I asked why the [speed was 88 MPH](https://scifi.stackexchange.com/questions/14812/in-back-to-the-future-why-was-the-speed-88-miles-per-hour), and it made a lot of sense.

Now I want to ask: Why does the Flux Capacitor need 1.21 jigowatts to travel in time? Is this a magic number?