The problem being no fuel was left due to a leak in the tank. So Doc wrote another letter to his futre self and said "include a spare can of gas in the boot", e

The problem being no fuel was left due to a leak in the tank. So Doc wrote another letter to his futre self and said "include a spare can of gas in the boot", end of film. "Mr Fusion" could product 1.21GW. For about a tenth of a second. It then needed a recharge, so about 33kWh, half that of a Model 3