Leisure Battery Size Calculator Guide | What Size Leisure Battery Do I Need?

Why choosing the correct leisure battery size matters

Choosing the right leisure battery size is one of the most important decisions in any campervan or off-grid electrical system. If the battery is too small, it will run flat quickly and daily use becomes frustrating. If it is too large, you may spend more money than necessary and add extra weight without much real benefit.

The goal is not to buy the biggest battery possible. The goal is to choose a battery capacity that matches the way you actually use your system.

• run appliances for the time you really need
• avoid damaging deep discharges
• support overnight and off-grid use more comfortably
• improve battery lifespan and charging efficiency

Step 1: list the devices you want to power

Start by listing everything you want your leisure battery to run. Every appliance draws power differently, and even small daily loads add up over time.

Typical campervan and off-grid loads include:

• LED lighting
• 12V fridge
• diesel heater fan
• water pump
• USB charging
• laptop charging
• 12V sockets
• router or Wi-Fi devices
• inverter-powered equipment

The more accurate your appliance list is, the more accurate your battery sizing will be.

Step 2: estimate how long each device runs per day

After listing the devices, estimate how long each one runs over a normal day. Runtime matters just as much as current draw. A small load running all day can use more energy than a larger load used only briefly.

Example daily usage might look like this:

• lights: 4 hours
• fridge: 24 hours cycling
• heater fan: 2 hours
• USB charging: 3 hours

This daily usage pattern becomes the foundation of your battery size calculation.

Visual example: how daily usage turns into battery capacity

A practical battery size calculation starts by adding together the daily amp-hour use of your main devices. This gives you a more realistic picture of how much battery capacity your setup needs.

Step 3: calculate total amp-hour usage

Leisure battery size is usually measured in amp-hours, written as Ah. This figure tells you how much energy the battery can store. Once you know how much energy your devices use each day, you can start estimating the battery size required.

Example daily usage:

• fridge = 45Ah/day
• lighting = 8Ah/day
• USB charging = 10Ah/day
• water pump = 5Ah/day

Total daily usage: approximately 68Ah per day

Use the leisure battery runtime calculator

The easiest way to estimate battery size accurately is to use a runtime calculator. It helps you match your appliance usage to the battery capacity your setup actually needs.

Step 4: account for battery type (AGM vs lithium)

Battery chemistry makes a huge difference to usable capacity. Two batteries with the same Ah rating can deliver very different real-world performance depending on whether they are AGM or lithium.

• AGM batteries typically use around 50% of their rated capacity comfortably
• lithium batteries typically use around 90% of their rated capacity

Example:

100Ah AGM battery → usable capacity ≈ 50Ah
100Ah lithium battery → usable capacity ≈ 90Ah

Typical leisure battery sizes explained

Battery size depends on your usage pattern, charging method and how long you want to stay off-grid. The examples below are a general guide for typical campervan and leisure battery systems.

How many days off-grid should your battery support?

Battery size depends not only on daily energy usage but also on how many days you want the system to operate between charging opportunities. This is often called autonomy.

• 1 day autonomy suits weekend travel with daily driving
• 2 days autonomy suits mixed touring setups
• 3 or more days autonomy suits more serious off-grid camping

If you have reliable solar charging or drive every day, you may need less battery capacity. If you park up for longer periods without charging, you usually need more.

How solar charging affects battery size

If your campervan has solar panels, the leisure battery may not need to provide all of your daily energy on its own. Solar can replace some of the energy used during the day and reduce the amount of battery capacity needed.

That said, solar output changes a lot depending on weather, season, panel size and how the vehicle is parked. UK winter performance is very different from summer performance.

• summer solar can significantly extend battery runtime
• winter solar contribution is much lower
• shade and parking angle affect charging output
• solar helps reduce battery demand but does not remove it

Why inverter use increases battery size requirements

Inverter use changes battery sizing dramatically because 230V appliances often draw far more power than people expect. Even occasional inverter use can increase battery demand sharply.

Devices such as kettles, coffee machines, hair tools, chargers and mains appliances can pull large currents from a 12V battery once inverter losses are taken into account.

• small inverter loads still increase battery demand noticeably
• high-power appliances require much more battery capacity
• inverter losses mean the battery sees more demand than the appliance rating alone suggests
• lithium batteries are often more suitable for inverter-heavy systems

Visual comparison: AGM vs lithium usable capacity

Rated battery size does not tell the full story. The key difference is usable capacity, which is why AGM and lithium batteries behave so differently in practice.

Common mistakes when choosing leisure battery size

• choosing battery size based only on price
• forgetting how much power a fridge uses over 24 hours
• ignoring inverter loads
• not allowing for winter usage or poor charging days
• assuming the rated Ah figure is fully usable
• not thinking about how often the battery will be recharged

A battery can look large enough on paper but still perform badly if real appliance use, usable capacity and charging conditions were underestimated.

Related campervan battery system guides

Electrical installation safety notice

This leisure battery size calculator guide is provided as a general reference for estimating battery capacity in campervan, motorhome and off-grid 12V electrical systems.

Correct battery sizing depends on installation conditions including daily energy consumption, charging method, battery chemistry, ambient temperature, seasonal usage and expected discharge depth.

Battery runtime estimates are also affected by appliance efficiency, inverter losses, charging availability, solar contribution and how regularly the system is driven or recharged.

Always confirm manufacturer specifications before selecting battery capacity for lithium or AGM leisure battery installations.