In most areas of the US, 3KW of solar panels are only going to produce 9KWh of electricity per day. That isn't much. It's not going to supply modern living with HVAC, and charge an electric vehicle
You aren't going to want to weld to the body panels, hard to control warping while you weld. And the big thing is things need to flex or else they fatigue crack. If you mount two things together by welding them and they don't flex the same way, something is going to break. There is a reason...