Using data I am getting from quick googles, a Tesla model S has 95 kWh of power max, with a range of 405 miles (~650 km). That gives us 4.26 miles per kWh (or 6.84 km/kWh). According to the city of LA, there are about 2.5 million cars registered to the city.

Let's assume in this perfect future, the number of cars is not increased and they have all been converted to cars that perform identical to this Model S data. Let's also assume each of these cars are required for daily work commuting, and assuming each Angelino commute the average I found of 41 miles which is about 9.6 kWh per day per car commuting or 24 million kWh total per day just commuting.

Assuming this data is correct and a solar panel can produce 2.4 kWh per day a daily commute requires 10 million solar panels operating at 100% every day. Assuming the average solar panel is 17.6 square feet, then the total area needed for solar panels to charge one car commute per day is 1 square km or 64ish city blocks.

However, if we replace all of these car commuters with a train, which we can say requires 0.05 kWh/km, that comes to 8.75 million kWh for the daily commute, or 36% of the power requirement using cars only. That doesn't even factor in the amount of infrastructure for supporting cars (roads vs rails, parking, public charge stations, mechanics, less power sources, etc).

Replacing every gas powered car with an electric powered one would reduce emissions. However, replacing car transportation with more efficient forms of transportation reduces carbon emissions even further. Again, these are spherical vehicles in a vacuum making a lot of assumptions, but I think my point stands.