Solar panel angle and placement: why your fixed roof panels aren't the whole story

Parked under a coolibah on the Fitzroy River last June, my 200W roof panel was pulling maybe 40W at midday. Full sun, decent battery, nothing wrong with the wiring. The problem was three metres of shade off one branch, and a roof that was flat to a sky where the sun sits low all winter across the top end. That's when the folding panel came out of the back of the Troopy and did the actual work for the rest of the week.

Fixed roof solar gets sold as the "set and forget" option, and for a lot of touring it's genuinely fine. But if you're free camping seriously, especially anywhere north of about the Tropic of Capricorn in the cooler months, or bush camping under trees rather than in open showground-style sites, the gap between what your roof panel is rated to produce and what it actually delivers can be enormous. Understanding why is most of the battle.

The angle problem nobody puts on the box

A solar panel produces its rated output when the sun's rays hit it dead square, at 90 degrees. Every degree off that and you lose output, roughly following the cosine of the angle of incidence. It's not linear and it's not dramatic at first, but by the time you're 40-50 degrees off square you're losing a third or more of potential output, and it gets worse fast after that.

Here's the bit that surprises new tourers: a flat roof panel is rarely square to the sun. At Broome in July, the sun sits around 35-40 degrees above the horizon at midday, and lower again morning and afternoon when a lot of your charging window happens. A horizontal panel on your roof is badly misaligned to that sun path for months at a time. Compare that to a portable panel you can tilt to face the sun directly, or even just prop up against the annexe wall at the right angle, and you can genuinely pull 20-30% more watts out of the same-rated panel on a winter's day in the Kimberley.

In summer it matters less. When the sun tracks higher and more overhead through the guts of the day, a flat roof panel is much closer to square-on and the difference narrows. This is why blanket advice about solar setup rarely works. Ask "do I need tiltable panels" and the honest answer is: it depends which season and which latitude you're actually camping in.

Shading is the silent killer

Angle costs you percentages. Shade costs you almost everything. Most cheap and mid-range panels are wired so that shading even a small corner, one tree branch shadow, a roof rack crossbar, an exhaust stack shadow moving through the day, can drop the whole panel's output by 50-80%, not just the shaded cell's share. This is down to how the cells are wired in series strings and where the bypass diodes sit in the junction box. Better panels with more sub-strings and diodes handle partial shade more gracefully, but none of them love it.

This is the other big argument for a portable panel, or at minimum for panels on some kind of tilting mount. You can walk it out from under the tree canopy and into full sun twenty metres away, something a roof panel bolted flat to your van can never do. I've lost count of the free camps along rivers and gorges, gorgeous shady sites, where the roof panel is basically decorative and the folding panel earns its keep sitting out on the open ground nearby.

Fixed roof panels: what they're actually good for

None of this means roof solar is a bad idea. It charges while you drive, which a portable panel obviously can't. It's immune to theft risk overnight, doesn't need setting up or packing away, and works fine in caravan parks and open camps where you're not fighting shade. For touring where you're moving most days and stopping in reasonably open sites, fixed roof panels covering your daily load is often plenty. Where fixed-only setups struggle is exactly the profile of touring I do: staying multiple nights in one spot, often in shaded riverside or gorge camps, often in the cooler, lower-sun months when the Kimberley and top end are actually pleasant to be in. If that's your style of trip too, don't assume the roof panel wattage on the spec sheet is what you'll see in the real world.

Sizing for the worst case, not the best

The system-sizing conversation usually gets pitched around best-case sun, and that's backwards. Size for your worst realistic camping day, shaded, overcast, winter sun angle, and treat anything better as a bonus. If your fridge, lights, water pump and charging habits need roughly 100-120Ah a day (a fairly typical solo Troopy-and-camper load with a compressor fridge and not much else), don't assume a 200W roof panel gets you there reliably. In genuinely good sun it might produce 60-80Ah across a day. In a shaded winter camp it could be a fraction of that.

My own setup is a modest 160W roof panel plus a 120W folding panel I deploy whenever I'm stopping for more than a night. That combination, plus a DC-DC charger topping up from the alternator on driving days, covers me through most of a Kimberley dry season without running a generator. It's not the biggest system I've seen on a Troopy, and I'd argue bigger fixed arrays are often oversold to people who'd get more real-world benefit from a decent folding panel and better camp selection.

Orientation matters even when you can't tilt

If you're stuck with fixed roof panels, orientation of the vehicle itself still matters more than people think. Parking so your roof panels face roughly north (in the southern states) or angled toward the sun's midday path gives you a real, free improvement over parking side-on or in whatever direction happens to line the van up with the view. It costs nothing and most tourers never think about it. Same goes for choosing a site with morning sun rather than deep afternoon shade if you're relying purely on roof solar, since the difference between a site that gets sun from 7am and one that only clears the tree line by 11am can be hours of charging time.

Portable panels: the trade-offs that matter

Folding panels aren't free lunch either. They're another thing to set up and pack away, another cable running across the ground that something can trip on or a dog can chew through, and another item that can walk off if you're not paying attention in a busier free camp. I run mine on a long enough lead that I can tuck it out of sight from the track, and I never leave it out unattended overnight in anywhere with much foot traffic. They're also more fragile in the wind. A gust that a roof panel shrugs off can flip a folding panel over or send it cartwheeling if it's not weighted down or staked. None of this is a reason to skip one. It's a reason to actually think about where and how you deploy it rather than just leaning it against a tyre and walking off.

What I'd actually check before your next trip

Before you assume your system is undersized, check the boring stuff first. A dirty panel loses more output than most people credit, road dust and salt spray on the Cape Leveque Road build up fast and cut output measurably. Check your controller's actual logged output against the panel's rated wattage on a clear day; if you're consistently seeing 60% or less in good sun with no obvious shading, something else is wrong, wiring, connections, or a tired panel. If you tow the Gibb River Road or spend time down the Cape Leveque Road in the cooler months, budget on the assumption that your roof panels are working at a discount, and plan a folding panel or a generator as your backup, not an afterthought.

For the full maths on sizing a system properly, including battery capacity and daily load calculations, our piece on how to size a caravan solar and battery system that actually works goes deeper than angle and shading alone. And if solar genuinely can't keep up on a run like the Gibb River Road Adventure, it's worth reading up on sizing your genset habit down before you buy more panel than you need.