Most batteries carry 10-year warranties; Enphase and Franklin stand out at 15. Nearly all guarantee ≥70% capacity at end of warranty. But the fine print has two traps:
Cycles: like a car's "5 years or 50,000 miles," some warranties cap full charge/discharge cycles (4,000-15,000). Daily-use batteries burn cycles fast — if you're cycling nightly (which is the whole point under net billing), a higher cycle limit or unlimited-cycle warranty matters enormously. Throughput: some cap total lifetime energy (e.g., 60MWh). Exceed either limit and the warranty is void even inside the time window. Backup-only batteries can ignore this; nightly-cycling batteries can't. This single paragraph has saved my clients more money than most sales pitches.