Surge

Correct information above.
Just keep in mind that with a disconnector on each battery, it makes it so much easier to isolate 1 of the batteries quickly for whatever reason, without having to disconnect its wires. 

Kuodos to Esener. It's the first Battery Spec that I've seen that actually specifies what BMS is inside the Battery.!

I went with 12 x JASolar 540W panels (two strings of 6) because they fit the available roof space better. I couldn't get three rows of 460W panels and two rows of larger 540W panels in landscape orientation both maximized the area covered as well as reduced the cost of the mounting kit. The string voltages are only 240V so it may not be in the max efficiency range of the Sunsynk 8KW MPPT controllers but I really don't feel that it is such a big issue. Weather plays a far bigger role than MPPT efficiency curves. Just use what fits your roof layout and stay within the electrical specs.
If a manufacturer came along with a 30% efficient panel then sure, that would be a game changer but at the moment solar PV panel effiencies are so close to each other it makes little sense to pick the biggest panels one can buy simply because it sounds better.

I went with 12 x JASolar 540W panels (two strings of 6) because they fit the available roof space better. I couldn't get three rows of 460W panels and two rows of larger 540W panels in landscape orientation both maximized the area covered as well as reduced the cost of the mounting kit. The string voltages are only 240V so it may not be in the max efficiency range of the Sunsynk 8KW MPPT controllers but I really don't feel that it is such a big issue. Weather plays a far bigger role than MPPT efficiency curves. Just use what fits your roof layout and stay within the electrical specs.
If a manufacturer came along with a 30% efficient panel then sure, that would be a game changer but at the moment solar PV panel effiencies are so close to each other it makes little sense to pick the biggest panels one can buy simply because it sounds better.

I'm not sure if the following is legal in terms of keeping municipal and rainwater/greywater separate but it's something I'd do without it bothering my conscience.
If you want to make the change over system automatic you could install a ball float valve (like the ones in your toilet cistern) inside and close to the bottom of your rainwater tank and connect it to the municipal supply. When the rain water level in the tank drops to say the 25% capacity the ball float valve will open and maintain the level at 25% using municipal water. Just determine where the crossover point needs to be by moving the ball float valve up or down inside the tank.
Your booster pump will never run dry unless there is no rain water and the municipal supply is off and you'll never need to worry about manual intervention. Alternatively install a float switch in the tank so that the booster pump can't run when the tank is below a certain level.
The ball float valve prevents rain water from feeding back into the municipal supply although the municipal pressure will almost always be higher than the head of water in the tank (except when there are water outages or if the tank is on a very high tank stand in which case you wouldn't need a booster pump). Regardless it's not possible to accidentally feed rainwater back into the municipal supply using a decent float ball valve.
N.B. Just make sure that the outlet from the ball float valve is in clear air (not underwater) when the valve starts to open. This prevents siphoning/feedback into the municipal supply when the municipal supply is off and the level in your tank is in the "top up" area. You can do this by bending the float arm downwards or installing the valve at a tilted angle so the float is lower down than usual.

The closest thing I can find and "off the shelf" is this https://www.sustainable.co.za/sustainable-580wh-power-box.html
It uses LiFePO4 batteries and a decent hybrid inverter from Victron. You can add solar to it later and even use ESS.
If you want to DIY you can probably source the components cheaper by hunting a bit.
e.g. The BlueNova BN13V series of LiFePO4 batteries are cheapest on Takealot. I suspect that they are being sold directly via BlueNova so you cut out the middleman.
I'm sitting on the fence because I can go the cheap 12V route and just use a LiFePO4 charger with single 280Wh LiFePO4 battery to power the ADSL router, wireless AP and lighting. That would give me about 6 hours of backup power for under R5000. Adding the 230V inverter option adds a lot of cost and instead of using the TV I can just use a smartphone to stream Netflix (I don't use Monochoice) or surf the web.
Choices ... choices ...

