The debate always continues about whether it's gigawatt, jigawatt or jigowatt.
It is Jigowatt in the BTTF universe.
The correct spelling is indeed "gigawatt". And although the Bobs got the spelling wrong the first time, they still chose to keep that spelling in the scripts for BTTF1 and the sequels. Hence I still consider that it is spelt "jigowatt" and not "gigawatt"; atleast in the BTTF universe.
Below is a transcript from the BTTF DVD commentary, Bob Gale (Writer/Producer of BTTF trilogy) explains about jigowatt.
Bob Gale: I should talk about "jigowatts" for a second.
The proper pronunciation, of course, is "gigawatts" [with a hard "G" sound], and when Bob [Zemeckis] and I were doing research, we talked to somebody who mispronounced it "jigowatts". We were actually completely unfamiliar with the term, and we thought that was how it was supposed to be said. It does come from the Greek 'gigas' [that Greek root is pronounced with a "J" sound, not a "G" sound], for gigantic, so I suppose it's not beyond the realm of possibility. But never having heard of it, we actually spelled it in the script "jigowatt". A "jigowatt" is actually supposed to be a gigawatt, a million watts. So the mystery of the gigawatts is now solved.
The fact that Bob Gale spelt it as "Jigowatts" in all the scripts should be authoritative enough.
If anybody is still not convinced, then the fact that Doc Brown also spelt it as "Jigowatts" in his letter, will help anybody realize that it is the official spelling in the BTTF universe.
(I know it might appear to be as "jigawatt" in the photo above, but it's the letter 'o' not 'a', just like it was spelt in the scripts)