At 12km/h it requires 5.59KW of sun energy to fall on the car (in total, not per sq meter as in original comment).

Actual figure given on website is 5m2 of panels on car. Assume 1Kw per m2 of sun power (near equator, no cloud, midday) @ 22% conversion efficiency gives 1.1kw electrical output from panels.

So one needs 5.59KW sun power input for 1.1kw output.