Here's the back-of-the-envelope calculation...
In 2014, the U.S. consumed 3,723,681 million kWh (per EIA data), or 3.7e+12 kWh.
Assuming that 1W of solar capacity generates 1kWh per year (in the Arizona desert this is closer to 2 kWh/W, and good exposure in Minnesota gives around 1.3 kWh/W, so this assumes that the solar panels have somewhat mediocre placement), we would need 3.7e+12W of installed solar capacity to generate all our electric usage over a year (today's installed capacity is about 2e+10W in the U.S. and we installed about 6e+9W in 2014).
At 15% efficiency, 1 m^2 of solar panel is about 150W of nameplate capacity. So we need 2.5e+10 m^2 of solar panels to get all our required capacity.
Assuming that in a large installation we need twice as much land as panel area (the extra land provides spacing between rows, access roads, etc.), we need 5e+10m^2 of land area to site our solar panels. That's 50,000 square kilometers, or a square approximately 225 km on a side. Or if you prefer, about 140 miles on a side.
By coincidence, 50,000 square kilometers is just a little bit smaller than the size of San Bernadino County in California, the largest county in the U.S., and just a little bigger than Coconino County in Arizona and Nye County in Nevada.
So I think the statement that you could cover a single county in Arizona or Nevada with solar panels and generate enough electricity for the entire U.S. is correct, especially since I used somewhat conservative assumptions. You might even be able to do it in Maricopa County in Arizona (about 25,000 km^2).
It's the storage which is the real problem, not finding the land for all those solar panels. We have plenty of land, but the storage technology is the weak point.
In 2014, the U.S. consumed 3,723,681 million kWh (per EIA data), or 3.7e+12 kWh.
Assuming that 1W of solar capacity generates 1kWh per year (in the Arizona desert this is closer to 2 kWh/W, and good exposure in Minnesota gives around 1.3 kWh/W, so this assumes that the solar panels have somewhat mediocre placement), we would need 3.7e+12W of installed solar capacity to generate all our electric usage over a year (today's installed capacity is about 2e+10W in the U.S. and we installed about 6e+9W in 2014).
At 15% efficiency, 1 m^2 of solar panel is about 150W of nameplate capacity. So we need 2.5e+10 m^2 of solar panels to get all our required capacity.
Assuming that in a large installation we need twice as much land as panel area (the extra land provides spacing between rows, access roads, etc.), we need 5e+10m^2 of land area to site our solar panels. That's 50,000 square kilometers, or a square approximately 225 km on a side. Or if you prefer, about 140 miles on a side.
By coincidence, 50,000 square kilometers is just a little bit smaller than the size of San Bernadino County in California, the largest county in the U.S., and just a little bigger than Coconino County in Arizona and Nye County in Nevada.
So I think the statement that you could cover a single county in Arizona or Nevada with solar panels and generate enough electricity for the entire U.S. is correct, especially since I used somewhat conservative assumptions. You might even be able to do it in Maricopa County in Arizona (about 25,000 km^2).
Even if the panel efficiency got to be 100%, you had the real estate and you used a tracking system, you could only power the country for about 8 hours max. What do you do after the sun went down? There is no technology that can store enough energy to supply the country for the other 16 hours.
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