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Your MPPT controller is the brain of your solar system — undersizing it means you're literally throwing away free energy on sunny days. The sizing rule is simple: take your total panel wattage, divide by battery voltage, and add a 25% safety margin for cold-weather voltage boost.
For a 12V system with 400W of panels: 400W ÷ 12V = 33.3A. Add 25% margin: 33.3 × 1.25 = 41.7A → you need a 50A MPPT. A 30A controller would cap your harvest at 360W — wasting 10% of your array on a good day. For 200W panels: 200 ÷ 12 × 1.25 = 20.8A → a 20A MPPT is technically sufficient but a 30A leaves room to add a panel later.
⚡ Expert tip
When in doubt between two MPPT sizes, always go one size up. A 30A controller handling 20A of actual current runs cooler, lasts longer, and leaves room for a future panel upgrade. The price difference between a 20A and 30A Victron SmartSolar is only $30-40.
Example calculation for your setup
Results based on a typical use case
| Appliance | Power | Usage/day | Wh/day |
|---|---|---|---|
| Compression fridge | 45W | 24h | 1080 |
| LED lighting | 20W | 4h | 80 |
| Water pump | 30W | 0.5h | 15 |
| Phone charging | 15W | 2h | 30 |
| Daily consumption | 1205 Wh | ||
Recommended solar panels
302 Wc
4h average sun hours
Adjust these values with the calculator below
VANPOWERCALC PROYOUR ENERGY PROFILE
YOUR ENERGY PROFILE.
This document contains the sizing of your future electrical installation, calculated based on your appliances.
Minimum Daily Consumption
0 WH / DAY
Inventory:
SYSTEM SIZINGPage 2
Battery
To guarantee 0WH without damaging your bank (80% max discharge):
0 AH
LiFePO4 12V
Solar
Minimum power required to recharge your consumption:
0 W
via MPPT
220V AC
Maximum power (with 25% safety margin).
0 W
No Need
Safety & CablingPage 3
12V Cable Sizing Guide
Use this professional reference table to select the correct gauge (mm²) for your cables. For 12V in a van, the maximum tolerated voltage drop is 3%. Always use multi-stranded flexible automotive wire.
| Current (A) | Round trip < 2m | Round trip 4m | Round trip 6m |
|---|---|---|---|
| 5A (LEDs, USB) | 1.5 mm² | 2.5 mm² | 4 mm² |
| 10A (Fridge, Pump) | 2.5 mm² | 4 mm² | 6 mm² |
| 20A (Heater) | 4 mm² | 10 mm² | 10 mm² |
| 50A (DC/DC Booster) | 10 mm² | 16 mm² | 25 mm² |
| 100A (Inverter) | 25 mm² | 35 mm² | 50 mm² |
Fuse Sizing
The fuse protects the wire, not the appliance. Always place it as close to the power source as possible (battery or busbar).
- Wire 1.5 mm² → Max fuse 10A
- Wire 2.5 mm² → Max fuse 20A
- Wire 4 mm² → Max fuse 30A
- Wire 6 mm² → Max fuse 40A
- Wire 10 mm² → Max fuse 60A
SCHÉMA ÉLECTRIQUE
PANNEAUX SOLAIRES
0W
REGULATEUR MPPT
BATTERIE AUXILIAIRE
0 Ah
Lithium LiFePO4
FUSIBLE
FUSIBLE
BOÎTE À FUSIBLES 12V
Pompe, Leds, Frigo...
CONVERTISSEUR 220V
NON REQUI
PRO GEARPage 4
SHOPPING LIST
Where to find this equipment? Here is the community-approved selection.
12V 6-way Fuse Box
Mandatory protection
Digital Multimeter
Test your connections
Heavy Duty Crimping Tool
For perfect lugs
Heat Shrink Tubing
Insulation and safety
GENERATED BY VANPOWERCALC PRO - UNLIMITED.
About this tool
The 25% margin isn't arbitrary — it covers the cold-weather voltage boost. Solar panels produce higher voltage in cold air (lower temperature coefficient). A panel rated at 18V Vmp at 25°C can hit 21-22V at -5°C. Your MPPT converts this excess voltage into extra current — which can exceed the nominal rating. Without headroom, the controller clips this bonus energy. The other spec to check: maximum input voltage (Voc). If you wire panels in series, their open-circuit voltages add up. Two panels with 22V Voc in series = 44V. In cold weather, Voc can spike to 48-50V. Make sure your MPPT's max input voltage (typically 75V or 100V for van controllers) can handle this. Exceeding it fries the controller instantly — no warning, no recovery.
Frequently asked questions
What size MPPT controller for 200W solar?▼
A 20A MPPT works but a 30A is better. The math: 200W / 12V × 1.25 margin = 20.8A. A 20A controller will clip on cold sunny mornings when the panels overperform. A 30A gives you room to add a third panel later without replacing the controller.
Can my MPPT controller be too big?▼
It can't be too big for your panels — a 50A controller on 200W of panels works perfectly fine, it just won't reach full capacity. The downside is cost: a 50A Victron is $80 more than a 30A. But if you plan to expand your array later, buying the bigger controller now saves money vs replacing it.
MPPT vs PWM: how much difference?▼
MPPT typically harvests 15-30% more energy than PWM in real-world conditions. The gap is biggest when panel voltage is much higher than battery voltage (cold weather, partially charged battery) and smallest when they're close. For any array over 100W, MPPT pays for itself within the first year.
What happens if I exceed my MPPT's max input voltage?▼
The controller dies immediately. No fuse, no protection — the internal MOSFETs blow. This happens when panels in series produce more Voc than rated, especially in cold weather. Always calculate worst-case Voc at the lowest expected temperature and stay 10% below the controller's limit.
Can I connect different panel wattages to one MPPT?▼
In parallel: yes, if they have similar Vmp (maximum power voltage). In series: not recommended — the weaker panel limits the entire string's current. If you're mixing panels, parallel is almost always the safer choice for a van setup.