In many cases this will mean taking out the existing inverter and replacing it with one that is compatible with a battery. There are exceptional cases where there is an existing system that is compatible with a battery, for example the solar edge system which is compatible with the new Tesla battery. There are other options available that mean a change of inverter is necessary, for example the new LG Chem battery storage system. A type of inverter called a Hybrid is needed in order to cope with the new battery so it would mean losing the existing inverter.
If you do not have any solar panels installed then you have the option of many battery storage systems. If you are particularly drawn to the Tesla battery then at present you would need the Solar Edge system and optimisers in order to be compatible. If you are not particularly drawn to the Tesla brand then there are other options available such as LG and Goodwe, which are considerably cheaper.
For either of the options above consideration should be made on guarantees, many companies now offer a 10 year warranty.
There are a few more decisions to make when deciding which system to opt for, the main one being if you want the system to keep working if there is a power failure. At present solar panel systems attached to the grid are designed to switch off if there is a power failure. If you want your system to continue to work when there is a power failure, it is important to specify this at the design stage as a different type of inverter is needed.