How to Size a Battery Bank for Your RV: A Practical Guide
Battery bank sizing is the question that trips up more RV electrical upgrades than almost anything else. Buy too little and you’re watching the gauge drop before dinner. Buy more than you need and you’ve hauled hundreds of extra pounds down the highway for no good reason. The right size is a specific number, and it’s not that hard to get to once you know the inputs.
The Formula
Daily energy use (Wh) × Days of autonomy ÷ Usable capacity factor = Battery bank size (Wh)
Then convert to amp-hours: Wh ÷ System voltage = Ah
Step 1: Daily Energy Use
| Load | Typical Draw | Hours/Day | Daily Wh |
|---|---|---|---|
| 12V compressor fridge | 40–60W avg | 24 | 960–1,440 |
| LED interior lights | 20W total | 4 | 80 |
| Rooftop vent fan | 25W | 6 | 150 |
| Furnace fan (propane heat) | 125W | 2 | 250 |
| Phone/tablet charging | 40W | 2 | 80 |
| Laptop | 60W | 3 | 180 |
| TV/streaming device | 80W | 2 | 160 |
| CPAP (no humidifier, 12V) | 40W | 8 | 320 |
| Water pump | 60W | 0.5 | 30 |
Realistic daily totals: Weekend van, minimal loads: 500–800 Wh/day. Small-to-mid travel trailer, no A/C: 900–1,400 Wh/day. Class B or C, full use: 1,200–2,000 Wh/day. 5th wheel or Class A, no A/C: 2,000–3,500 Wh/day. If unsure, 1,200 Wh/day is a reliable baseline for a mid-size rig.
Step 2: Days of Autonomy
How many days do you want to run without any charging input? Most RV systems target 2 days of autonomy as the baseline — it covers one fully cloudy day, the most common scenario that would otherwise drain a system.
Step 3: Account for Battery Chemistry
LiFePO4: ~95–100% of rated capacity is usable. A 200Ah bank gives you ~190–200Ah of real energy.
AGM: Safe depth of discharge is ~50%. A 200Ah bank gives you ~100Ah of usable energy. Discharging deeper accelerates degradation significantly.
This is the most important number in RV battery sizing. A 100Ah LiFePO4 battery does the same real work as a 200Ah AGM battery — at roughly half the weight.
Step 4: Run the Math
LiFePO4 example (1,200 Wh/day, 2 days autonomy): 1,200 × 2 ÷ 0.95 = 2,526 Wh. At 12V: 2,526 ÷ 12 = ~210Ah → round to 200–250Ah.
AGM same scenario: 1,200 × 2 ÷ 0.50 = 4,800 Wh. At 12V: 400Ah of AGM — that’s why AGM-based RV banks often have four or more large batteries.
Quick Reference by Rig Type
| Rig Type | Est. Daily Use | 2-Day LiFePO4 Bank | 2-Day AGM Bank |
|---|---|---|---|
| Weekend van, light loads | 600 Wh | 100Ah | 200Ah |
| Small trailer or camper | 900 Wh | 100–150Ah | 300Ah |
| Mid-size trailer, Class B | 1,200 Wh | 200–250Ah | 400Ah |
| Class C, full use | 1,800 Wh | 300–400Ah | 600–750Ah |
| 5th wheel or Class A, no A/C | 2,500 Wh | 400–550Ah | 800–1,000Ah |
Weight: The Factor Vans and Trailers Can’t Ignore
To get 400Ah of usable capacity: AGM requires ~800Ah rated capacity at ~520 lbs. LiFePO4 requires ~420Ah at ~120–130 lbs. That’s a 400 lb difference — in a van or lightweight trailer, that’s often the difference between being within GVWR and not.
Don’t Forget the Battery Monitor
A shunt-based monitor like the Victron SmartShunt measures actual current in and out, giving you accurate state of charge at all times — far more reliable than voltage-based gauges, which are notoriously inaccurate with lithium batteries. Install one from the start.
Browse the full lithium battery collection at Blue Marine, or schedule a free consultation with our team to right-size your bank for your specific rig and load profile.
Related reading:
The Complete RV Solar and Battery Upgrade Guide
LiFePO4 vs AGM Marine Battery: Which Should You Buy?
Rigid vs Flexible Solar Panels: Which Is Better for Boats and RVs?
