Battery storage for solar panels works by diverting electricity to charge the batteries when the system detects you are using less than you are generating. This will continue until either the batteries are full, or you start using more electricity than you are generating. At that point, the batteries will kick in so you start using the stored electricity before buying more from your supplier.
There are two main types of battery storage system; AC-coupled and DC-coupled.
AC-coupled batteries are connected at the property’s distribution board so any energy feeding into the battery goes through the solar inverter before going into the battery. This means that any energy generated by the solar PV, stored in the battery and used in the property is converted from DC to AC through the solar inverter, back to DC to be stored in the battery and finally back to AC to be used within the property. All this converting means that there are losses within the system and that less energy comes out of the system as than was put in. This ratio of energy in to energy out is known as the efficiency of the battery system and varies between different types of batteries. In general, current battery systems are about 80-90% efficient.
DC-coupled batteries are connected between the solar panels and the solar inverter. The advantage of this is that there is no need to convert the energy generated by the solar panels as it is already in DC form and ready to be stored. There are, however, restrictions on the lengths of the strings connected into the battery and the type of inverter which means that DC-coupled storage systems need to be carefully designed before installation.