Jerome will probably have a better answer, but a power chord is simply a root note and the 5th above the root (a 5th is 7 half-steps above the root). Sometimes the root tone is doubled one octave above. Power chords simply omit the 3rd from the normal chord structure.
I believe the way to notate a power chord is with the root note and a 5 - - - so, an 'A' power chord would be A5. A 'G' power chord would be G5, etc.
These Root + 5th chords are also called 'open', 'bare', or 'empty' fifths in traditional musical naming. Shape Note singing (like the old Sacred Harp & Southern Harmony hymnal stuff) uses open 5th harmonies extensively.
"That darn Pythagorean Comma thing keeps messing me up!"http://en.wikipedia.org/wiki/Pythagorean_comma