Design Info tab doesn't account for the body you're orbiting having a different parent star than the system parent.

To recreate, first download and open any system that includes multiple star systems. Then in the designer, open the Design Info window and under the Environment tab, choose a planet that orbits closely to one of the stars that isnt the technical system parent. Now place a solar panel/array on your craft, keeping the Design Info window open. While the solar panel/array is highlighted in the designer, look under the Solar Panel(/Array) tab in the Design Info window. The reported power production will be much lower than in simulation because it is going off of the system parent instead of the immediate parent star.