I'm not sure if the following is legal in terms of keeping municipal and rainwater/greywater separate but it's something I'd do without it bothering my conscience.
If you want to make the change over system automatic you could install a ball float valve (like the ones in your toilet cistern) inside and close to the bottom of your rainwater tank and connect it to the municipal supply. When the rain water level in the tank drops to say the 25% capacity the ball float valve will open and maintain the level at 25% using municipal water. Just determine where the crossover point needs to be by moving the ball float valve up or down inside the tank.
Your booster pump will never run dry unless there is no rain water and the municipal supply is off and you'll never need to worry about manual intervention. Alternatively install a float switch in the tank so that the booster pump can't run when the tank is below a certain level.
The ball float valve prevents rain water from feeding back into the municipal supply although the municipal pressure will almost always be higher than the head of water in the tank (except when there are water outages or if the tank is on a very high tank stand in which case you wouldn't need a booster pump). Regardless it's not possible to accidentally feed rainwater back into the municipal supply using a decent float ball valve.
N.B. Just make sure that the outlet from the ball float valve is in clear air (not underwater) when the valve starts to open. This prevents siphoning/feedback into the municipal supply when the municipal supply is off and the level in your tank is in the "top up" area. You can do this by bending the float arm downwards or installing the valve at a tilted angle so the float is lower down than usual.

I'm not sure if the following is legal in terms of keeping municipal and rainwater/greywater separate but it's something I'd do without it bothering my conscience.
If you want to make the change over system automatic you could install a ball float valve (like the ones in your toilet cistern) inside and close to the bottom of your rainwater tank and connect it to the municipal supply. When the rain water level in the tank drops to say the 25% capacity the ball float valve will open and maintain the level at 25% using municipal water. Just determine where the crossover point needs to be by moving the ball float valve up or down inside the tank.
Your booster pump will never run dry unless there is no rain water and the municipal supply is off and you'll never need to worry about manual intervention. Alternatively install a float switch in the tank so that the booster pump can't run when the tank is below a certain level.
The ball float valve prevents rain water from feeding back into the municipal supply although the municipal pressure will almost always be higher than the head of water in the tank (except when there are water outages or if the tank is on a very high tank stand in which case you wouldn't need a booster pump). Regardless it's not possible to accidentally feed rainwater back into the municipal supply using a decent float ball valve.
N.B. Just make sure that the outlet from the ball float valve is in clear air (not underwater) when the valve starts to open. This prevents siphoning/feedback into the municipal supply when the municipal supply is off and the level in your tank is in the "top up" area. You can do this by bending the float arm downwards or installing the valve at a tilted angle so the float is lower down than usual.

I'm not sure if the following is legal in terms of keeping municipal and rainwater/greywater separate but it's something I'd do without it bothering my conscience.
If you want to make the change over system automatic you could install a ball float valve (like the ones in your toilet cistern) inside and close to the bottom of your rainwater tank and connect it to the municipal supply. When the rain water level in the tank drops to say the 25% capacity the ball float valve will open and maintain the level at 25% using municipal water. Just determine where the crossover point needs to be by moving the ball float valve up or down inside the tank.
Your booster pump will never run dry unless there is no rain water and the municipal supply is off and you'll never need to worry about manual intervention. Alternatively install a float switch in the tank so that the booster pump can't run when the tank is below a certain level.
The ball float valve prevents rain water from feeding back into the municipal supply although the municipal pressure will almost always be higher than the head of water in the tank (except when there are water outages or if the tank is on a very high tank stand in which case you wouldn't need a booster pump). Regardless it's not possible to accidentally feed rainwater back into the municipal supply using a decent float ball valve.
N.B. Just make sure that the outlet from the ball float valve is in clear air (not underwater) when the valve starts to open. This prevents siphoning/feedback into the municipal supply when the municipal supply is off and the level in your tank is in the "top up" area. You can do this by bending the float arm downwards or installing the valve at a tilted angle so the float is lower down than usual.