Answer:
First of all, input electricity is just indicating the direction of that energy, in this case, it refers to the electrity that is being received in a sub-system. On the other hand, output electricity is the energy that is being produced to another sub-system.
For example, in this case the power plant represents the production of energy, which offer output electricity because produces the energy for other sub-system. Then, those sub-systems receive this energy, that's why at this level it becomes input energy.
Being said that, we can say that energy is the primary type of input and output, it's what is being used in the power grid.