Looking at the entry on Merriam-Webster.com they give two valid pronunciations ji-g-wät and gi-g-wät . They even have a neat audio clip of each being spoken.

I am certainly happy that they used the original, older sounding pronunciation of the word since the story involved time travel and Doc Brown conceived of the Flux Capacitor on November 5th in 1955 after slipping and hitting his head on the toilet.

Ironically, most references point to the early 1960’s as the first time the term was used. That’s okay at the start of the film as it’s set in 1985, but in 1955, the younger Doc Brown should have said “1.21 gigawatts? What’s a gigawatt Marty?

The saying over the years has taken on a life of its own. I’ve heard people say it without even knowing where it came from. What’s even more fun is the debate that follows on whether a gigawatt is an actual term. People would rather debate the pronunciation of the word then ponder the possibilities of time travel. Here’s hoping for a Back to the Future 4. Let’s make it a reality while we still can.
So, how much is 1.21 gigawatts you ask? Well, a gigawatt is equal to one billion (10 9) watts or 1 gigawatt = 1000 megawatts. If your typical 100 watt incandescent light bulb (at least what used to be typical before they outlawed them here in the US) draws 100 watts of energy, 1.21 gigawatts would be able to light over 12 million 100 watt light bulbs. 1.21 gigawatts is also equivalent to 1,621,400 horse power.