@MC3OZI've been suggesting since the Semi reveal that Megachargers should be grid connected solar farms with big batteries.
The grid connection is for selling electricity to the grid, including peaking electricity - that should be profitable.
Megacharging and Supercharging for 7 cents per kWh would be profitable.
I think Elon was just giving an idea of how much solar was needed to power the US, not suggesting locating it all in one place.
IMO Solar farm Megachargers need to be built on cheap land near major highways about 250 miles apart, if they were built, they would be very significant in aggregate.
My view is based on a a very literal interpretation of things Elon has said, it seems that Tesla are at least open to the idea of using solar to power fast charging.
while not Tesla focused per se, an 83 megawatt Solar farm is being built (about 1.5 square miles) a few miles from the intersection of Interstate highways I-81 and I-64, 30 miles west of Charlottesville, Virginia and about 150 miles SW of washington DC that purportedly will have batteries, they are also building Ventura battery project in California while may be largest in the world for a short while.
ie, this stuff is hitting non-linear log growth phase