There's also an important use for the diminished fifth power chord. A diminished fifth is a tritone, the most important entity on a dominant 7th chord, formed between the major third and the minor seventh. As the bass player is probably already providing the root, you can use this power chord a major third above (4 halfsteps/frets ahead) the dominant 7th chord's root.
Now, the funny thing about the tritone is that it divides the octave exactly in half, so there are TWO dominant 7th chords sharing the same tritone, their roots being a diminished fifth apart. In practical terms this means you can also use the diminished fifth power chord a minor second below (1 halfstep/fret behind) the dominant 7th chord's root.
If this subject interests you, as the active harmonic principle of a dominant 7th chord is the tritone, jazz players often replace it by its substitute, a diminished fifth apart. For instance, try a ii-V-I chord progression such as Dm / G7 / C. Pretty standard, huh? Now try replacing the G7 by a Db7, like this: Dm / Db7 / C. Notice how the substitution softens the chord's root movement (it becomes chromatic) while retaining the harmonic functionality. This operation is rarely used in rock context, but you might like to experiment.
Ok, enough of that. Let's look up the several voicings for the augmented fifth power chords, with the roots on the 6th, 5th, 4th, and 3rd strings:
And the voicings for the diminished fifth power chords:
Enjoy!
