Undersizing your battery bank is one of the most common — and most expensive — mistakes in a van build. Buy too little capacity and you're rationing power by day three of a trip. Buy too much and you've overspent by a thousand dollars on batteries you didn't need and added dead weight you'll carry forever.
Here's the actual math, not a rule of thumb.
Step 1: Calculate Your Daily Amp-Hour Usage
Amp-hours (Ah) are the standard unit for battery capacity. Convert every appliance's watt-hour draw to amp-hours using:
Amp-Hours = Watt-Hours ÷ System Voltage
For a 12V system with 1,000 Wh of daily usage:
1,000 Wh ÷ 12V = 83.3 Ah/day
If you're running a 24V system (common in high-power builds with A/C), the same 1,000 Wh only draws 41.6 Ah/day — one reason larger power setups often move to 24V or 48V to reduce wire size and current losses.
Step 2: Apply Depth of Discharge (DoD)
You can't use 100% of a battery's rated capacity without shortening its life — how much you can safely use depends on battery chemistry:
| Battery Type | Safe Depth of Discharge | Usable of 100Ah rating |
|---|---|---|
| Lead-acid (flooded) | 50% | 50 Ah |
| AGM | 50% | 50 Ah |
| Lithium (LiFePO4) | 80–100% | 80–100 Ah |
This is the single biggest reason lithium batteries dominate modern van builds — you get nearly double the usable capacity from the same rated size, plus thousands more charge cycles.
Step 3: Build In Days of Autonomy
"Days of autonomy" is how many days you want to run on battery power alone with zero solar or alternator input — useful for parked, shaded, or overcast stretches.
Required Battery Capacity = (Daily Ah ÷ DoD%) × Days of Autonomy
For 83.3 Ah/day usage on lithium (90% usable) with 2 days of autonomy:
(83.3 ÷ 0.90) × 2 = 185 Ah
Round up to the nearest standard size — a 200Ah lithium battery bank covers this build with a small margin.
Real-World Sizing by Use Case
| Use Case | Daily Wh | Recommended Lithium Bank |
|---|---|---|
| Weekend trips, minimal fridge | ~500 Wh | 100 Ah |
| Moderate full-time, no A/C | ~1,000 Wh | 200 Ah |
| Remote work + induction cooking | ~1,800 Wh | 300–400 Ah |
| A/C, power tools, heavy use | 2,500+ Wh | 400–600 Ah |
Don't Forget Charging Capacity
A larger battery bank is only useful if you can recharge it fast enough. Match your battery capacity to a charging system (solar + DC-DC + shore power) that can realistically refill it within a day of moderate sun or driving — otherwise you're just carrying dead weight that takes three days to top off.
Frequently Asked Questions
How many amp-hours do I need for van life? Most full-time builds without A/C land between 150–300 Ah of lithium capacity. Calculate your specific daily Wh usage rather than copying someone else's number — appliance choices vary widely.
Is 100Ah enough for van life? For minimal weekend use, yes. For full-time living with a compressor fridge and daily laptop use, 100Ah usually runs out by day 2 without solar input.
Should I get one large battery or multiple smaller batteries? Either works if wired correctly, but fewer, larger batteries typically mean simpler wiring and fewer failure points. Match total capacity to your calculated need either way.