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Mr Fusion explained ...

The #MrFusion Home Energy Reactor is the name of a power source used by the #DeLorean time machine in the #BacktotheFuture trilogy.

It can be seen for the first time at the end of Back to the Future when "Doc" Emmett Brown pulls into the McFly's driveway after a trip to the year 2015.

The Mr. Fusion Home Energy Reactor converts household waste to power for the time machine's #fluxcapacitor and time circuits using nuclear fusion, presumably cold fusion.

In the film, Mr. Fusion allows the DeLorean time machine to generate the required #1.21 #gigawatts needed to travel to any point in time. The energy produced by Mr Fusion replaces plutonium as the primary power source of the DeLorean's time travel, allowing the characters to bypass the arduous power-generation requirements

The plutonium fission reactor was most likely left installed underneath Mr Fusion as a backup power source.

The Mr Fusion can provide enough power to the flux capacitor but is not used to power up the DeLorean itself, which makes use of an ordinary gasoline combustion engine to reach the #88mph speed necessary for it to time travel.
A limitation that proved itself crucial in the third movie when Doc and Marty find themselves stuck in 1885 and unable to return with the DeLorean out of gas (due to a fuel leak).

The vehicle's hover system is powered by Mr. Fusion and is capable of bringing the DeLorean up to the required 88 mph; the combustion engine was also probably left on board as a backup. However, the flight systems were destroyed as a result of a lightning strike, leaving Marty to rely on the original combustion engine, which in turn was disabled.