Short answer: A 13,500 BTU RV AC drawing ~1,400 W for 4 hours needs about 486 Ah of 12 V lithium, roughly 5 × 100 Ah LiFePO4 (about 10 AGM batteries), plus a 3,000 W inverter.
Energy = 1,400 W × 4 h = 5,600 Wh. Through a 90%-efficient inverter that's 6,222 Wh from the battery. ÷ 12.8 V = 486 Ah. Most 100 Ah LiFePO4 batteries also limit continuous current to ~100 A, and the AC pulls ~120 A at 12 V, so you need several in parallel anyway.
| Hours of AC | LiFePO4 100Ah batteries |
|---|---|
| 1 | 2 |
| 2 | 3 |
| 4 | 6 |
| 8 | 11 |
A soft-start kit cuts the startup surge, not the running energy. Running time depends heavily on outside temperature and the duty cycle of your AC. Try your own numbers in the battery bank calculator.
Safety: These are general estimates, not a design. Follow your equipment manufacturer's instructions and local electrical code, and consult a qualified electrician for anything connected to shore power, a house, or high-current batteries.
Eight hours at ~1,400 W needs about 970 Ah of 12 V lithium (10 × 100 Ah). It's possible but expensive; many RVers run the AC on a generator or shore power instead.
Two 100 Ah batteries give about 1 hour of AC at most, and their combined BMS current must handle ~120 A.
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