The 4-Step Sizing Method
Sizing a van electrical system isn't guesswork — it's arithmetic. Follow these four steps in order and you'll know exactly what to buy before spending a single dollar.
Step 1: Power Audit (→ Battery Size)
List every device, its wattage, and daily usage hours. Multiply to get Wh/day. Divide by 12.8V for Ah/day. Multiply by 2 for two days of autonomy. That's your minimum battery capacity in Ah. Example: 1,000 Wh/day ÷ 12.8V = 78 Ah/day × 2 = 156 Ah minimum → buy a 200Ah LiFePO4.
Step 2: Battery Size → Solar Panel Wattage
Your solar must replenish what you use each day. Take your daily Wh consumption and divide by the sun-hours for your typical location (4-6 in summer, 2-3 in winter for most US/EU locations). 1,000 Wh ÷ 4 sun-hours = 250W minimum panel wattage. Add 25% for real-world losses (heat, angle, partial shade): 250W × 1.25 = 313W → buy 400W of panels for comfortable margin.
Step 3: Solar Wattage → Charge Controller Size
Take your total panel wattage, divide by battery voltage: 400W ÷ 12.8V = 31A charge current. Your MPPT must handle at least 31A output. A Victron 100/30 is right on the edge; a 100/50 gives headroom. For the input voltage: count your panels in series and multiply by Voc. 2 × 40V = 80V — within the 100V limit of the SmartSolar 100/50.
Step 4: Load Wattage → Inverter Size
List every AC device you'll run simultaneously: laptop charger (65W), blender (600W for 30 seconds), small microwave (900W). The inverter must handle the largest simultaneous load plus a startup surge (typically 2× for motors). If you'll run a microwave and laptop together: 900 + 65 = 965W continuous, with a 1800W surge. A 2000W pure sine wave inverter covers this with margin. Don't buy a 3000W inverter "just in case" — larger inverters have higher idle consumption that drains your battery 24/7.
YOUR ENERGY PROFILE.
This document contains the sizing of your future electrical installation, calculated based on your appliances.
Inventory:
Battery
To guarantee 0WH without damaging your bank (80% max discharge):
Solar
Minimum power required to recharge your consumption:
220V AC
Maximum power (with 25% safety margin).
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
BOÎTE À FUSIBLES 12V
Pompe, Leds, Frigo...
CONVERTISSEUR 220V
NON REQUI
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
Comparison table
| Component | Sizing Formula | Example (1000 Wh/day) | Recommended |
|---|---|---|---|
| Battery | Wh/day ÷ 12.8V × 2 | 156 Ah minimum | 200Ah LiFePO4 |
| Solar | Wh/day ÷ sun-hours × 1.25 | 313W minimum | 400W panels |
| MPPT Controller | Solar W ÷ battery V | 31A minimum | Victron 100/50 |
| Inverter | Max simultaneous AC load × 1.5 | 1500W | 2000W pure sine |
About this tool
Sizing a complete van electrical system combines three engineering disciplines: load analysis, energy storage chemistry, and electrical circuit design. Most van builders approach it backwards — buying panels because they look good on a roof, then wondering why the battery is always dead. The correct sequence is loads first, battery second, solar and charging third.
Load analysis starts with a spreadsheet. List every electrical device you plan to use: the brand and model (not generic) because a Dometic CFX3 45 fridge draws 2.5-3.5A average while a cheap no-name 45L draws 4-6A for the same cooling. Multiply watts by hours-per-day for each device to get Wh/day. Add all values. This is your design target.
Battery sizing: target 2× daily consumption for 2 days autonomy (standard), or 3× for 3-day autonomy (recommended for rainy-season van life). A 100Ah LiFePO4 = 1200Wh usable. If your daily total is 700Wh, 100Ah gives you 1.7 days autonomy — marginal. Step up to 150Ah (1800Wh) for a comfortable 2.5-day buffer.
Solar sizing: take daily consumption, divide by your location's average peak sun hours, multiply by 1.25 for real-world losses. 700Wh ÷ 4 PSH × 1.25 = 218W minimum. Two 120W panels give exactly 240W — sufficient. Two 200W panels give 400W — comfortable with headroom for cloudy days.
Charge controller: MPPT output amperage = solar watts ÷ battery voltage × 1.1 = 400 ÷ 12 × 1.1 = 36.7A. Use a 40A MPPT controller (Victron SmartSolar MPPT 100/50). Wire panels in series (if 12V nominal panels: open circuit 40V) — within the 100V controller limit.
DC-DC charger for driving: Victron Orion-Tr Smart 12-12|30 as standard. Install between starter and leisure battery. Adds 30A = 360W charging while engine running.
Distribution: positive busbar + ANL fuse + fused circuits per load. Negative busbar connected to battery negative only (no chassis ground for the leisure circuit — keeps the system clean and prevents galvanic corrosion).
Electrical system planning before you buy anything: the biggest efficiency gain in van electrical design happens before a single component is purchased. Map the physical location of every load in the van (sink = water pump, bed area = lighting + heater control, work desk = laptop + monitors + USB), then calculate the cable run from your battery bank location to each load. Shorter runs require thinner cable and smaller fuses — for a load circuit, cost savings of €5-15 per circuit add up across 8-12 circuits in a typical build.
Fuse and breaker strategy: The main ANL fuse (or class T fuse for high-current installs) protects the battery from a short circuit on the main bus. Size it for 125% of the maximum sustained load your system will draw simultaneously. Downstream circuit breakers protect individual circuits. A 15A circuit breaker for a 12V laptop circuit costs €8 and allows safe resettable protection — preferable to a blade fuse that requires a spare parts inventory.
Two things most VanLife beginners forget: 1) Grounding. Every load needs a return wire back to the battery negative — do not use chassis ground for 12V DC circuits in modern vehicles with CAN-bus electronics. 2) Shore power safety. If you install a 230V shore power inlet, include a 30mA RCD (residual current device) on the incoming line inside the van — a basic safety device for any AC installation.