Power chords - determining key?

With power chords, how does one determine the key? I've read a little bit, and know that an E5 is formed of E & B notes, but how does that help with determining the key?

For example, looking at a Green Day song, I see intro: "E5 B5 C#5 G#5 A5 E5 B5", and then verses are pretty much "A5 B5 E5", and chorus "E D C#5 A B", but who knows this tab i found could be totally wrong. But as an example, can the key be determined from that?

Thanks!

Without hearing which chord is the key centre, we have to deduce the most likely key, not only from the chords contained in the song but also from the order in which they occur.

The chords themselves indicate the key of A major as they all naturally occur in that key. However, the order of the chords is such that the key of E major is more likely to be the true key. The song has got one chord foreign to the key of E, which is D major but that could indicate a modal harmony (mixolydian mode) which is not uncommon in rock songs. Instead of the chords being derived from the E major scale (E F# G# A B C# D#) they are derived from E mixolydian mode (E F# G# A B C# D) - This doesn't affect the tonal centre or key.

Listening to the last chord of any song is one of the strongest indicators of a song's key, however that doesn't seem to apply in this case if the last chord is B - unless you feel that the B naturally wants to lead back to E but fades out instead.

Edit - I just noticed the last chord is B major, not B5. B major occurs in the key of E but, unlike B5 doesn't occur in the key of A, which provides more evidence that the key of the song is E major. It would also mean that the chord, D major is better interpreted as having been borrowed from the parallel minor key E minor, rather than indicating the mixolydian mode.