The most common solar mistake in RV builds: undersizing. A system that looks adequate on paper — 400W of panels, 200Ah of battery — fails in practice because the sizing was based on ideal conditions. This guide walks through the real math with three rig profiles and shows you exactly what to buy.
Step 1: Calculate Your Daily Load
List every electrical load in your rig. For each one, estimate daily watt-hours: watts × hours of daily use. Include inverter losses (~10% overhead on AC loads).
| Load | Typical Watts | Hours/Day | Daily Wh |
|---|---|---|---|
| 12V residential fridge | 50W avg | 24h | ~120 Wh |
| Laptop (15") | 45–65W | 8h | 360–520 Wh |
| External monitor (24") | 25–35W | 8h | 200–280 Wh |
| LED lighting | 10–20W | 4h | 40–80 Wh |
| Phone + tablet charging | 15–20W | 2h | 30–40 Wh |
| Wi-Fi router/hotspot | 10–15W | 16h | 160–240 Wh |
| CPAP (no heat) | 30–40W | 8h | 240–320 Wh |
| 12V fan (Fantastic Fan) | 20–40W | 6h | 120–240 Wh |
| Inverter losses (~10%) | — | — | Add 10% to AC loads |
Step 2: Calculate Required Solar
Solar panels produce their rated wattage only at peak sun (1,000 W/m² irradiance, 25°C cell temperature). Real-world conditions cut that significantly. The formula:
Solar needed = Daily load (Wh) ÷ (Peak sun hours × 0.75 derating factor)
The 0.75 derating accounts for panel temperature losses (~10%), wiring losses (~5%), and charge controller inefficiency (~10%). Peak sun hours vary by location: Arizona gets 6–7 hours, the Pacific Northwest gets 3–4 hours in winter, the Southeast gets 4–5 year-round.
Using 4.5 peak sun hours (national average): for a 600 Wh/day load, you need 600 ÷ (4.5 × 0.75) = 178W minimum. Size up to 300W for cloudy day buffer.
Three Rig Profiles with Real Numbers
| Profile | Daily Load | Solar (panels) | Battery Bank | Cloudy Day Reserve | Hardware Cost (est.) |
|---|---|---|---|---|---|
| Weekend Warrior | 200–300 Wh | 200W (2×100W) | 100Ah LiFePO4 | ~1 day | ~$800–1,200 |
| Light Full-Timer | 400–500 Wh | 400W (2×200W) | 200Ah LiFePO4 | ~1.5 days | ~$1,800–2,500 |
| Working Full-Timer | 600–800 Wh | 800W (4×200W) | 400Ah LiFePO4 | ~2–3 days | ~$3,500–5,000 |
Seasonal Adjustment Factors
Solar output drops significantly in winter at northern latitudes. If you travel year-round, size for your worst-case season:
- Sun Belt (AZ, NM, TX, FL) year-round: Use 5.5 peak sun hours. Standard sizing holds.
- Pacific Northwest winter: Use 3.0 peak sun hours. Increase solar by ~50% or plan generator backup.
- Mountain states (CO, UT, WY) winter: Snow accumulation on panels is the bigger problem — panel angle and a quick brush matters more than raw size.
- Northern US/Canada winter: Solar supplemental only. Plan on shore power or generator for primary charging Oct–Mar.
Charge Controller: MPPT vs. PWM
Always use MPPT (Maximum Power Point Tracking) for any system over 100W. PWM wastes 20–30% of available solar power. Victron SmartSolar and Renogy Rover are the standard choices. Size the charge controller to handle 125% of your panel wattage (for future expansion) and the expected short-circuit current of your array.
Bottom Line
Size bigger than you think you need. Solar panels and lithium batteries have gotten cheap enough that the cost of undersizing — buying again in 6 months — is worse than sizing correctly the first time. A working full-timer needs at least 600W of panels and 300Ah of LiFePO4. If you have roof space for 800W, use it. The incremental cost is small compared to the peace of mind on consecutive cloudy days.