You only need the low E,A and the D string, sometimes, to do basic power chords depending if the root is on the 6 or 5 string. It looks like a barre chord but, in reality the index is only holding down the 6 or 5 string. The rest of the strings are being damped so most of the index finger is resting on the strings with no pressure. Your other finger or fingers hold down the other one or two strings.
Go to the lesson on
C.A.G.E.D chords. The E chord has the root,1, on the 6 string and the 5 note on the 5 string.
Slide the chord up one fret and just barre across the 6 string first fret while holding down the 5 string third fret and you have an F powerc chord,. Just move that configuration up and down the neck to change chords, The note relationships remain constant.
Same with using the 5 string for the root of the power chord. Look at the A chord in
CAGED. The root on the 5 string and the five on the 4 string. Those are the two most common positions for playing power chords.