Small Solar Panels will only work when there is sunlight that is able to hit the panel. It will not work if there is terrain blocking direct view of the sun, or if underground.
A Small Solar Panel will fill one bar of battery in 8 seconds.
It is 2x more power and space-efficient to fill a Medium Storage with eight Small Solar Panel than to use a single Medium Solar Panel. However, if the Storage is not flattened only half of the panels will receive sunlight.
Media
Solar Panel Icon
A small solar panel and a (regular) Solar Panel for comparison.
A small solar panel in one of the side slots of the backpack.