[Retric]

20,000km2 sounds huge but:

The US currently uses 40% of the 84 million acres or 340,000km2 crop for ethanol production which is arguably pointless. So, if we swapped just corn ethanol for solar cells we would have 6 times more land area than the entire state of Massachusetts covered in solar cells (135,000km2). And if we feel ethanol is really necessary we only need 8% of the total corn crop leaving 32% for corn ethanol.

If this still seems like a huge issue the soybean crop is far larger. http://www.epa.gov/oecaagct/ag101/cropmajor.html

PS: As to storage, we don't need 100% solar wind and hydro can make a huge difference. Pumped storage is also far cheaper at scale; it's just impractical when scaled down to home use.