OK, well all that info gives people here more attuned to the costs of DIY systems in your neck of the woods a bit more meat to chew on when it comes to advice on what sort of system expansion you might consider.
Two-fold: have a minimum viable situation on my first (very sun-protected) floor during a prolonged power outage. This would require running a 12A portable AC unit and basic electronics and a few lights. Second, during normal times, when I have grid power, save a bit of money each month on my utility bill. I want my system to stay off-grid, so I've decided to simply plug a bunch of fridges and extension cords to electronics into my system to achieve this.
If you are happy with the rest of your system and it's only storage you are considering, then I would be investing in power/energy monitoring if you haven't already and run the system with the devices you want to see how it performs.
Simulate a grid outage. Simulate your otherwise regular off-grid operating mode.
Gather data on how the system performs. You have 4.8 kWh of storage now (call it 4 kWh effectively useable on the AC side). Nothing like hard data and actual use to know whether something is up to the task.
In regular off-grid operating mode it might be that if you experience poor solar weather conditions for an extended period then you choose to rely on the grid rather than expand your system to cope with multiple consecutive days of low PV output. IOW the off-grid system handles the chosen loads most of the time but on occasions you are using the grid for backup.
Only you can do the calculus on whether spending the $ on extra storage to cover for those less frequent occasions is worthwhile, or just accept that the system will do it's thing 80-90% of the time and the grid handles the rest, or whatever ratio makes sense for you. Local grid tariff regimes vary widely so it's hard to say from here in Australia.
For grid outage backup scenarios then you'll also need to have in mind for how long you want coverage for, and consider why backup might be required, e.g. is this mostly a storm/weather related grid outage scenario (ours are)? How long will they typically last for, or how long do you want to be able to cover? If it's weather related then that has implications for whether your solar PV is going to be sufficiently secured and also whether it will be able to supply all that much energy during inclement weather to recharge batteries. You might need generator as backup, at least to do a battery charge for periods during the daytime (might not be socially acceptable where you are to run a generator at night).
You may also want to consider how much storage capacity to keep in reserve. This will be a bit of a subjective assessment on your part, and it might be an adjustable target. e.g. in storm season you might choose to always have a minimum capacity of X kWh on hand, but during times of the year when outages are less likely you are OK with letting the battery state of charge drop to a lower level. Reserve capacity costs more as much of the time it is not being cycled and so is not helping to reduce grid import costs.
e.g. I will have two storages - the 18 kWh of lead acid I currently have for grid outage backup, and the 10 kWh of LiFePO4 I am adding for regular off-grid cycling for running the regular power and lighting circuits for our two homes (but not the large power draw devices such as ducted aircon, electric oven, induction stovetop etc). I don't expect the home to be able to run off-grid the whole time, so at times energy demand will be supplemented by the grid supply.
The energy required to keep lights, refrigeration and basic power devices, e.g. phone charging, computer, internet connection, maybe a TV, running isn't a difficult sum to do but it would probably help you to do some power monitoring of the more energy hungry of those such as the fridge and any other devices you want to operate during an outage. A plug in power monitor is pretty cheap.
Some older devices might be worth updating if they suck too much power. A while back I sold off an older Sony TV and got a new replacement. The new one uses 100W less power and is way less of an energy drain. Fridges are another - modern units (and inverter driven) are way more energy efficient than older fridges.
I made the assessment I needed to cover in the vicinity of 500-600 W on average, with peaks over 3 kW, and to have coverage for > 12 hours.
By
load management I meant will you be actively managing loads so as to ensure you keep them within the capacity of the system to deliver power, as well as shifting loads to times of day when your PV array has excess capacity to burn? There are various ways to do this, from changing habits, to automation and smart devices. e.g. my hot water system uses a smart power diverter which controls the power the water heater consumes such that it avoids importing energy from the grid.
Primarily the point being to think not only about the energy supply side but also the energy demand side. By getting demand under control you may not need to spend nearly as much $ on the supply side.