Recommended battery
0Ah
12 V leisure battery
0 Ah at 24 V about 0 Wh
How this was worked out
Daily energy
Saved on this device
Battery settings
Change days, chemistry, or extra margin. The sizes above update as you type.
How long you want to run if there is little or no solar or hook-up. Two days is a common start.
Battery chemistry
A small buffer for weather, ageing, and days that use more than usual. Set 0 to skip.
How to use
- Daily energy is taken from Daily Power if you have used it. You can type your own watt-hours instead.
- Choose how many days you want to last without charging. Two is a common start.
- Pick LiFePO4 or AGM / lead-acid. That sets how much of the rated battery you can safely use.
- The big figure is amp-hours at 12 V — how most UK leisure batteries are sold. It updates as you type.
FAQ
What does “days without charging” mean?
It is how long you want the batteries to last if the solar is poor or you are not on hook-up. Two days covers a dull weekend for many people. Add more if you camp for longer without charging.
Why is a LiFePO4 battery smaller than AGM?
You can use more of a lithium battery’s rated size. This calculator plans LiFePO4 at 80% usable and AGM / lead-acid at 50%, so the AGM battery needs to be larger for the same daily use.
12V or 24V — which Ah number do I use?
Shop with the big 12 V Ah figure if your leisure battery is 12 V — that is how most UK packs are sold, such as 12 V 460 Ah. Use the 24 V Ah chip only if the leisure system is 24 V. The same energy is half the Ah at 24 V (Wh ÷ 24 versus Wh ÷ 12). Do not mix them.
Should I buy the exact Ah figure?
Treat it as a planning size, then round up to a 12 V leisure battery you can actually buy. Weather, fridge run-time, cable losses and battery age all change the real figure. Leave a margin and get a qualified installer for safety-critical work.
Can I change the daily watt-hours here?
Yes. Use “Enter a different figure” if you do not want the Daily Power total. Your Daily Power list stays as you left it. Switch back any time to use that saved total again.
Wh vs Ah — what’s the difference, and why show both?
Watt-hours (Wh) measure energy. Amp-hours (Ah) are that energy divided by voltage: Ah = Wh ÷ volts. UK and EU leisure batteries are usually sold in Ah at 12 V, so that is the big number. We still show 24 V Ah and the Wh total so you can compare systems.
What’s the difference between usable capacity and the size printed on the battery?
The printed size is the full rated capacity. You should not plan to use all of it. This calculator treats LiFePO4 as 80% usable and AGM / lead-acid as 50% usable, so the size to buy is larger than the energy you will actually use. That keeps a reserve and helps the battery last longer.
Can I put batteries in parallel or series?
In simple terms, parallel keeps the same voltage and adds Ah; series raises the voltage (for example two 12 V batteries to 24 V) and does not add Ah. Matching batteries, cables and protection matter. This page only estimates a total size — get a qualified installer to design a parallel or series setup.
How should I set days without charging for hook-up versus wild camping?
If you are on hook-up most nights, one day or less can be enough as a buffer. For wild camping or poor solar, two days is a common start; add more if you stay off-grid longer. Raise the days figure and the 12 V Ah size grows with it.
Does this include the starter battery?
No. This is for the leisure battery that runs the fridge, lights and other living loads. Do not count the vehicle starter battery as part of this size, and do not flatten it to power the habitation side.
Will solar change how big a battery I need?
Solar can top the battery up in the day, so you may get away with fewer days without charging in good weather. Size the leisure battery first for the dull days you still want to cover. Then open Solar to turn the same daily watt-hours into a recommended panel size.
What about winter and cold weather?
Cold weather can cut available capacity, and AGM / lead-acid usually suffer more than lithium. Fridges and heaters also run longer in winter. Treat this as a mild-weather planning estimate and leave extra margin, or ask an installer, if you camp in frost.
Do I need a BMS, or can I use a drop-in LiFePO4?
Most leisure LiFePO4 batteries sold as a complete 12 V pack include a BMS inside. “Drop-in” means it is built to replace a similar-size leisure battery, but chargers, cables and protection still have to suit lithium. This page only estimates size — check the maker’s instructions and use a qualified installer for safety-critical work.
Guides
Short reads on Motorhome Tools. They do not change the numbers on this calculator.