Solar Panel Sizing Guide for Overlanders

The Solar Sizing Problem

Every overlander who's looked at solar panels has asked the same question: "How many watts do I actually need?" The answer isn't on a spec sheet — it depends on what you're running, where you're camping, and how long you stay parked.

I've seen people bolt 400W of solar onto a rig that draws 30Ah per day, and I've seen people try to run a fridge and inverter off a single 100W panel in the Pacific Northwest in December. Both are mistakes. Let's do the math properly.

Step 1: Calculate Your Daily Load

Before you think about panels, you need to know what you're feeding. List every device that draws power from your auxiliary battery and estimate daily usage:

• 12V compressor fridge (50–65L): 2–4A draw, cycles ~40% of the time = ~1–1.6A average × 24h = 24–38Ah/day
• LED light bar (interior): 1–2A × 4 hours = 4–8Ah/day
• Phone/tablet charging: ~2A × 3 hours = 6Ah/day
• USB fan: 0.5A × 8 hours = 4Ah/day
• Laptop (via inverter): ~5A (at 12V) × 2 hours = 10Ah/day

A typical overlanding setup without heavy inverter use draws 35–55Ah per day. Add a laptop or CPAP machine and you're looking at 50–80Ah. Be honest with yourself about what you actually run — overestimating leads to buying panels you don't need, but underestimating means dead batteries on day two of a boondocking trip.

Step 2: Understand Real-World Panel Output

This is where most people get tripped up. A "200W" solar panel does not produce 200 watts. That rating is under Standard Test Conditions (STC): 1000W/m² irradiance, 25°C cell temperature, AM1.5 spectrum. In the real world, you'll see 70–85% of rated output under good conditions.

A more realistic calculation:

• 200W panel, ideal summer conditions (Southwest US): ~160–170W actual × 5–6 peak sun hours = 800–1020Wh = 67–85Ah at 12V
• 200W panel, shoulder season (mid-latitudes): ~140–160W × 3–4 peak sun hours = 420–640Wh = 35–53Ah
• 200W panel, winter or Pacific Northwest: ~120–140W × 2–3 peak sun hours = 240–420Wh = 20–35Ah

Peak sun hours (PSH) is the critical variable. Arizona in July gets 7+ PSH. Oregon in December gets 1.5. Plan for where and when you actually camp, not best-case scenarios.

Step 3: Size Your Array

The formula is straightforward:

Panel wattage needed = (Daily Ah load × 12V) ÷ (Peak Sun Hours × 0.8 efficiency factor)

For a 50Ah/day load camping in shoulder season with 4 PSH:

(50 × 12) ÷ (4 × 0.8) = 600 ÷ 3.2 = 188W minimum

Round up for margin. A 200W array handles this load. If you winter camp regularly, double it — or accept that you'll supplement with alternator charging on drive days.

For a deep dive on the best panels and charge controllers for overlanding rigs, see our best solar setups for overlanding roundup.

Fixed vs Portable Panels

Fixed (Roof-Mounted) Panels

Pros:

• Always deployed — no setup required
• Charges while driving (supplementing alternator)
• No theft risk
• Clean installation with permanent wiring

Cons:

• Can't angle toward the sun — you lose 20–40% vs an optimally tilted panel
• Vehicle must be parked in sun (no shade camping)
• Competes for roof space with racks, tents, and gear
• Panel temperature runs higher on a dark roof, reducing output

Portable (Ground-Deployed) Panels

Pros:

• Can be angled and repositioned to track the sun
• Park in shade, deploy panels in sun — best of both worlds
• No permanent modifications to vehicle
• Easy to upgrade or replace

Cons:

• Must be set up and taken down each time
• Theft risk if left unattended
• Cables running to vehicle can be tripped over or damaged
• Must be stowed during travel

My recommendation: fixed panels for your baseline load, portable for supplemental power. A 200W fixed panel handles daily essentials while driving and at camp. A folding portable panel (100–200W) lets you boost charging on cloudy days or when you're parked in the trees.

Charge Controllers: PWM vs MPPT

If you're running more than 100W of solar, use an MPPT charge controller. Period. MPPT (Maximum Power Point Tracking) controllers convert the higher voltage output of solar panels to the optimal charging voltage for your battery, recovering 15–30% more energy than a PWM controller. The Victron SmartSolar and Renogy Rover are both excellent choices.

Size your charge controller to handle your total panel wattage with headroom. A 200W array needs a 20A controller minimum; a 400W array needs 30–40A.

All-in-One Power Stations as a Solar Solution

If a full custom solar installation sounds like more than you want to tackle, portable power stations with solar input offer a plug-and-play alternative. The Jackery Explorer 1000 pairs with folding solar panels and includes a built-in MPPT controller, inverter, and battery management — no wiring required. The EcoFlow Delta 2 is another strong option with faster solar charging input (up to 500W).

The tradeoff: you'll pay more per watt-hour of storage compared to a DIY system, and capacity is limited compared to a proper aux battery bank. But for weekend warriors or people who don't want to cut into their vehicle's wiring, they work.

Seasonal Strategy

Don't try to build a solar system that covers worst-case winter scenarios — you'll overspend and overweight your roof. Instead:

• Summer/shoulder season: Solar handles everything. 200W fixed is usually sufficient for moderate loads.
• Winter/cloudy conditions: Rely on alternator charging during drive days. A DC-DC charger at 30–40A can replenish 80–100Ah in 2–3 hours of driving.
• Extended stationary camping: Deploy portable panels to supplement. A 200W portable + 200W fixed gives you resilience even in mediocre sun.

Common Mistakes

• Ignoring partial shade: Even 10% shade on a series-wired panel can cut output by 50%+. Use panels with bypass diodes, and consider parallel wiring for partially shaded roofs.
• Forgetting cable losses: Use 10 AWG minimum for runs over 10 feet from panel to controller. Thinner wire loses meaningful power.
• Mounting panels flat: A 10–15° tilt improves output and lets rain wash off dust. Flat-mounted panels collect grime that reduces output over time.
• No battery monitor: Without knowing your actual state of charge, you're guessing. A shunt-based monitor pays for itself in peace of mind.

Size your solar for what you actually use, plan for the conditions you actually camp in, and always have alternator charging as your backup. Solar is the best upgrade you can make to an overlanding rig — when it's sized right.