Can be problematic. Some solar panels, only need a small partial shadow to reduce output to almost zero. The rails around the side of the roof rack may cast enough shadow for this to occur. Some panels are much better than others in this respect but you would either have to pay more for them or get lucky. Not a problem if mounted across the top of the roof rack, but makes them higher & raises centre of gravity. Best to either alter the roof rack or use roof bars. It’s a good strategy if you can get the panel(s) un-shadowed. I’m a big fan of roof mounting over portable.