Shaded solar panels will obviously have a reduced solar harvest than a solar panel in full sun, but the amount of the reduction in output isn't nearly so obvious. Counterintuitively, some types of panels that are only partially shaded may have worse output than a panel that is evenly shaded. Shade comes from the obvious sources like clouds and trees, but roof-mounted solar panels are often shaded by other things on the roof, such as roof vents and cargo racks.
“ The response of a series parallel array to shade will be complicated.[1]”
Solar panels are "current sources": their voltage pops up into the normal range in any kind of meaningful light (>= 20% insolation), but current will suffer.)
"Partial shading" means that light is falling on the panel, but not evenly. Some cells are receiving more light than others. To prevent generated power from rushing into a shaded string of cells and overheating them, panels have bypass diodes between the strings. These diodes electrically isolate the shaded strings to protect them. (Ideally each individual cell would be protected by its own diode, but diodes incur electrical losses and add cost and complexity.)
Avoiding solar harvest loss
If your panels are mounted on your vehicle, avoid parking in shade. However, in warm weather direct sunlight makes keeping cool much more difficult. As such, it's generally preferable to park so that your panels are fully shaded instead of partially shaded. When planning your build, ensure that your panels aren't going to be shaded by other things mounted on the roof. Roof vents are the most common culprit.
Amorphous/thin-film panels are less affected by shade.
A dedicated solar charge controller for each panel can reduce shade losses.
With PWM controllers, parallel panel configurations typically yield more power in partial shade than serial.
With MPPT controllers and lower voltage serial panel configurations, where the total Vmp is <=2x battery bank voltage, parallel will probably still yield more. But there is another approach, which is to bring the shaded cells/strings back online by bringing the rest of the panel down to their level. It sounds counterproductive, but with MPPT controllers and in some higher-voltage series configurations (say Vmp is >=3x bank voltage), it works. This occurs because the MPPT has a broader range of voltages to sweep and can find other power peaks (panel voltages) that are low enough to bring the shaded cells back online but still high enough to charge the battery bank. Effectively, the MPPT is evenly "shading" the entire panel voltage-wise in order to get the maximum output from it in partial shade conditions.[2]