Clearly, the best spot to produce solar energy is right at the US/Mexico border. The map shows above 40 MW/h per day per m² in the area.

Border length is around 3 200 km. Assuming that the average height of solar panels is 2 meters, that gives us 6 200 000 m² of solar panel coverage.

The new Solar City panels, announced by Elon Musk, claim a 22.5% efficiency (and at a low cost per unit).

40 mW/h m² day × 6 200 000 m² × 22.5% × 365 days = 21 024 000 000 MW/h per year

21 000 terawatt hours per year. Current electricity consumption by US is 5000 TWh per year. Mexico — 232 TWh. Canada — 570 TWh. The whole world is consuming around 25 000 TWh per year.