Most people buy too much battery. If your goal is backup during 1-2 outages a year, you need 5-10 kWh, not 27. If you're chasing TOU arbitrage, the math only works when the peak/off-peak spread is > $0.15/kWh — which is rare outside California, Massachusetts, and parts of New York.
What drives payback
Utility export compensation. If your utility pays you retail rates for solar export (true net metering), a battery's economics are weak. If they pay wholesale ($0.03-0.05/kWh), the battery suddenly pays off.
TOU rate structure. A 3-hour peak window with $0.40/kWh rates makes a battery print money. A flat-rate plan does not.
Incentives. California's SGIP, New York's NY-SUN, and a few utility programs can cover 30-100% of battery cost for specific customer classes.
Payback for backup-primary batteries should also value outage avoidance — food loss, comfort, work-from-home continuity. Our default $200/yr is conservative.