How Much Solar Power Does The US Need?

If the US wishes to replace its nasty coal-fired electrical generation complex and scary nuclear facilities, along with riverine hydropower, it needs to come up with about 4 million Gwh of power from other sources, according to these folks. If the source is solar power just how much area of solar panels would be needed? 

Here are the numbers: 

Total acres of solar panels:  11,200,000

Total square miles of solar panels: 17,500

What this means is the solar panels needed to supply the current US demand for electricity would cover an area almost exactly equal to the area of Estonia. The area would be larger than the area of Denmark, the Netherlands, Switzerland or Belgium.