What effect does solar power harvesting have on the weather? (details inside )

Whether sunlight falls on solar cells or the earth, the effect will be the same.

“Effect be on natural systems that operate with the energy on of the sun” Do you mean plants?
One side-effect of solar power systems is shade. Any plants under a solar panel will get less sunlight.

Otherwise the biggest difference might be that the albedo of the natural landscape like a desert that is light-colored might be a lot higher than that of a solar panel which will likely be dark and keep a lot more of the sun’s energy close to the ground.

No effect whatsoever, other than what would normally occur with or without the cells.

The only way I could see it having some kind of effect is if the solar panels were mounted on a platform in space in such a manner as to prevent the sunlight from reaching the earth and even then it would have to be a mighty big set-up or significantly far enough from earth to cast a tremendous shadow.