That's an interesting one. I would write a world plugin that as starters scales the max voltage with the dot product of the solar panel's surface normal and the unit vector that represents the direction of the sun at the location of the solar panel surface normal. The bigger challenge (in my opinion) is how to take effects of objects that locally block the sun - if that is what you also need - I guess it would require getting access to the scene graph somehow, and I dunno how at this time.