assuming average conditions, somewhere on the order of 20 kw at cruising speed. More is needed to accelerate.

On the earth's surface, we get about 1kw per meter squared of energy from the sun. That's before factoring in inefficiencies in the panels, inefficiencies due to panels not oriented correctly, the battery charging system etc. Actual usable energy generated is significantly less than that.

So, with 100% efficient panels, you would still need, in theory, at minimum around 20 meters squared of perfectly efficient, perfectly oriented panels. Probably about 3 to 4 times more, in practice. There just is not enough surface area on a car.