Alternative systems are closer to 5 years payback if you have access to off peak energy. 3-4%/year degradation is very high, battery packs available in the UK from Fogstar are rated at 8,000 cycles with a 95% depth of discharge.

FWIW my high voltage battery pack has almost 365 cycles and is still at 100% state of health.

FWIW i was just going off that fact that the warranty on the battery mentioned in the article only guaranteed >=60% after 10 years...