Electrical System Sizing Guide
Sizing a van electrical system comes down to answering one question: how much power do you use in a day, and how will you replace it? Get this right and your system runs smoothly. Get it wrong and you’re constantly running out of power or spending money on capacity you don’t need.Step 1: Calculate Daily Power Consumption
List every electrical device you’ll use and estimate how many hours per day each runs. Then calculate amp-hours (Ah) for each. Formula: Watts ÷ Voltage = Amps. Amps × Hours = Amp-Hours (Ah)Example Load Calculation
| Device | Watts | Hours/Day | Daily Ah (at 12V) |
|---|---|---|---|
| LED lights | 20W | 5h | 8.3 Ah |
| Fridge (compressor) | 45W | 12h (cycling) | 22.5 Ah |
| Vent fan (MaxxAir) | 15W | 8h | 10 Ah |
| Phone charging | 15W | 3h | 3.75 Ah |
| Laptop | 60W | 4h | 20 Ah |
| Water pump | 60W | 0.5h | 2.5 Ah |
| Total | 67 Ah/day |
Step 2: Size Your Battery Bank
You don’t want to drain your batteries to zero — that kills their lifespan (even lithium). A safe depth of discharge for lithium (LiFePO4) batteries is around 80%. Lead-acid should only go to 50%. Formula: Daily Ah ÷ Depth of Discharge = Minimum Battery Capacity Using the example above:- Lithium: 67 Ah ÷ 0.80 = 84 Ah minimum
- Lead-acid: 67 Ah ÷ 0.50 = 134 Ah minimum
- Lithium (2 days autonomy): 67 × 2 ÷ 0.80 = 168 Ah → a 200Ah battery bank is your sweet spot
- Lead-acid (2 days autonomy): 67 × 2 ÷ 0.50 = 268 Ah → two 6V golf cart batteries in series, or similar
Step 3: Size Your Solar Array
Solar panels need to replace what you use each day. But panels don’t produce their rated wattage all day — real-world output depends on climate, angle, shade, and season. Rule of thumb: In Southern California, a rooftop panel produces roughly 4–5 peak sun hours per day on average. In the Pacific Northwest or winter months, that drops to 2–3. Formula: Daily Ah × Battery Voltage ÷ Peak Sun Hours ÷ System Efficiency (0.85) = Minimum Solar Watts Using the example (67 Ah/day):- 67 Ah × 12V = 804 Wh/day
- 804 ÷ 4.5 sun hours ÷ 0.85 efficiency = 210W minimum
Step 4: Alternator Charging
Don’t forget — your van’s alternator is a charging source too. A DC-DC charger (like a Victron Orion) pulls power from the alternator while you drive and feeds it to your house batteries. A 30A DC-DC charger puts about 30Ah per hour of driving into your batteries. If you drive 2 hours, that’s 60Ah — nearly a full day’s consumption for the example above. For people who drive regularly, alternator charging can be your primary power source, with solar as backup. For people who stay parked for days, solar becomes more critical.Step 5: Wire Sizing
Wire gauge depends on two things: current (amps) and wire length (round trip distance). Higher current = thicker wire. Longer distance = thicker wire. The goal is to keep voltage drop under 3% on any run.Quick Reference: Common Wire Gauges
| Circuit | Typical Current | Recommended Gauge (up to 10ft) |
|---|---|---|
| LED lights | 2–5A | 14 AWG |
| Vent fan | 2–3A | 14 AWG |
| Fridge | 5–8A | 12 AWG |
| Water pump | 5–10A | 12 AWG |
| Inverter (2000W) | 170A+ | 2/0 AWG |
| Battery to bus bar | 100A+ | 4–2/0 AWG |
Putting It Together
For the example 67 Ah/day system:| Component | Spec |
|---|---|
| Battery bank | 200Ah lithium (LiFePO4) |
| Solar | 300W (2× 150W panels) |
| Charge controller | 30A MPPT |
| DC-DC charger | 30A |
| Inverter | 1000–2000W pure sine |
| Shore power charger | 20A (optional) |