Stop Guessing — Here’s the Exact Math
The most common van life solar mistake: buying a “200W panel and 100Ah battery” because someone online had that setup, without knowing if it actually matches your power consumption. The result is either a chronically dead battery (not enough) or thousands of dollars of unused equipment (too much). This guide gives you the exact framework to size your system correctly.
Step 1: Calculate Your Daily Power Consumption (Wh/day)
List every electrical device you’ll use and estimate daily runtime:
| Device | Watts | Hours/day | Wh/day |
|---|---|---|---|
| 12V compressor fridge | 40 (avg) | 24 | 960 |
| Laptop | 60 | 8 | 480 |
| Phone charging | 15 | 2 | 30 |
| LED lighting | 20 | 4 | 80 |
| Fan (ceiling vent) | 25 | 8 | 200 |
| Water pump (12V) | 60 | 0.3 | 18 |
| Diesel heater controller | 10 | 8 | 80 |
| Inverter standby | 5 | 24 | 120 |
| Total (example) | 1,968Wh |
This is a typical van life daily consumption: ~2kWh/day for a solo or couple setup with a fridge, laptop work, and basic lighting. Add more for: multiple laptops, drone charging, external monitor, coffee maker, hair dryer, or anything else you can’t live without.
Step 2: Calculate Battery Bank Size
Formula: Daily consumption × Autonomy days ÷ Usable depth of discharge = Battery capacity needed
Key Variables
- Autonomy days: How many days without sun do you need? For van life, 2–3 days is standard (covers most overcast stretches)
- Depth of discharge (DoD): LiFePO4 batteries can safely discharge to 100%, but for longevity, use 80–90% as your planning target
Example Calculation
Daily consumption: 2,000Wh
Autonomy: 2 days
DoD: 90%
Battery needed = 2,000Wh × 2 ÷ 0.90 = 4,444Wh ≈ 4.5kWh
In 12V terms: 4,444Wh ÷ 12V = 370Ah → round up to 400Ah at 12V
Alternative: 200Ah at 24V (more efficient for larger inverters)
Battery Size Recommendations by Use Case
| Use Case | Daily Use (Wh) | Recommended Battery | Typical Setup |
|---|---|---|---|
| Minimalist (no fridge) | ~500Wh | 100Ah 12V | Laptop + phone + lights |
| Weekend warrior | ~1,000Wh | 200Ah 12V | Small fridge + basics |
| Full-time solo | ~1,500–2,000Wh | 300–400Ah 12V | Fridge + laptop + fan |
| Full-time couple | ~2,500–3,000Wh | 400–600Ah 12V | 2 laptops + fridge + extras |
| Digital nomad heavy | ~3,000–4,000Wh | 600Ah+ or 24V system | Multiple screens + AC fan |
Step 3: Calculate Solar Panel Array Size
Formula: Daily consumption ÷ Peak sun hours ÷ System efficiency = Solar watts needed
Peak Sun Hours by Region
Peak sun hours = hours per day your panels produce their rated output. This varies by location and season:
- Southwest US (AZ, NM, NV): 5.5–7 hours (ideal for van life solar)
- West Coast, Mountain West: 4.5–6 hours
- Midwest, Southeast: 4–5.5 hours
- Pacific Northwest, New England: 3–4 hours (challenging in winter)
- UK/Germany: 2–3 hours
System Efficiency Factor
Real-world solar systems are 75–85% efficient due to: MPPT controller losses (~3%), wiring losses (~2%), battery charging losses (~5%), temperature derating (~5–10%), and panel orientation losses (van panels are fixed, not always at ideal angle). Use 0.80 as your efficiency multiplier.
