https://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage https://en.wikipedia.org/wiki/Superconducting_magnetic_energ... You build a big inductor,

https://en.wikipedia.org/wiki/Superconducting_magnetic_energy_storage https://en.wikipedia.org/wiki/Superconducting_magnetic_energ... You build a big inductor, and because it's superconducting current just goes round and round. At room temperature you have no refrigeration losses, you can build more just as fast as you can kick out superconductor. It would be 100% efficient, have 0 self-discharge, and enormous power capacity and infinite power cycles.