A E5 (E Power Chord) chord contains the notes E, B. It is built from the formula 1 - 5, with E as the root.
Is a power chord really a chord?
Strictly, no — a chord needs three different notes, and a power chord has two. Musicians call it one anyway because it does a chord's job: it establishes a root and a harmonic centre. It just deliberately refuses to say whether that centre is major or minor.