How much land for solar?

The latest rather feeble tactic I've seen from a denialist is to assert that we can't switch to renewables because they'd take up too much land.  It's nonsense of course.

From Ramez Naam

We’ll probably never power the world entirely on solar, but if we did, it would take a rather small fraction of the world’s land area: Less than 1 percent of the Earth’s land area to provide for current electricity needs.

The second to last column tells us that, weighted by how much electricity they actually produce, large solar PV facilities need 3.4 acres of total space (panels + buildings + roads + everything else) for each Gwh of electricity they produce. 

That leads to an output estimate of 0.294 Gwh / year / acre, and virtually the same total area, around 50,000 square kilometers in the US, or 0.6% of the continental US’s land area.   

[....] 

So, consider that: 

1. The built environment in the US (buildings, roads, parking lots, etc..) covered an estimated 83,337 square kilometers in 2009, or roughly 166% of the area estimated above. (Likely this area would not be as efficiently used, of course. But it could make a significant dent.) 

2. Idled cropland in the US, not currently being used, totaled 37.2 million acres in 2007, or roughly 150,000 square kilometers, roughly three times the area needed. 

3. “National Defense and Industrial” lands in the US (which includes military bases, department of energy facilities, and related, but NOT civilian factories, powerplants, coal mines, etc..) totaled 23 million acres in 2007, or roughly 93,000 square kilometers, nearly twice the area needed to meet US electricity demand via solar. Presumably much of that land is actively in use, but it gives a sense of the scale. 

4. Coal mines have disturbed an estimated 8.4 million acres of land in the US. That works out to around 34,000 square kilometers, not too far off for the estimate from solar, and doesn’t include the space for coal power plants. And coal currently produces around only 40% of US electricity and hasn’t been above 60% in decades. To scale coal to 100% of US electricity would have required far more land than is required to meet that same demand via solar. Other analysis says the same: Counting the size of coal mines and their output, solar has a smaller land footprint per unit of energy than coal. 

And the solar estimate of ~50,000 square kilometers, of course, is with solar systems already deployed. It doesn’t take into account the possibility of future systems with higher efficiencies that could reduce the land footprint needed.

Read more here.

It's a bit more complicated with wind farms.  According to this piece from NREL (doing for wind what this piece did for solar):

Excluding several outliers, the average value for the total project area was about 34 ± 22 hectares/MW, equal to a capacity density of 3.0 ± 1.7 MW/km2.

This implies an output of  about 3.1 GWh/acre/year from a wind farm.  But unlike solar farms, farming can continue on the ground even as the turbines turn overhead.  If you read the NREL article in full, you'll see the problems they had estimating actual land use, and their calcs are correct only if you assume the land can't be used for anything else.  This snippet from this article about UK wind farms says:

A standard wind farm of 20 turbines will extend over an area of about 1 square kilometre, but only 1% of the land is used for the turbines, electrical infrastructure and access roads. The rest of the land can be used for farming or natural habitat.

My guess is, if you allow for the fact that most of the land in a wind farm has two uses, once again, even if we were to get 100% of power from wind farms, it would still only take about the same as solar: under one percent of land area.

Moral of the story: no, we will not be constrained by the availability of land from switching to renewables.