- Marty: "Wait a minute, what are you doing Doc?"
- Doc: "I need fuel!"
- — Marty and Doc while refueling Mr. Fusion with garbage
Also known as the fusion generator, the Mr. Fusion Home Energy Reactor converted household waste to power the time machine's flux capacitor and time circuits using nuclear fusion. (It is thought that this is cold fusion.) Mr. Fusion allowed the DeLorean time machine to generate the required 1.21 jigawatts to travel through the space-time continuum. The energy produced by Mr. Fusion replaced plutonium as the primary power source of the DeLorean's time travel and flight capabilities.
The Mr. Fusion was first seen when Emmett Brown returned back from 2015. He rummaged through Marty's trashcan to find a banana peel, a Miller beer can (pouring in the remains of the beer, and then dropping the can in), and some other pieces of garbage which he dropped into the unit before closing it. This suggests that the Mr. Fusion unit could run on pure trash.
When Marty, Doc, and Jennifer arrived in the year 2015, a few logos are spotted for Fusion Industries on the power generators in the alley they landed in. The generators resembled the shape of a very small truck.
Behind the scenes
The ending of Back to the Future suggested that in the future, mankind's energy problems would be solved by a device that would convert any matter into energy, and that the "Mr. Fusion" (seen on the back of the DeLorean) could, in theory, someday be able to power automobiles. However, vehicles seen in Back to the Future Part II don't have a Mr. Fusion, and the Texaco service station in 2015 is still in operation. However, Fusion Industries most definitely did exist in 2015 because, in Back to the Future: Part II, Doc parked the DeLorean in an alley next to a Fusion Industries generator. In Back to the Future Part III, Doc confirms that the fusion device only powered the flux capacitor and the time circuits, and that the DeLorean time machine still had an internal combustion engine that ran on gasoline.
In the novelization, Doc has to take care in 2015 to keep the DeLorean obscured, citing that many of his modifications – specifically the installation of a Mr. Fusion Home Energy Reactor – are not exactly considered "street legal".
- The Krups Coffee grinder model number 223A, marketed under the name Coffina, was visible in the 1979 film Alien, where it served as a futuristic kitchen appliance aboard the spaceship Nostromo.
- In the final scene of Back to the Future (1985), the Krups Coffina grinder served as new piece of hardware from the future, the Mr. Fusion Home Energy Reactor, as a throw-away joke at the end of a movie that wasn't expected to spawn a sequel. Mr. Fusion's in-universe purpose was to offer a safer, more energy-efficient alternative to the radioactive and (extremely scarce) plutonium required earlier in the film to power the DeLorean's time circuits. As Mr. Fusion operated on garbage, it was clearly a futuristic, eco-friendly alternative.
- The Mr. Fusion was constructed by mounting a Krups Coffina grinder on top of a custom-made base made from the outer shell of a Singer magnetic disc memory core, now obsolete, which was a feature in some computers made in the 1960s. In the film, it's possible to see that the large grey window (the grinds collecting cup) along the trailing edge the the unit doesn't lie flush with the housing. During the closeup (before Doc opens the fuel compartment) the control knob from the grinder is visible, including its original number markings on the control knob on the backside of the unit. Visible also, lower down on the back of Mr. Fusion, is the Coffina's on/off switch.
- The grey circle behind the Fusion Industries logo is a clear-backed decal applied to the original Coffina's translucent window, which allowed a user to gauge how many unground coffee beans remained in the bean chamber. From the back it's possible to see through the clear collecting cup and see the spout where the ground coffee grains would fall from. (Visible also in the film is the top edge of the camera being reflected in the shiny 2015 bar code license plate.).
The opening scene in Part II employs a few different camera angles from its Part I equivalent. As with other differences between the otherwise identical scenes, someone had the foresight to paint over the numbers on the Coffina's control knob during the Part II filming, and it became a featureless peg on the back of Mr. Fusion; the the "Mr. Fusion" decals on the Part II unit had a lot less finesse than in Part I. Near the end of Back to the Future, Part III (1990), a shot of the DeLorean's back deck shows Mr. Fusion prominently, with the translucent grey cap for the Coffina grinder apparently missing, and allowing viewers to see down into to the top of the grinder's shallow bean collecting cup. (If this were a Coffina grinder instead of a Mr. Fusion reactor, one would pour the coffee beans into the top of the unit. The coffee grind collecting cup, the unit's lid, and the back control knob are missing, and the side window (with the Fusion Industries logo) appear to be blacked out from the inside.
- Fun fact: Back to the Future series producer Bob Gale has an extra Mr. Fusion unit sitting on his desk, acting as a lamp.
- Back to the Future trilogy
- Back to the Future: The Ride
- Super Back to the Future Part II
- Back to the Future Part III (video game)
- The Earth Day Special (Non-canonical appearance)
- Super Back to the Future Part II
- Universal Studios: Theme Park Adventure
- Back to the Future: The Card Game
- Star Trek/Legion of Super Heroes #5 (Non-canonical appearance)
- Back to the Future: The Game