Resource: gentle intro to solar
Solar power can be extremely helpful for car dwellers, especially those living in cargo vans or other vehicles with large, high, and flat rooflines. Solar is more challenging for cardwellers due to limited roof space. SUVs are somewhere in the middle.
Framed panels can easily be hidden between the rails of a roof rack. While suboptimal, flex panels can be adhered flat on the roof. Neither are obvious to passers-by.
Your vehicle may already have a rack (or at least rails), making the job easier. A simple two-bar universal rack is about $150, and a nice locking two-bar rack from someone like Inno is about $300-$450. When rack-mounting panels, mount them flush with or on top of the rack. This will prevent the rack from shading the panels. To prevent theft the panel could be attached with tamper-proof fasteners. When racks are present, mounting a panel or two on them is often the first choice:
Portable panels are panels that store away and are deployed as needed.
Pro: Panel can be in sun while car is in shade. Can be secured when not in use. Can be aimed at the sun.
Con: Expensive per watt. Newbies commonly underestimate how much power is required. Voltage drop from long wiring runs. Must be aimed at the sun1), which would require repositioning every couple of hours. Portability increases risk of theft. Can be urinated on by dogs.2)
Each type of portable panel has it's own strengths and weaknesses.
These are the type of rigid-frame panel that folks mount on their roof, but usually smaller to keep weight and storage size down.
Pro: Cheapest per watt. Toughest panel.
Con: Heaviest portable. Hardest to stow due to frame thickness, panel size, and rigidity.
Briefcases are typically 2-4 small panels hinged together. The suitcase folds up after use.
Pro: Fold for easier storage. Built-in handle makes carrying easier.
Con: Most expensive per watt. Relatively heavy. Can be fiddly. Subject to mechanical wear from setup and teardown. Some have a built-in charge controller, resulting in very long runs between controller and battery.3)
Semi-flexible panels are basically the panel part of a framed panel without the frame. In the past these were usually amorphous4) cells but now it is common to see mono and, to a lesser degree, poly.
Pro: Very light. Extremely thin, which allows it to be slid into narrow storage.
Con: Can blow away in wind. Much easier to damage than framed panels; consider the lifetime of these panels to be a few years rather than decades. Roughly 2x as expensive as framed panels.
Low yield panels (10w-25w) encased in hard plastic. Most often seen in backpacking scenarios or on dashboard “battery maintainers”.
Con: Most expensive Watt/$. Lowest output. Can overcharge batteries that are unused.
Batteries are bulky and relatively heavy; finding a place to install the house battery bank may be the biggest power-related challenge facing cardwellers.
From front to rear:
Deep cycle batteries appropriate for off-grid use are discussed in this article. There are some differences for cardwellers:
Solar panels don't connect directly to batteries. They go through a solar charge controller that coordinates the panels' production and the battery's 12) demands.
Part of the article linked above covers how to choose a controller type and what size (rating) to select.
If you have a solar generator a solar controller of some kind is built into it.
In any case, read the specs carefully to make the panels you like will work with it. The critical piece is something like maximum input voltage. If the max is 23.0v then you could use a panel with a Voc of 20v, but could not run two panels in series for 40Voc.
While cars tend to have smaller alternators than vans they can still be a source of significant power compared to solar. Examples: