I did some calculations a few years back on my blog: http://andrews.blogtownhall.com/2008/07/23/g-d_save_us_from_simple_solutions.thtml I used the total annual energy consumption, the earth average per square meter of solar energy, the then best yield from experimental photovoltaic panels of 20%, and the amount of solar panels needed to yield the yearly consumption of the US would cover California, and that did not account for cleaning, failures, space for infrastructure and so on. I think if we are worried about climate change, covering a large part of the nation with solar panels would change the climate much more than CO2 ever could.