Capacitors are fairly good at keeping charge - there isn't much loss over time. In fact, capacitors used in high voltage equipment should generally have a discharge resistor wired across them to deliberately discharge them slowly, to prevent them being a hazard to anyone repairing the equipment.
Superconductors don't help with this (much - just maybe with the wires leading up to a capacitor). Superconductors allow much better inductors instead. You can also store energy in an inductor, but it's different because in a capacitor the charge stays put and in an inductor the current is constantly flowing.
you can just pump electricity into the superconductor capacitor and it wouldn't have any loss it would just stay there until discharge.