The Medium Solar Panel is a power generation item.
The Medium Solar Panel only produces power when exposed to sunlight. The two panels are folded upward over the center when it is not exposed to sunlight. The Medium Solar Panel doesn't have any power output cables by itself, therefore it must be placed on a Platform with a 2-slot connection in order for the produced power to be used.
A Small Solar and a Medium Solar Panel for comparison.
Medium Solar Panel on its Pre-Release version.
A closed up Medium Solar Panel.
Trivia
The Pre-Release version had a similarity to pathfinder from the movie "The Martian". It has 3 panels, which were changed to 2 along with a new model later in Pre-Alpha.