I'm confussed: I know what an E chord is, but where does the E5 come from? I can't find it in any chord book I have.
The 5 stand for the perfect fifth from the root note which is E. So the fifth is B.
E5 is just an E power chord. In fact all chords with a letter and a five next to it (E5, F5, F#5 etc.) are just power chords.
Yeah, it's just an E plus the 5th note of the E scale. All scales are 7 letters. I'll use C as an example because it has no sharps or flats:

C  D  E  F  G  A  B
1  2  3  4  5  6  7
8  9  10 11 12 13 14

The 8th note of a scale is always the octave, which is the same note one pitch up from the starting note. The good news is this formula works for all chords, so in the future if you see a C9 you'll know it's just a C chord plus the 9th note of the C scale. It works the same for 7ths, 13ths, and so on.

Hope that helps!
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5 chords are power chords
power chords = root note, 5th of scale (based off of root), and then octave

Root: C
5th: G
Octave: C

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