As per request  I will share my view on running batteries on a Solar system. 
Now just a little background on my setup. I am running a Axpert inverter that is a "offgrid" system and it is from that view point that I write this. If your need is much higher then there could be a difference but I doubt it. Also I have not looked into grid tie as the legalities and the cost of it always scared me abit..

The basic design that most people will have with the Axpert inverter is a 1 , 3 , 4 combination. 1 Inverter , 3kw Panels and 400ah batteries and as a rule this works fairly well considering the amount of sunshine we have and all. 
Now you would want to run your batteries preferably on about 20% soc as to get the maximum TIME out of them and as a general rule will imply that you will draw about 3 kwh per evening from your batteries but just for fun lets make it 4 kwh per night that you would use. 

Now current price on a Trojan Bank would set you back R 41 678 for a 450ah bank at 48v. So let's round it down to R 40 000.
Prices varies from as low as R 1.40 per unit for Eskom to as High as R 4.00 here on the farm so let's take a middle price of R 2.70 per unit. 

So: 4 units per night at R 2.70 = R 10.80 x 31 days = R 334.80 per month to run the house on grid at night. But if you had the batteries you would not pay anything as you have paid for the kwh in advance. Eskom were allowed a price increase of about 2% this year but let's say that they will stay with inflation of 6% per year.

If you take the R 40 000 that you would have forked out on a large battery bank and decide to invest it , let's say on a fixed interest rate of 7%. (IMO I would not do it that way as this will be a long term investment) 

The figures show that you will deplete the investment in about 13 years 

If you are going to borrow the money a roundabout prime rate as would need to pay the interest you will still paying the batts by the time they expire in about 10 years time IF everything went very well with the battery maintenance. 

 

The figures would suggest that it would be much better to get a small bank just to have everything run for a while if the power goes out and the rest of the time you use Eskom at night and panels in day time. You will have very little risk compared to running a full bank with equalizing , water levels , soc and battery temperatures just to name a few and IF Eskom rates change to the level that Batteries does warrant the risk then go for it. 

Even at R 4 per unit the figures shows that the investment will last 7 years 

and if you're a paying a loan it will take you 8 years to pay it off and that is at a rate of 4 kwh per night. We are currently doing 3 units a night  and that makes even a bigger difference. 

The long and the short of it all is when you look into solar know one thing for sure you will have to make so lifestyle changes or Solar will do it for you!

Hope this helps someone!

Paul

4Kwh? That's very generous...  How much does your house actually use during the night, on average?

We are on about 2.5 to 3 kwh per night.But in the calculations I wanted work on the max rather than the min amount per night.