Power chords just contain a first, a fifth, and an octave. Depending on the chord progression, the third (major or minor) can be suggested, but since the third isn't actually being played, you have a wider range of available keys.

Say you have E5, D5, A5. You could play any number of modes over that - E minor/harmonic, E phrygian, E mixolydian, etc.

Try recording that progression in a loop and then solo over it using each of those modes for 4 bars. This is what Satch does a lot (called pitch axis). You'll find that the feel of the progression changes for each mode that you play. This is because when you change modes/key over that progression, the suggested major/minor tonalities of the chords shift. Have fun. :)