it was, is, and always will be gigawatts.
Someone - or a group of people - simply pronounced it wrong.
The power required is pronounced in the film as "one point twenty-one jiggawatts."
While the closed-captioning in home video versions spells the word as it appears in the script, jigowatt, the actual spelling matches the standard prefix and the term for power of "one billion watts" : gigawatt.
Though obscure, the "j" sound at the beginning of the SI prefix giga- is still an acceptable pronunciation for "gigawatt."
In the DVD commentary for Back to the Future, Robert Zemeckis stated that he had thought it was pronounced this way because this was how the scientific advisor that he had for the film pronounced it.