Joules are units of energy; watts are units of power (energy per unit time).
In electrical terms, a joule equals one watt-second. This is the energy released in one second by a current of one ampere through a resistance of one ohm. It is a very tiny amount of energy. 1 megajoule is the equivalent of .278 kilowatt hours.
It seems fairly obvious that the use of terms unfamiliar to the public is meant to make this achievement seem more impressive than it really is.