it seems clear that said battery is lithium ion, and that it contains electronics to charge and discharge AC, at 50Hz (UK).
as i read the spec sheet, it is guaranteed for 10 years.
this probably assumes regular maintenance, whatever that is
i think the questions about electrolytic caps is somewhat moot - since the battery as a whole is guaranteed,
all the internal parts are likewise guaranteed.
i think the better question is, what does payback mean, what assumptions went into it, and how was it calculated?
my brother installed solar panels on an outbuilding on his farm with two(?) of this sort of Tesla battery in Kentucky.
he believes that about 10 months out of the year, his meter will run "backward"
for the two or three months its been working, he's very happy with the system
he said payback on the solar was about 5 years, but once you add the batteries, forget it.
i think his primary goal was a house and barn-wide uninterruptible power supply