Motorhome & caravan edition · free planner
How big a leisure battery and how much solar for the nights you spend off hook-up?
Tick what you run, say how many nights off hook-up, and get the battery, solar, inverter, cable and fuse sizes. Free, no signup, worked out for UK sun.
- UK irradiance figures by season
- Battery, solar, inverter, cable and fuse sizes in one pass
- Warns about PME hook-up, cold lithium charging and undeclared upgrades
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Worked example
A typical two-night wild camp
- Compressor fridge, LED lights, water pump, heater fan, phones, a bit of TV: about 850 Wh a day.
- Two nights off hook-up on lithium: a 200 Ah bank. On lead-acid, 300 Ah, because you can only use half of it.
- 500 W of solar to put a day back in spring or autumn. In winter, plan on driving or hook-up.
The sums behind it
- Fusing
- Main fuse within 18 cm of the battery positive (ABYC E-11 §11.10; BS EN ISO 10133 for boats), rated at or below the cable's ampacity source
- Voltage drop
- 3% maximum on the inverter run, copper resistivity 0.0175 Ω·mm²/m, two-way run length source
- UK sun
- Peak sun hours from the EU PVGIS dataset for central England, flat panel, derated 30% for dirt, angle and cloud source
- Usable capacity
- LiFePO4 80% depth of discharge, lead-acid 50%; manufacturers' cycle-life curves
- Inverter
- 88% conversion efficiency, 10 W idle draw, 25% headroom on the loads likely to run together
Before you buy
How many nights can I go without hook-up on a 100 Ah leisure battery?
It depends on what you run. A compressor fridge, lights, pump and phones use around 700 to 900 Wh a day. A 100 Ah lead-acid battery gives you about 600 Wh before it starts to suffer, so roughly one night. The same 100 Ah in lithium gives about 960 Wh, so a night and a bit. Put your own kit into the planner above for your number.
Can I charge my motorhome from a socket at home?
Not from an ordinary socket if your house is on a PME earthing system, which most UK homes are. A fault can make the vehicle body live. Use a hook-up unit with its own earth, or an installer-fitted supply. The planner flags this on the motorhome edition.
Is lithium worth it in a motorhome?
If you spend two or more nights off hook-up regularly, usually yes: you can use 80% of the capacity instead of 50%, it weighs about half, and it charges faster. Check the battery has low-temperature charge protection, declare the upgrade to your insurer, and check your payload.
How much solar do I need?
Enough to put back one day's use in the season you actually travel. UK sun gives about 4.3 peak hours in summer, 2.6 in spring and autumn, 1 in winter. The planner uses those figures with a 30% derate for dirt, angle and cloud.
Who made this
Off-Grid Sums is a Bloor Built product. The sums were written by a controls engineer who runs a camper off-grid year round and has spent thirty years on industrial control and power systems. It is a planning aid: check the output against your own kit, and use a competent installer and your Boat Safety Scheme examiner for the final word.