Sizing a van electrical system has a correct order of operations — get the sequence wrong and you end up sizing components around each other's mistakes instead of around your actual needs. Here's the full process, tied together.

The Correct Order

1. Calculate daily load (Wh) → 2. Size battery bank (Ah) → 3. Size charging sources
(solar + DC-DC) → 4. Size inverter (for peak simultaneous load) → 5. Size wire and fuses

Each step depends on the one before it — sizing an inverter before knowing your battery capacity, for example, risks choosing an inverter that can draw more current than your battery bank can sustainably deliver.

Step 1: Calculate Daily Load

Start with a full appliance-by-appliance daily watt-hour calculation (see our electrical load calculator guide). This single number — your total daily Wh — drives everything downstream.

Step 2: Size the Battery Bank

Convert daily Wh to amp-hours at your system voltage, then divide by your battery chemistry's safe depth of discharge, then multiply by your desired days of autonomy (see our battery bank sizing guide for the full formula). This gives you your target battery capacity in Ah.

Step 3: Size Your Charging Sources

Decide how you'll recharge that battery bank — solar, DC-DC (alternator) charging, shore power, or some combination (most modern builds use at least two). Size solar wattage against your daily Wh and realistic sun hours (see our solar wattage guide); size a DC-DC charger against how much driving time you realistically get between parked stretches.

Step 4: Size the Inverter

This is a separate calculation from your daily Wh total — inverter sizing is about peak simultaneous load (what you might run at the same time), not total daily energy use. Add up your worst-case combination of appliances running together, check both continuous and surge/startup wattage, and size the inverter above that (see our inverter sizing guide).

Step 5: Size Wire and Fuses for Every Circuit

Once you know your battery bank, inverter, and major appliance amp draws, size the wire gauge for each specific circuit based on that circuit's amp draw and run length (see our wire gauge guide), and fuse each circuit to protect the wire, not the appliance (see our fuse sizing guide).

Why Doing This Out of Order Causes Real Problems

  • Sizing an inverter before a battery bank: you might buy an inverter capable of drawing more current than your battery bank and wiring can sustainably deliver, causing voltage sag or overheating under load.
  • Sizing solar before knowing daily load: you either oversize (wasted budget, unnecessary roof space) or undersize (chronic battery deficit) relative to what you actually need.
  • Wiring circuits before finalizing appliance choices: leads to re-wiring runs when appliance draw turns out different than assumed during rough planning.

A Worked Example, Start to Finish

  1. Daily load: 1,000 Wh/day (moderate use — fridge, lighting, laptop, water pump)
  2. Battery bank: 1,000 Wh ÷ 12V = 83.3 Ah/day → ÷ 0.9 (lithium DoD) × 2 days autonomy ≈ 185 Ah, rounded to 200Ah
  3. Solar: 1,000 Wh ÷ (4 sun hours × 0.8 efficiency) ≈ 312W, rounded to 400W for margin
  4. DC-DC charger: 40A charger to restore ~40Ah/hour of driving — a few hours of driving substantially recharges the 200Ah bank
  5. Inverter: worst-case simultaneous load (fridge + laptop + induction cooktop) ≈ 1,700W continuous → 2000W inverter with margin
  6. Wire/fuses: each circuit sized individually against its own amp draw and run length — main battery-to-inverter run at ~166A continuous needs heavy gauge wire and an appropriately rated fuse near the battery terminal

Frequently Asked Questions

What order should I size my van electrical system in? Load calculation first, then battery bank, then charging sources (solar/DC-DC), then inverter, then wire/fuses for each circuit — each step depends on the results of the one before it.

Can I just buy a pre-sized electrical kit instead of calculating this myself? Pre-sized kits can work well if they roughly match your actual calculated daily load and peak simultaneous use — but without doing your own load calculation first, you can't verify whether a kit's stated capacity actually fits your specific appliance mix and usage pattern.

How much should I oversize my electrical system for safety margin? Many builders add 15-20% margin across battery and solar sizing to account for real-world losses and unexpected usage days — beyond that, oversizing mainly adds cost and weight without proportional benefit.