Power chords can be moved up and down the fretboard. In effect, we are transposing the power chord when we do this. When we move to a different fret we use the root note to determine its name.
Take a look at this power chord pattern. The note on the low E string is its root note:
Now, by forming this pattern on the 1st fret we are creating an F power chord because the root note is on an F note.
We can play this pattern all the way up the fretboard creating different power chords.
Here it is with the root note on the 3rd fret. The note on the 3rd fret of the low E string is a G, so that makes this power chord a G.