Van conversion edition · free planner
Size your van's 12V and solar system before you buy a single part
Tick what you'll run, say how many days off-grid, and get battery, solar, inverter, cable and fuse sizes. Free, no signup, no £3,000 kit needed to get a design.
- Starlink, laptop and induction hob are in the list
- Cable gauge and fuse sizes for the inverter run
- Checks panel wiring against the controller's input voltage
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Worked example
Working from the van, three days off-grid
- Fridge, lights, roof fan, pump, phones, laptop six hours, Starlink eight hours: about 1,450 Wh a day.
- Three days on lithium: a 460 Ah bank. That is why 'a 100 Ah battery and a panel' runs out on day two.
- 550 W of solar in summer to break even. The inverter for a laptop alone is small; add an induction hob and it is 2 kW and 200 A of cable.
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
Do I need a kit or can I buy the parts myself?
You can buy the parts yourself. The kit companies give you a free design because the margin is in the hardware. The planner gives you the sizes so you can price components from anyone.
Series or parallel for my panels?
Parallel unless you have checked your controller's maximum input voltage. Two panels in series can put over 40 V into a controller rated for 25 V and destroy it. The planner warns about this on the van edition.
What size inverter for a laptop and Starlink?
Small. A laptop charger and a 12V Starlink Mini need no inverter at all if you power them from 12V. The inverter question only gets big when you add a kettle, induction hob or hair dryer, which the planner sizes for you.
What fuse goes where?
A main fuse within 18 cm of the battery positive, sized to the cable. A fuse on the battery side of the inverter cable, sized to the inverter's current plus 25%. A fuse or breaker on the solar input. Never put a washer between the fuse and the lug.
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.