It is not totally certain, but it is probably an anagram of his real surname, Arouet, in a roundabout way: Calling himself Arouet le Jeune (Arouet the younger, as his father had the same first name as he, François), and not distinguishing between U/V, and J/I (as in classical Latin), you get AROVET Le Ieune → VOLTAIRE.

It has nothing to with electricity. "Volt" as the unit for electrical potiential is named after the scientist and inventor of the battery, Alessandro Volta, who was born in 1747. Voltaire started using his famous pseudonym in 1718.