Barbara Mitchell
Most solar sizing guides will tell you to “calculate your daily watt-hours, then add 20% for buffer.” That advice isn’t wrong, but it’s incomplete enough to get you into real trouble. I’ve watched people park a shiny new rig loaded with 400 watts of rooftop panels and then scratch their heads when they’re running on empty by 7 PM. The math wasn’t the problem. The assumptions were.
Here’s the honest version, from someone who’s been running off solar as a primary power source for going on six years.
The Number Everyone Wants First
Fine. If you’re full-timing and want a single ballpark: 800 to 1,200 watts of solar is the range where most people land without constantly white-knuckling their battery state of charge. That’s not a magic number, and I’ll explain why yours might be lower or significantly higher. But if someone asks me over coffee what they should budget for in terms of panel wattage, that’s what I say.
400 watts, which used to be the popular “starter” recommendation, is survivable if you’re running minimal appliances, living somewhere consistently sunny, and don’t use air conditioning. I ran 400 watts for my first eight months on the road. It worked until it didn’t. One cloudy week in the Pacific Northwest nearly wiped out my food when the compressor fridge started cycling erratically from low voltage. After that I bumped to 800W and slept better.
The ceiling matters too. There’s a practical limit based on your roof square footage and your battery bank capacity. Dumping 2,000 watts into 200Ah of lead-acid is just going to cook your batteries faster. Panel wattage and battery capacity need to grow together.
Actually Calculating What You Need
| Load Item | Daily Consumption | Notes |
|---|---|---|
| 12V compressor fridge (40% duty cycle) | ~280Wh | Ambient temp dependent |
| LED lighting (4 hours) | ~40Wh | Conservative usage |
| Laptop (4 hours) | ~120Wh | Typical work load |
| Phone and tablet charging | ~30Wh | Daily tops |
| Diesel heater fan | ~30-80Wh | Variable by settings |
| Water pump | ~20Wh | Minimal usage |
| Ceiling fan (6 hours) | ~60Wh | Warm weather |
| Subtotal (no CPAP) | ~580-610Wh | Conservative baseline |
| CPAP with humidifier off | ~30-60Wh/night | Adds ~650-670Wh total |
| Rooftop AC (13,500 BTU, 4 hrs/day) | ~5,000-6,000Wh | Requires 1,500W+ panels or generator |
Skip the vague “add up your appliances” advice. Here’s how to do it properly.
List every device you run, its wattage, and how many hours per day you actually use it. Not what the manual says. What you actually do. A 12V compressor fridge (something like the BougeRV 12V compressor fridge that most serious full-timers use) draws roughly 30-50Wh per hour but only runs maybe 30-40% of the time depending on ambient temperature. That’s roughly 200-350Wh per day just for the fridge.
A realistic full-timer load looks something like this:
- 12V compressor fridge running at 40% duty cycle: ~280Wh/day
- LED lighting for 4 hours: ~40Wh/day
- Laptop, 4 hours: ~120Wh/day
- Phone and tablet charging: ~30Wh/day
- Diesel heater fan (Webasto, Espar, or a cheap Vevor knock-off): ~30-80Wh/day depending on settings
- Water pump: ~20Wh/day
- Ceiling fan, 6 hours: ~60Wh/day
That’s a conservative 580-610Wh daily for a no-air-conditioning setup. Add a CPAP machine (with humidifier off, about 30-60Wh/night) and you’re pushing 650-670Wh.
Now the solar math. A 100W panel in good sun produces roughly 350-400Wh on a solid day. Six peak sun hours is optimistic for most of the country for most of the year. Four hours is more honest. So a 100W panel is giving you maybe 300-350Wh daily on average across seasons. Divide your daily consumption by that per-panel output, round up, and that’s your rough panel count.
600Wh daily ÷ 320Wh per 100W panel = ~1.9 panels. Call it 2, add your 20% buffer, and you’re at 240W minimum under ideal conditions. But ideal conditions don’t last all year. Double it to 480W and you’re genuinely comfortable most of the time. That’s why 400W feels tight and 600-800W feels right for a modest load without AC.
