What is the average power requirement for the United States, in watts? (Hint: 1 = 1 )?

The total annual U.S. energy consumption is about 2 x 10^20 joules.
With current technologies and solar collectors on the ground, the best we can hope for is that solar cells will generate an average (day and night) power of about 230 . What total area would we need to cover with solar cells to supply all the power needed for the United States? Give your answer in both square meters and square kilometers.