How to Calculate Your Campervan Battery Autonomy

Running out of power two nights into a trip is the fastest way to ruin van life. The good news: battery autonomy is simple maths once you know the four numbers that matter. This guide walks you through the exact formula — then shows how to size your battery, solar and alternator with confidence.

1. Calculate your daily consumption

Everything starts with how much energy you draw in 24 hours. For each appliance, multiply its power in watts by the number of hours you use it per day. That gives you watt-hours (Wh). Add them all up, then convert to amp-hours (Ah) — the unit batteries are sold in.

Here is a typical two-person campervan:

*A fridge cycles on and off, so it runs roughly 10 effective hours over 24h. That total of 795 Wh ÷ 12V ≈ 66 Ah per day.

2. Find your usable battery capacity

A battery's nameplate capacity is not what you can actually use. Each chemistry has a safe depth of discharge (DoD) — go below it and you damage the cells. Multiply nominal capacity by the DoD to get usable amp-hours.

• AGM / GEL: 50% DoD → a 100Ah battery gives 50Ah usable.
• Lithium (LiFePO4): 80–90% DoD → a 100Ah battery gives 80–90Ah usable.

This is why a 100Ah lithium battery realistically replaces a 160–180Ah lead-acid bank — at a third of the weight.

3. Add solar & alternator recharge

Autonomy is not just the battery — it is the balance between what you take out and what you put back. Two sources usually refill the bank.

Solar

A solar panel's daily yield depends on your latitude and season. A practical rule: multiply total watts-peak by the average peak-sun-hours for your region, then apply ~75% for real-world losses (MPPT, heat, angle).

Example: 300Wp in Southern Europe (≈5 sun-hours) ≈ (300 × 5 × 0.75) ÷ 12 ≈ 94 Ah/day. In Northern Europe in winter, expect a third of that.

Alternator

When you drive, a DC-DC (B2B) charger tops up the leisure battery. Multiply its dedicated amperage by driving hours and apply ~70% efficiency.

Example: a 30A charger over 2h of driving ≈ 30 × 2 × 0.7 ≈ 42 Ah/day.

4. Compute your real autonomy

Now subtract your daily recharge from your daily consumption to get the net deficit the battery must cover. Then divide usable capacity by that deficit.

Using our example — 66 Ah consumed, 94 Ah of summer solar — the battery actually gains charge during the day. Autonomy is effectively unlimited in good weather. On a cloudy day with no driving and only 20 Ah of solar, the deficit is 46 Ah/day; a 100Ah lithium battery (80Ah usable) then lasts about 1.7 days before needing a recharge.

AGM vs GEL vs Lithium: which to size for?

For full-time travel, lithium wins on usable capacity, lifespan and weight — and its higher upfront cost is usually lower per cycle over ten years.

Common sizing mistakes

• Using nameplate capacity instead of usable Ah. Always apply the DoD.
• Ignoring the fridge duty cycle. It does not run 24h flat-out — but it does run all night.
• Sizing solar for summer only. Winter yield can be 60–70% lower; size for your worst realistic month.
• Forgetting inverter losses. 230V appliances through an inverter add ~10–15% overhead.