Let's start with 2 note power chords with the root on the E string:

This chord is an E power chord or E5. The 2 note power chord uses only the root and the fifth (E and B in this case). Play only the 2 lowest strings and use only down strokes (strum the strings with your right hand going down from the fat E). If we slide this power chord 1 fret further, like we did with the bar chords, we get an F power chord:

If we go 2 frets further from here we get a G power chord:

I think you got the picture by now. Here's the guitar neck with all the notes from the E string:

Besides 2 note guitar power chords there are also 3 note power chords. They use the root, fifth and again the root but one octave higher and they look like this:

The same principle as with bar chords and 2 note power chords goes for 3 note power chords: slide it up the neck to get other chords. If we slide 1 fret further we get an F power chord:

And this goes on, look at the guitar neck above for reference.

There's an alternative way to fret 3 note power chords: instead of using finger 3 and 4, use your little finger to fret 2 strings at once. This example is an F power chord fretted in the alternative way:

Let's have a look at 2 note power chords with the root on the A string:

Play only strings 4 and 5, don't play the low E and other open strings. We can slide this up 2 frets, what gives us a B power chord:

One fret further gives us a C power chord:

There are also 3 note power chord with the root on A:

Slide this up and we have a B:

And ...

Here's the guitar neck with all the notes of the A string for easy reference:

That was it, now plug in you axe, turn on the distortion, turn that volume up and start banging those guitar power chords!