Solar panels are items that produce power out of nothing when they are exposed directly to the light of the sun.
Right from the start of the game you can craft a small solar panel in your backpack's printer, using 1 stack of Copper. This kind of panel can be placed on any one slots, but in the backpack, it will only produce power when placed on one of the side slots (see image on the right).
When exposed to light, the panel will orient itself automatically toward the sun (if possible) and produce 0.5 units/sec, or 8 secs/bar worth of energy.
After researching the schematic you can also craft another kind of solar panel in the Printer module, using 2 stacks of copper. It can be placed on any two adjacent slots except in the backpack or on Storages.
When exposed to light, the two panels are open and produce 2 units/sec, or 2 secs/bar worth of energy. This is 4 times more than the solar panel you can craft in the backpack.
When exploring, you can come across a large, crashed solar array with a bent girder segment embedded in the surface of the ground (see image on the right).
This solar panel can be completely dug out then hauled to your base when attached to a rover or truck with a winch.
Exposed to sunlight, this array yields a minimum of 1 bar/sec of power. Solar arrays only have panels on one side, and must be facing upwards to work. Upon placing solar arrays at your base they are prone to movement, especially during storms. Currently, upon saving and re-entering the game solar arrays will become stationary. This is great for holding them in place, but can be a issue if not placed properly beforehand.