All a "power chord" is, is a triad (or diad, or whatever it's called) of notes... here is an example of a G power chord (in two different positions). Most commonly, power chords are played on the E, A, and D strings.

Full Power Chord : Root Note, Fifth, Octave
Half Power Chord : Root Note, Fifth

eg.
e |--------------------|
b |--------------------|
g |--------------------|
d |--5--G-------------|
a |--5--D------5--D---|
e |--3--G------3--G---|

e |--------------------|
b |--8--G--------------|
g |--7--D------7--D---|
d |--5--G------5--G---|
a |--------------------|
e |--------------------|