eb (flat) - also thought of as lowered by a half-step.
Root - C

The "Power Chord" has the advantage of being ambiguous since it's neither major nor minor because of the missing third.

The C "power" chord, would be the Root and 5th notes.
5th -   G
Root - C

Across all Root notes based on the white keys on a piano, a simple chord chart would be:
5th -    G      A      B      C      D      E      F# (sharp)
Root - C      D      E      F      G      A      B

Of course, chords could also be formed with the Root notes of C#/Db, D#/Eb, F#/Gb, G#/Ab, A#/Bb, which happen to be the black keys on a piano.

[chajin]

Put more simply, it’s what medieval music was until sometime during Ars Nova, before the Black Death. Back then, the power chord was what longhairs still call it, the perfect fifth—”perfect” because it could be tuned to a pure 3/2 frequency ratio. By the time of Howlin’ Wolf equal temperament had gummed up all musical tuning for two centuries, and so the the power of the power chord resided, not in its tuning, but in it sense of emptiness that drove the music forward to some choral conclusion.

[doorgunner69]

No wonder I got nowhere trying to learn to play

[Nullygrub]

As a guitarist, I was taught to play power chords as root, fifth, root with the second root note being an octave higher than the first. When you add distortion, the resulting harmonics produced will fill in the rest of the chord.