The Malta energy storage system takes electricity, converts and stores that electricity as heat, and then converts it back to electricity to be redistributed on the electric grid. In charge mode, the system operates as a heat pump, storing electricity as heat in molten salt. In discharge mode, the system operates as a heat engine, using the stored heat to produce electricity.
40% of the solar and wind power that is generated is wasted. Malta uses steel containers to hold heated salts. This could store 40% of the solar and wind energy that is wasted and return 25% of that amount or 10%.
It seems like they will use the electricity to heat salt and then convert heat stored in the salt back to electricity when needed. This would likely be 35-55% effective in either direction. The total system might be 10-15% efficient. However, the energy would otherwise be wasted.