Panels Only vs Panels + Battery: Which Pays Off?

Adding a 10 kWh battery to an 8 kW solar system increases install cost from 24,000 to 33,000 dollars (after ITC: 16,800 vs 23,100). The battery makes sense in three cases: (1) you have time-of-use rates with a steep peak, (2) your export rate is much lower than retail (e.g., California NEM 3.0), or (3) you need backup for outages. Otherwise, panels alone deliver shorter payback. Panels-only payback: 7-9 years. Panels + battery payback: 9-12 years if TOU/low export, 14+ years on flat retail.