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.
Technically, a power chord isn't a chord at all. It's an interval. Chords are made up of three or more notes, whereas a power chord only has two.
The "first" and "fifth" denotes where that note lies in a given major scale. The simplest example is in C, where the scale is C D E F G A B. The power chord is the first and fifth notes in that scale, thus C (the lower sounding note, or root) and G (the higher sounding note.)
Other common examples would be in G major (G A B C D E F#) where the first is G and the fifth is D. E Major (E F# G# A B C# D#) would be E and B. F major (F G A Bb C D E) would be F and C.
Most of us Non-Musicians would know what we were looking at, even if the power chord were properly diagramed.
If we heard it, many of us would get it.
Simplest explanation.
Doe Ray Me Fa Sew La Tea Doe
Doe being the First.
Sew being the Fith.
Play both notes cranked on 11 and you have a power cord.