There's at least a couple of ways to use solar panels...augment the power from the grid when the sun is shining, or, if self-contained, be able to store enough energy for your peak and average power needs when the sun is not shining or you need more than you can make at the instant. That usually means batteries to store it. At Niagara Falls between NY state and Ontario, Canada, they use some excess power, when available, to pump water to a reservoir so that they can keep water flowing over the falls for the tourists during the day. IN reality, they can almost stop the flow over the falls when they direct it to the turbines and to the reservoir. There are some major tunnels upstream of the falls that direct water to the turbines below the falls.
The area on the top of the i3 isn't all that much. Modern panels need about 10sqin to produce one watt. The i3 seems to use around 250W/mile. At 60mph, that's one minute. That means 250*60 minutes for an hour, or 15,000W. So, to power the car, at 10sqin per watt, that's an area of 150,000 sqin, or over 1,000sqft...how much do you think you have on the roof?
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above