Could existing substations even come close to that kind of load? 8 cars recharging at MW rates would saturate most substation transformers in my area.
How many MWs would a carpark roofed with solar panels generate? My calculations say you need about 4k square meters to get 1MW whilst the sun is shining bright. That's about the size of medium sized supermarket carpark. Double that if you cover the building as well. You might get a good 2MW at peak, enough to allow a few cars charging at once. Not much good at after the afternoon sun has gone down and the after work rush is happening at the supermarket.
Given most drivers will be charging at home, I wonder what percentage of cars we can expect to be wanting to charge whilst shopping?
Unless you have enormous battery banks, the first few charges will deplete them.