Add a rooftop air conditioner and the entire equation changes. A standard 13,500 BTU RV AC pulls 1,200-1,500W while running. Running it four hours a day adds 5,000-6,000Wh to your daily total. That requires either a very large solar array (1,500W+), an inverter/charger paired with a substantial lithium battery bank (400Ah minimum, realistically 600Ah), or a generator to bridge the gap. People who tell you they run AC full-time on solar alone without a generator aren’t lying, but they’re either in Phoenix with 2,000W of panels, or they’re only running it for an hour at dusk. There’s no shame in a quiet Honda EU2200i as a backup.
Panel Type, Roof Space, and the Rigid vs. Flexible Debate
7 Things We Wish We Knew BEFORE Starting RV Life FULL-TIME (1 month on the road) · Lloyd & Mandy on YouTube
Monocrystalline panels are the right choice. Polycrystalline are cheaper per watt but less efficient in partial shade and take more roof space per watt. Flexible panels are convenient for curved roofs but they run hot (which kills output and shortens lifespan), and I’ve had two delaminate within three years in desert heat. If your roof can take rigid panels, use rigid panels.
Space is often the real constraint. A standard Class C or travel trailer roof has maybe 80-120 square feet of usable area once you account for vents, air conditioner units, and roof racks. A typical 200W rigid monocrystalline panel is about 17-18 square feet. Do the math on your actual roof before you spec out 1,600W and realize you can only physically fit 800W.
Class B vans are the hardest case. A 170-inch wheelbase Sprinter has a narrow roof, and if you’re also running a roof vent fan and staying low-profile for parking garages, you might max out at 400-600W of panels. Van lifers often compensate with a foldable portable panel like the Jackery SolarSaga 200W that they can deploy at a campsite and stow while driving. Works fine if you’re disciplined about it.
A good solar charge controller matters more than most people think. MPPT controllers (Victron SmartSolar is the one I trust) squeeze 20-30% more power out of your panels than PWM controllers, especially in partial shade and cold weather. Don’t cheap out here. A Victron SmartSolar 100/30 runs around $120-150 and will outlast three budget alternatives.
Battery Bank: Solar’s Neglected Partner
You can have 1,200W of solar and a 100Ah battery and still run out of power. Panel wattage means nothing if you don’t have the storage to hold what you generate.
Lithium (LiFePO4) is the current standard for full-timers with good reason. You can use 80-100% of the rated capacity, they charge fast, and they handle being at partial state of charge without the sulfation damage that destroys lead-acid in the same situation. A 200Ah LiFePO4 bank (roughly $500-800 for a decent drop-in like Battle Born or a quality BMS-equipped unit from Ampere Time) is a reasonable minimum for a 600-800W solar system. For anything bigger, go 400Ah.
Get a proper battery monitor. The Victron BMV-712 is about $100 and tells you exactly what’s going in and out of your battery in real time. Guessing at state of charge based on a single voltage reading is how you deep-cycle lead-acid into an early grave or wonder why your lithium bank died sooner than expected.
What the Influencers Get Wrong
Sources
- BougeRV 12V compressor fridge
- Jackery SolarSaga 200W
- solar charge controller
- Victron BMV-712
- quality surge protector
Here’s the contrarian take: most solar build videos are filmed in summer, in the Southwest, at full sun. Then they’re watched in January in Oregon.
The Pacific Northwest, the Upper Midwest in winter, the Gulf Coast in rainy season, the Northeast in November through March. These regions cut your effective solar production by 40-60% compared to the summer Arizona numbers everybody uses for their “proof of concept” videos. If you’re full-timing and you move around, design your system for bad sun conditions, not the best ones. That means more panels than the math says you need on a perfect day.
I’ve also seen too many builds skip a quality surge protector and water filtration system. Not directly solar-related, but if you’re running a big inverter system and plugging into shore power occasionally, an unprotected hookup can fry your inverter/charger. Ask me how I know. (I don’t love talking about the $600 mistake from my second year on the road.)
The right system size is the one that matches your real habits and your real travel geography, not the YouTube build that looks great in June outside Moab. Start with an honest load calculation, design for your worst-sun month rather than your best, and leave room to expand. Roof space and battery capacity first, then fill in the panels. That order matters more than the watt count.
Photo: Giant Asparagus via Pexels
