Guitar players; how do you do power chords?

Just wondering whether you barre the chords or use your 1, 2 and 3 fingers to press each note individually. I only ever barre the E power chord since it's open E and the 2nd frets of the A and D strings (B and E notes). Might be a newbie thing since I can't barre very well yet but who knows. Thanks for any answers.