Anyone ever consider what the "Mr. Fusion" on the back of the DeLorean (and presumably a power source used all over in 2015) is really
capable of? Forget that it's capable of providing cold fusion in a small space and is "stable" enough to be mounted on a car. But what we see it do is simply incredible, on some levels it's more powerful than any other natural fusion reaction in the universe.
In massive stars hydrogen is fused together to form helium, then helium to form carbon, and some of the other elements are mixed in there but once a star (if its big enough) gets to iron it cannot go any further. At that point there's not enough energy in the star to fuse two iron molecules together to from the next element, at this point the star is so heavy and dense that it collapses into itself likely forming a black-hole.
When we see Doc use the Mr. Fusion at the end of BTTF 1/beginning of BTTF 2 we see him dump inside the Mr. Fusion chamber some eggs, beer, an aluminum can, styrofoam and some other pieces of garbage. "Somehow" the Mr. Fusion is capable of taking these materials that not only contain elements much more complex than hydrogen or helium but alos contains chemical compounds. Rather than pretty much doing nothing because it doesn't have the power to, the Mr. Fusion is able to take the stuff dumped into and convert it into raw, pure, energy. (Also I'm sure some number crunching of what we see Doc put into the device would equal far more than 1.21 GW.)