You need to at least half, but most likely more, your estimate if you talk about cars that live in the cities. For solar panels efficiency, two main things tha

You need to at least half, but most likely more, your estimate if you talk about cars that live in the cities. For solar panels efficiency, two main things that matter are proper angle to the sun and amount of shade. Parking in the city, you cannot choose to align your panels with a sun. And you’ll be surrounded by tall structures (buildings, trees, etc) that will shade your car. You’ll get nowhere near close to an efficiency of a house rooftop solar.