Root on A string
x355xx
C5 is the C power chord. Its notes are C and G. No third means no major or minor quality. Mute unused strings carefully.
x355xx
8-10-10-x-x-x
Numbers show frets from the low E string to the high e string; x means don't play that string.
Every C and G on the first 12 frets. The filled dots are the root, C. Build your own shape from any nearby group.
Other C chords
Other power chords