If you buy the 100 or 105AH "Solar Storage" batteries that look like car batteries, but which are called "Solar Storage", they will last about 2 years and then even though they say that are "sealed" I had 8 of these batteries and managed to refill them after two years to that they lasted 4 years. But their performance wasn't great.
18 months ago I upgraded to "Deep Cycle" batteries, 260 AH 12 Volt SonX batteries. The 4 SonX cost about R20,000 instead of the first set which cost about R12,000. But they are firstly 520 AH at 24 Volts instead of 420 AH at 24 Volts and secondly their performance is excellent. They last a lot longer than the original batteries.
And then the specification says they should last 7 years, but I'm hoping they will last longer than this, but even 7 years will be excellent. Note that in a battery system design, one should design the system to not use more than 30% of the battery capacity on a regular (daily) basis and only use 50% on a very irregular basis. This way the batteries are looked after and will last the specified time.
Having said all this, the inverters should last 12 years and the most of the photovoltaic (PV) panels are guaranteed for 30 years, with the PV panels performance deteriorating to 90% of their rated capacity after 12 years and to 80% of their rated capacity after 30 years.
So relative to inverters and PV panels, batteries don't last as long and therefore need to be replaced more often and therefore are relatively more expensive. But the benefits they give you far exceed their costs, in my opinion.
One should also note that if you have a "battery system", you need more components than on a "grid tie system" without batteries and therefore a system with a battery costs more than just adding the additional battery bank. And there is more maintenance on a battery system although the systems can be programmed to look after themselves and send warning system messages or emails or sms's if there is a problem.
Set 1: Price R12,500: Lasted 2 years. The second 2 years battery performance was really poor. Equals R6,250 per annum.
Set 2: R20,000: Should last 7 years: R2857 per annum.
These calculations exclude interest and the Cost of Capital, but I think the point is made.