Battery Size Calculator

How to size a leisure battery without guessing

The honest way to size a battery is to start from what you actually use, not from a number someone quoted you down the pub. Work out your watt-hours for a normal day, take off what the solar puts back, and leave yourself some headroom for a flat-grey week. That is the whole job, and it is what this tool does in the background while you tick off appliances.

Start with your daily power use

Every load in the calculator has a realistic wattage and a duty cycle built in, so a fridge is counted on the time it actually runs rather than flat out all day. Add your fridge, your lights, the water pump, phone and laptop charging, any heater fan, and an inverter if you cook or boil a kettle off the battery. The running total is your daily watt-hours, and that is the number everything else hangs off.

Peak loads matter as much as capacity

Capacity is only half the picture. A motor mover, an inverter or a big pump pulls a large current for a short time, and the battery's BMS has to pass that without tripping. The calculator checks your biggest single load against each battery's continuous rating and warns you when you are close, so you do not end up with a battery that has the energy but cuts out under load.

Solar, the alternator and longer off-grid stays

If you have solar, drop your panel wattage in and the calculator takes the daily yield off your load before it sizes anything. Want a proper solar figure for your latitude, the month and the weather? Run our solar yield calculator and bring the number back. Charging from the engine on the move is the other big one, and our charger compatibility guide covers the DC-DC and mains options. Set the autonomy slider for how many days you want between proper charges.

Which TITAN fits, and where

Under a van seat means a DIN case, and most of our range is DIN-fit. In a caravan with under-bench storage, or building a fixed off-grid bank, you have room for the bigger units. Still weighing up lithium against your old leisure battery? The lithium vs lead-acid page lays out the real difference in usable capacity and lifespan.

Common questions

How do I size a lithium leisure battery for my campervan or motorhome?

Add up everything you run off the battery and how long each runs for, which gives you watt-hours per day. Take off whatever your solar puts back, divide by 12 for amp-hours, and leave some headroom so you are not flattening the battery every night. Lithium gives you almost all of its rated capacity, so you can size much closer to the label than with lead-acid. The calculator does all of this and shows the smallest TITAN that covers your load.

What size battery fits under a motorhome or campervan seat?

Most vans keep the leisure battery under the driver or passenger seat, which suits a DIN-format case. The TITAN 80Ah, 105Ah, 120Ah, 150Ah, 180Ah and the 260Ah DIN Type 019 are all DIN-fit. The bigger units like the 230Ah, 280Ah and 460Ah are taller and longer, so they tend to go in caravans with under-bench storage or a fixed off-grid install.

Is a 100Ah lithium battery enough for a campervan?

For a lot of weekend setups, yes. A 100Ah lithium battery gives you about 1.28kWh of usable energy, which covers a compressor fridge, lights, the water pump and charging your devices for a day or two. If you run a diesel heater hard, use an inverter for cooking, or stay off grid for longer, step up to 150Ah or more or add solar. Put your kit into the calculator and it will tell you straight.

How long will a 100Ah lithium battery run a 12V fridge?

A typical 12V compressor fridge averages 40 to 50W once it has cooled down, so it draws roughly 3 to 4 amps on and off through the day. A 100Ah lithium battery will run it on its own for two to three days before a recharge, and longer with any solar topping it up. A hot day and a full fridge will pull that down.

Can a lithium battery run a caravan motor mover like a Powrtouch?

Yes. A single-axle mover draws around 80A in short bursts, dual-axle around 85A and quad-axle up to 90A. Any TITAN with a BMS rated 100A continuous or more handles a single-axle mover comfortably. For dual and quad-axle movers go for the 150Ah, 180Ah or larger, which leave plenty of headroom for the peak.

How much solar do I need to offset my daily battery use?

As a rough UK average across the year a panel returns about 3 to 4 good sun-hours a day after controller and wiring losses, so a 200W roof panel averages roughly 600Wh a day. That covers a fridge and lighting through summer. For a proper figure by month and weather, use our solar yield calculator and bring the number back here.

What does 'reserve at journey end' mean on the calculator?

It is how much you want left in the battery when you pack up. 15% is a sensible default, so you get home with a buffer, the BMS never sees a full drain, and the cells last longer. Set it to 0% to use everything, or higher to size a bigger battery and look after the cells.