Watch the 1 5 1 Structures online guitar lesson by Brad Carlton from Guitar Lab: Open String Universe
This chord is A root 5th root structured rooted off the 7th fret of the 5th string. The 2nd string open is a B which is the 5 in the key of E. The 1st string open is an E which is the root in the key of E. The total formula of this voicing is 1 5 therefore it is an E5. The formula 1 5 is a subset of the following scales: ionian, dorian, phrygian, mixolydian, aeolian, melodic minor, overtone, mixolydian b6, phrygian dominant, half-whole diminished. The included chart shows this chord/scale relationship for the power chord "5" harmony.