Well... I suggest maybe you should post why exactly you want to know the key?
Anyway, first you should try to determine which chord makes it feel your song is "solved", more "like home". That would help determine what key (E, D, .... whatever). But them you need to know the quality: major or minor?
Technically speaking, power chords don't allow us to determine the quality (no third degree). But, try playing the song with E (minor), D (major) and C (major)... Does it fit? It may be E minor then...
Well... but look: this is a very simplistic way to look at the matter...
I use an app call Suggester. You put in the chords and it tells you the key (with a % match) and then suggests other chords. I just entered these as major chords. According to it, you are in ..drum roll ...B Dorian scale (100%); G maj/Emin/Bmin (88%).
Awesome thanks a lot gcat
The chord progression definitely lends itself to Emin. Best reference for that particular progression would be the part after the solo in Stairway to Heaven, where Jimmy Page plays Amin, G, F and then back up. You can just play the power chords and it works perfectly.