Electric energy storage systems (EESS) store excess energy generated by a linked solar PV system or can be charged from the grid. Battery systems can be connected via either an AC coupled inverter or a DC coupled Hybrid inverter, this would be dependant on whether solar PV is available.
AC Coupled systems convert electrical energy AC from the grid into DC via the inverter so that it can be stored in the battery for later use. Typically at cheaper rates. When this energy is needed the inverter will convert the DC back to AC via the inverter for use in the property. DC Coupled systems have less conversion loses than an AC coupled system. Solar PV panels generate DC energy which can then be stored directly into the DC battery without conversion the only conversion would be when the energy is required from the battery and the DC would be converted to AC via the inverter. For grid to battery storage the principles are the as using an AC coupled system.