A 1 square meter solar panel generates 200W (when aimed perfectly at the sun).
Since the car's top is a flat surface (as opposed to tilted towards the sun), that's cos(30) * 200W, or 173W.
A typical compact car (Honda Civic) has a surface area of about 8 square meters if it were a perfect rectangle.
That's 8 * 173 = 1.384 kW, or 1.86 horsepower under wildly optimistic circumstances.