Sizing a van solar system is mostly arithmetic, but the inputs trip people up. This calculator does the maths for you. Enter your daily loads in watts and hours, set your sun hours and how many days of autonomy you want, and you get a panel, battery and MPPT recommendation that matches.
The defaults below are sized for a typical full-time mid-size van: 12V system, 12V fridge, LED lights, a Maxxair fan, laptop and phone charging. Edit the rows to match your own appliances.
Solar sizing calculator
All inputs editable. Results update when you click Calculate.
Most van builds are 12V. 24V only if you are running a very large system.
Conservative AU/US year-round average is around 4. Drop to 3 for cloudy regions or winter use, lift to 5 for AU summer.
Cloudy days the battery alone needs to cover. 2 is comfortable for full-time, 1 is minimum for weekend use.
| Appliance | Watts | Hours per day | Wh per day |
|---|
Recommended bank size
Recommended panel wattage
Recommended MPPT current
How this calculator works
The maths is simple, the conservative bits are deliberate.
- Daily Wh. Sum of (watts x hours) for each appliance you run.
- Daily Ah at your system voltage.
Wh / system V. - Battery Ah.
daily Ah x autonomy days x 1.25. The 1.25 multiplier keeps depth of discharge below 80% on LiFePO4 so you get the full rated cycle life. - Panel watts.
daily Wh / sun hours / 0.7. The 0.7 accounts for real-world losses: heat, panel angle, MPPT conversion, cable resistance. - MPPT amps.
panel W / system V x 1.25. Then rounded up to the next standard size (40A or 60A).
If you want to sanity-check the maths or learn the why behind it, read the solar sizing guide and the battery sizing guide.
When to upsize beyond what the calculator says
The calculator is sized for typical year-round use. There are three cases where you should bump the panel wattage or battery Ah a step up:
- Cold-climate winter use. Sun hours can drop below 2 in deep winter. Either set sun hours to 2 in the input, or run the calculator for summer and add 50% to the panel total.
- Heavy laptop or 240V appliance use. Inverters are around 85% efficient. If you run a kettle, microwave or air fryer through an inverter, those loads add up fast and the calculator under-estimates them unless you account for the inverter loss in the watts column.
- Full-time off-grid for weeks. Push autonomy to 3 days minimum. A single bad weather week can drain a 2-day bank.
Want the full sizing logic in a PDF?
The free DreamBuild starter pack includes a 1-page sizing cheat sheet with the same formulas, plus full guides for electrical, plumbing and the conversion itself. Drop your email below.