Payback Time of Nickel Iron Batteries

What is the payback time of nickel iron electrical storage batteries for homes based on electricity stored - compared to grid cost of energy?

Notes

• Note this report - [1]
  • Conclusions - 100kWhr of energy used total to drive 100k mile lifetime of battery in 1980. Comparison to 28 mpg vehicle? Or 1kWhr per mile. Similar figure for all battery systems from nickel iron to lead acid. It is interesting to note that nickel iron is even considered, as in 2020's standard, nickel iron is considered insufficiently energy dense. Point - not much difference in operating energy requirements - production energy is much smaller than usage energy (10x-30x smaller)
• Nickel Iron at Nickel-Iron Battery Quotes costs 60 cents per watt-hr at the 19.2 kWhr nominal scale. About $12k total. Product itself is $8k, and if open source - we can probably get it down in cost to a little more.