Hydrogen or heat. The former would be stored like natural gas currently is stored, underground. We store months of natural gas consumption.

Heat (at 600 C) is potentially even cheaper to store, with a cost of storage capacity as low as $0.10/kWh(th) of capacity. This could yield 365/24/7 heat for $3/GJ, competitive even with cheap natural gas.

https://austinvernon.substack.com/p/building-ultra-cheap-ene...

https://standardthermal.com/

Round trip efficiency if you go back to electricity is nothing great, but this is not important for very long term storage, where capex is king, not RTE.