Geiger

Hey there have to agree that redundancy is much more important than less cables - cables can be tidied up -  as noted for redundancy, swap out or expansion, all are valuable to consider - the only time one larger battery becomes a consideration is when there is an unavoidable space consideration and one large battery is your only option, but then buy big and well known which equals very expensive - think LG Chem.

Dual as redundancy is the key.

Again, thank you all for the responses. @hoohloc, yeah, will not DIY, thanks again.

I agree with the point about redundancy. I've always had a single 10kWh battery, first Revov, now Freedom Won. There was a query about the Revov. They wanted to put it on their test bed. Not an unreasonable requirement, but it left me with no battery at all for a couple of days. This is the exact scenario that @hoohloc describes.
As regards extending the battery, you can go from two 5kWh batteries to three. You can upgrade in smaller, more affordable increments. If you go for 10kWh then you have to extend to 20.
Downside: 2x5 usually costs more than 1x10 of the same range, and you may need to buy some extra cables to interconnect the batteries. 
My system is what it is now, but if I were starting afresh, I'd give serious thought to 2x5. So well done on asking a good question. 

Just know that you have to phone a friend to lift a 10kWh battery.

Redundancy is a good point to consider. Another point i guess is that should you need to upgrade - with a 5kw you'd only need another 5kw but with a 10kw you'd need one of those - its pricier

Think of Redundancy, you want to have back up when your battery fails. If you go for a 10kw and the battery fail for some reason and has to be sent in for repairs or warranty swap, you are doomed. If you go for two 5kw, one fails and you still have one to fall on or use while waiting for the repairs. Also, for trouble shooting, if you have an issue and suspect the battery has a problem, you can always test by using one battery at a time to isolate the issue. If you have only one battery then you will have to pay someone to come and assist you. Two is always better than one, no matter how you look at it and do not worry about any ''complexity in balancing'' leave that to BMS. You are not planning on DIY battery, right? Also, amount of wiring is nothing to worry about as the wiring linking the batteries come included when you purchase them. I'm talking based on Pylontech, not sure about other brands 

If i could do it again id go for a single higher capacity battery - reduces the amount of wiring and complexity in balancing (ensuring the multiple smaller ones charge and discharge relatively uniformly 

Hi
im noob to this forum and also to solar.
just completed my install of system as per the subject line.
 
quite neet…waiting to monitor the results.
 

Thank you all for the responses thus far. Will look into the specific models etc. Not just rushing in which is why this forum is so valuable to us.

Do you have any evidence to support this statement? Because that implies Dyness is inferior to Pylon

Note the warranty on the Pylontech UP5000 differs from the US5000 (5 yrs vs 7+3), a battery with a longer warranty should have a better resale. Can't comment on the Dyness.  The pylontechs tend to be compatible with more inverters.
 My personal call would be 2 batteries and solar panel. The reason
Solar panels will reduce you electrical bill and help with payback
If the grid is off for an extended period and weather permitting you will be able to recharge the batteries
You and family will learn how to live with solar, eg run power hungry appliances when the sun is shining, reduce consumption when there is poor solar power, you will get a feel for extra panels needed etc in future
Adding another battery is relatively simple (could be DIY less than a day) getting solar installed is a much lengthier process, and may be more expensive if done on its own rather than part of a complete job.  You would also need a COC (dont think thats required if you add a battery)
You may be able to find good 2nd hand batteries at a significant lower price vs new.

Here is a link to compare Dyness and Pylontech : Dyness Vs Pylontech Comparison - Solar Energy Warehouse

Sup Geiger. If you are spending R200k on solar, R2k for a better app, control and support is worth it.
Check that the batteries can output 4kW continuously.
If you like Eskom, buy another battery and leave the panels for the haters.