It requires 50MJ of electric energy to convert oxide to 1 Kg of aluminum, containing 31MJ of energy. 60% efficiency, not that bad. Electricity is not the only component of conversion cost, there are also costs of smelter construction and running. Market price of aluminum is $1.9/kg, including all costs and manufacturer's margin. Assuming this cost, it is less expensive to run electric car on aluminum than ICE / gasoline car.