Buying too small means dead batteries by noon. Buying too big means wasted money and weight. Sizing your RV battery bank is just arithmetic — here's the method we use on every build.
Step 1: List your daily loads
Write down everything you run in a typical day and roughly how many amp-hours (Ah) it draws. Common estimates:
| Device | Typical draw |
|---|---|
| Compressor fridge (24h) | 35–45 Ah |
| LED lights (evening) | 3–6 Ah |
| Water pump | 2–4 Ah |
| Laptop / devices | 8–12 Ah |
| Furnace fan (cold night) | 10–20 Ah |
Step 2: Apply the usable-capacity rule
Lithium gives ~90–100% usable capacity; AGM only ~50%. So a 100Ah lithium = ~90–100Ah usable, while a 100Ah AGM = only ~50Ah usable.
Step 3: Worked example
Say your total is 60Ah/day and you want 2 days of buffer without charging:
- Lithium: 60 × 2 = 120Ah needed → one 100Ah + one 50Ah, or a single 200Ah.
- AGM: 120Ah ÷ 0.5 = 240Ah needed → roughly two 120Ah AGM batteries.
Notice the AGM option is double the weight and cost for the same runtime — another reason most full-timers choose lithium.
Start here — HumsiENK 100Ah LiFePO4 (Bluetooth)
Common mistakes
- Using rated Ah as usable Ah on AGM — you'll halve your real runtime.
- Ignoring the charger — a lead-acid-only converter will undercharge lithium.
- No monitor — without one you're guessing and will over-discharge.
Disclosure: RV Battery Hub is reader-supported and may earn from qualifying links at no extra cost to you. Size estimates are starting points; verify with your actual gear.