How big of a solar plant would be needed to replace all of the US electricity with solar power? [Ray]
This was proposed by Elon Musk in 2015 and he came up with a PV array that would have an area of 10,000 km2. Several people have fact checked his answers since then and here is a link: http://blogs.ucl.ac.uk/energy/2015/05/21/fact-checking-elon-musks-blue-square-how-much-solar-to-power-the-us/.
The assumption started with a 2013 EIA estimate of US electricity consumption of 3725 TWh or 3,725,000 GWh. Converting this to power gives 425GW are needed. Assumptions are then made for incident solar power (1KW/m2 in Nevada desert) and solar panel efficiencies . Net is that 10,000km2 of solar PV would generate ~500GW or enough to power the US electricity needs. This is 100km x 100km on a side or 62mi x 62 mi which is about 3844 sq miles.
There was some additional discussion that practical installation of the panels along with changes in land slopes could increase the area to 2x-4x the 10,000km2 estimate. In reality one would not build this all in one location due to access into the electrical grid but this gives you some idea of what is required.