PRACTICAL ANSWERS HUB · CAMPERVANS, MOTORHOMES & CARAVANS
What size leisure battery do I need?
Estimate the battery-bank capacity you need from your expected daily use, how many days you want to stay off-grid and your battery chemistry. Use the calculator below, then read the guide before buying.
Page summary: Free leisure-battery sizing calculator and practical guide for campervans, motorhomes and caravans, covering AGM, lead-acid, gel and LiFePO₄.
Size your battery bank
RECOMMENDED MINIMUM NOMINAL CAPACITY
Enter your usage to calculate a planning target.
Example: 40 Ah per day for two days
AGM / lead-acid
40 Ah × 2 days = 80 Ah required. If you plan around 50% usable capacity, that becomes 160 Ah nominal. Add a 15% buffer and the target is about 184 Ah.
LiFePO₄
The same 80 Ah requirement at 80% usable capacity becomes 100 Ah nominal. Add a 15% buffer and the target is about 115 Ah.
What counts as daily use?
Daily use is the total energy your leisure-battery system supplies over 24 hours. Lighting and phone charging may be modest; compressor fridges, TVs, laptops, pumps, heating controls and inverter-fed appliances can increase the total quickly.
If you do not know your daily use, start with our Leisure Battery Runtime Calculator. Better still, measure your actual system with a shunt-based battery monitor.
Why chemistry changes the answer
The Ah printed on a battery is nominal capacity. Your practical usable capacity depends on the discharge floor you choose and the battery maker’s limits. This page uses 50% usable capacity as a planning default for flooded, AGM and gel lead-acid, and 80% for LiFePO₄. You can override that in the calculator.
Victron’s SmartShunt documentation uses a 50% discharge floor as the default for lead-acid and recommends setting lithium lower. That supports using different planning assumptions for the two chemistries.
What about solar and alternator charging?
This calculator sizes the battery for the number of days you enter without depending on uncertain recharge. Reliable solar, mains hook-up or alternator charging can reduce the storage you need, but weather, short drives and winter solar output can change the answer significantly.
Before you buy
- Check the battery physically fits and is suitable for the mounting position.
- Confirm every charger supports the battery chemistry.
- Check continuous and peak current limits for inverters and other heavy loads.
- Do not size cables or fuses from Ah capacity alone; they must suit circuit current and installation conditions.
- If building a multi-battery bank, follow the manufacturer’s rules for matching and interconnection.
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Equipment worth checking after you know your target capacity
These are broad Amazon UK searches, not claims that one product is suitable for every installation. Use your calculated Ah figure as a starting point and verify the manufacturer’s specifications before buying.
LiFePO₄ leisure batteries
Useful when you want more usable capacity for a given nominal Ah rating. Check BMS current, charger compatibility, low-temperature protection, size and warranty.
AGM leisure batteries
A familiar lead-acid option for many existing systems. Compare C20 capacity, dimensions, weight, warranty and the battery maker’s cycling guidance.
Battery monitors with shunt
A shunt measures current flowing in and out of the battery, giving a much more useful state-of-charge estimate than voltage alone.
Related PAH tools
How to size a leisure battery
Start with the energy you expect to use each day, multiply by the number of days you want to operate without charging, then allow for the usable portion of the battery rather than treating the full headline capacity as available. Round the result upward rather than designing a system that only works when every assumption is perfect.
Worked example
If your equipment uses 700 Wh per day and you want two days of autonomy, you need about 1,400 Wh of usable energy. At 12.8 V that is roughly 109 Ah usable; if you plan around 80% usable capacity, the nominal bank would be about 137 Ah, so a practical design would normally move to the next suitable standard size after checking the battery maker’s limits.
Leisure battery sizing FAQs
Should I size from Ah or Wh?
Wh is usually the clearer starting point because it describes energy directly. Ah only becomes meaningful when the system voltage is known.
Do solar panels let me buy a smaller battery?
They can, but solar production varies with season, weather, orientation and location. For off-grid planning, treat expected charging separately from the battery capacity needed to cover poor-generation periods.
What about inverter losses?
Include them when estimating mains loads. A 100 W AC appliance can require more than 100 W from the battery once inverter losses and standby consumption are included.
Sources and assumptions
Planning assumptions are informed by Victron Energy’s SmartShunt documentation. See SmartShunt settings and SmartShunt operation. Always follow the specifications for your own battery and charging equipment.