I recently saw this installation of solar panels in San Luis Valley of Colorado.
http://www.reuters.com/article/pressRelease/idUS32317+18-Dec-2007+BW20071218
http://www.slvdweller.com/uploads/bizXLCentralSolar.jpg
Roughly 80 acres in size capable of supplying 1500 homes. Quick calculation says 8000 acres would theoretically supply 150,000 homes. 8000 acres would cover 12.5 square miles, or an area the size of about 3 miles by 4 miles. Of course, then the question becomes, how do you store the energy efficiently when the sun does not shine, either because of clouds or night time? So the real output on less than perfect conditions is probably less, and such sources will need to be used as supplementary or complimentary sources of energy generation until an efficient means of storage is perfected.
Also, if solar collectors are manufactured in great numbers, what is the situation with raw materials, usable life, recycling, disposal, etc.? And what is the impact on the environment if installed over wide areas? All of these things are of interest to me, but seeing the installation tells me we are making progress, and I am enthusiastic about it.