"How many amp-hours do I need?" is the most common van electrical question. The answer is always: it depends on your loads. Here's the formula.
The calculation
Step 1: Daily watt-hours
List your loads, their wattage, and daily hours of use:
| Load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| 12V fridge | 50W avg | 24h (duty cycle) | 60Wh |
| Laptop | 65W | 4h | 260Wh |
| Phone charging | 15W | 2h | 30Wh |
| Fan (MaxxAir) | 25W avg | 6h | 150Wh |
| LED lighting | 20W | 3h | 60Wh |
| Daily total | 560Wh |
Add 15% buffer for hidden draws: 560Wh × 1.15 = 644Wh/day
Step 2: Convert to amp-hours
Wh ÷ voltage = Ah drawn per day 644Wh ÷ 12V = 53.7Ah/day
Step 3: Size for autonomy
How many days without charging do you want?
- Weekend: 2 days → 107Ah drawn
- Full-time: 3 days → 161Ah drawn
Step 4: Account for DoD
LiFePO4 at 80% DoD: divide by 0.8
- Weekend: 107Ah ÷ 0.8 = 134Ah minimum → choose 200Ah
- Full-time: 161Ah ÷ 0.8 = 201Ah minimum → choose 200–300Ah
Quick reference by use case
| Use case | Loads | Recommended battery |
|---|---|---|
| Weekend, no cooking | Fridge, phone, lighting | 100Ah LiFePO4 |
| Weekend + induction cooking | Above + cooktop | 200Ah LiFePO4 |
| Full-time, no cooking | Fridge, laptop, fan, lights | 200Ah LiFePO4 |
| Full-time + induction cooking | All above + cooktop | 300Ah LiFePO4 |
| Full-time + remote work (desktop) | All above + high laptop/screen use | 400Ah LiFePO4 |
Why solar matters for sizing
If you have 300W of solar producing 900Wh/day on average, and your daily load is 600Wh, your battery only needs to bridge overnight (8 hours) when solar isn't producing. That's a smaller battery requirement than if you were purely off-grid with no solar.
In practice: solar allows you to run a system with less battery than the pure autonomy calculation suggests — but you still need enough battery to get through a night and handle cloudy days.
The amp-hours vs watt-hours confusion
- Amp-hours (Ah): Current × time. A 100Ah battery delivers 100A for 1 hour, or 10A for 10 hours, etc.
- Watt-hours (Wh): Power × time. A 100Ah, 12V battery stores 1,200Wh.
When sizing, work in watt-hours (because appliances are rated in watts) and convert to amp-hours at the end. Mixing the two units without converting is a common mistake.
Related
Frequently Asked Questions
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Roam Wired turns campervan research into practical plans. We publish transparent guidance, a free electrical design tool and marketplace resources for UK and US van builders.
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