There are two ways a battery system could work for you:
The normal way is a connected battery system that basically takes any unused power from the installed solar system to charge up its connected batteries. Once the batteries have a charge in them this can be used to seamlessly power the building.
The second way to connect battery’s is without a solar system. This allows you to buy from your electricity supplier at a lower rate out of peak times (like the old economy 7 tariffs we once had). Then, once the batteries have power in them, this can be used to seamlessly power the building.
In fact, during the low solar months if you have a solar system installed and connected to batteries you can still use the “grid” during the cheaper off peak times to charge them up if your solar isn’t getting the job done.
This means you are not then paying the electricity company for this power or in the case of a battery system without solar you are paying a lower rate to charge the batteries (at night normally) and then using this power when the cost of electricity is higher (in the day). This then in turn helps keeps your electricity costs down.