Example Calculation
Daily consumption: 2,000Wh
Peak sun hours: 5 (Southwest US average)
System efficiency: 80%
Solar watts = 2,000 ÷ 5 ÷ 0.80 = 500W of panels
Solar Panel Sizing by Setup
| Daily Use | Location | Solar Needed | Panel Config |
|---|---|---|---|
| 500Wh | Southwest | 125W | 1 × 160W (enough headroom) |
| 1,000Wh | Southwest | 250W | 2 × 160W or 1 × 280W |
| 2,000Wh | Southwest | 500W | 3 × 175W or 2 × 280W |
| 2,000Wh | Pacific NW | 833W | 4 × 220W (may not fit on small van) |
| 3,000Wh | Southwest | 750W | 4 × 200W or 3 × 280W |
Step 4: Choose Your Charge Controller
The MPPT charge controller is the brain of your solar system. Size it to handle your panel array’s max current:
MPPT vs PWM
Always use MPPT for van life. PWM controllers are 15–20% less efficient — on a 400W array at 5 peak sun hours, that’s 240Wh wasted per day. MPPT controllers pay for themselves in 3–6 months.
Sizing the Controller
MPPT controller amps = Total panel watts ÷ Battery voltage × 1.25 (safety factor)
- 400W panels on 12V: 400 ÷ 12 × 1.25 = 41.7A → Use a 40A or 60A MPPT
- 600W panels on 12V: 600 ÷ 12 × 1.25 = 62.5A → Use a 80A MPPT
- 600W panels on 24V: 600 ÷ 24 × 1.25 = 31.25A → Use a 40A MPPT
Top MPPT Recommendations
- Victron SmartSolar 100/30 ($110): Up to 30A, 12/24V, Bluetooth monitoring. Best for up to 400W on 12V.
- Victron SmartSolar 100/50 ($160): Up to 50A, 12/24V. Best for 400–600W on 12V.
- Renogy MPPT 60A ($130): Budget option for 400–700W on 12V. Good for cost-conscious builds.
Step 5: Choose Your Inverter
The inverter converts 12V DC to 120V AC for your household appliances. Size it to handle your largest load simultaneously:
- 400W inverter: Laptops, phone charging, LED lighting — no large appliances
- 1,000W inverter: Coffee maker (800W), CPAP, TV
- 2,000W inverter: Microwave (1,200W), hair dryer, power tools
- 3,000W+ inverter: Everything except large AC units
Always buy a pure sine wave inverter — modified sine wave damages sensitive electronics (laptops, phone chargers, medical devices). The Renogy 2000W Pure Sine ($200) or Victron Multiplus 12/2000 ($500, with charger built in) are the top choices.
Complete System Recommendations
Budget Weekend Setup (~$1,200)
- Battery: 200Ah 12V LFP — Ampere Time ($449)
- Solar: 2 × 175W rigid panels — Renogy ($280)
- MPPT: Victron SmartSolar 75/15 ($65)
- Inverter: Renogy 1,000W pure sine ($120)
- Wiring + fuses: ($150)
- 12V fridge: BougeRV CR45 ($299)
Full-Time Solo Setup (~$2,500)
- Battery: 400Ah 12V LFP — 2 × Renogy 200Ah ($1,098)
- Solar: 3 × 200W rigid panels — Renogy ($420)
- MPPT: Victron SmartSolar 100/50 ($160)
- Inverter: Victron Multiplus 12/2000 ($500)
- Wiring + fuses + busbar: ($200)
- 12V fridge: Iceco JP50 ($299)
Full-Time Couple / Digital Nomad (~$4,500)
- Battery: 600Ah 12V LFP — 3 × BattleBorn 200Ah ($2,097) or switch to 24V for efficiency
- Solar: 4 × 200W rigid panels ($560)
- MPPT: Victron SmartSolar 100/50 × 2 ($320)
- Inverter/Charger: Victron Multiplus 24/3000 ($750)
- Wiring + fuses + Cerbo GX: ($500)
- 12V fridge: Dometic CFX3 55 ($649)
Common Sizing Mistakes
- Undersizing the battery for your fridge: A residential fridge draws 1.5–2× more power than a 12V compressor fridge. If you’re keeping the residential fridge, double your battery estimate.
- Ignoring cloudy day buffer: Don’t size for “average sun” — size for 2–3 consecutive cloudy days. The Pacific Northwest can go 5+ days without significant solar production in winter.
- Forgetting parasitic drain: Bluetooth devices, solar controller display, inverter standby, and battery monitor collectively draw 5–15W continuously — add 100–300Wh/day to your estimate.
- Mismatching panel and controller voltage: Panel VOC (open circuit voltage) must be below the MPPT’s maximum input voltage. Always check specs before wiring.