What some guitarists call a power chord is nothing but a root/fifth interval and IMHO should not be called a chord.
And what makes a fifth interval so powerful anyway? Are two notes more powerful than 3?
I am a guitarist, so I can add some explanations.
When we refer to a power chord, it don't make any reference to the construction of the chord (the fifth interval) but to the physical reference of the sound, that is the loudness of the chord.
Indeed, power chords are always played on a guitar with many effects pedals - mainly distorsion pedals, fuzz pedals, overdrive pedals, etc...
So you get a very powerful sound. We use that in hard-rock music, metal music, even in pop-rock music ...
We are here very far from classic definition of the chords, we are also very far from the standard instruments (brass, woodwinds, harp, piano, etc): we are in the guitar world...