Funny you should mention windmills.
My first observation is that you can do other things with the land you have wind farms on. Note the emphasis on
wind farms.
Pretty standard engineering calculations at this point shows that a typical wind farm generates 1Mw per 0.75 acres.
Now there are 915 million acres of farmland in the United States (USDA).
Now through the magic of multiplication, if through some reason we decided that all farms should also be wind farms, we'd have a generating capacity from just wind turbines on farms of 686 million megawatts.
I note that the current generating capacity of the entire United States is about 1.1 million megawatts.
Of course, you do need to take into account the capacity and utilization factors. Even making a very conservative assumption that any given wind turbine would only be generating 15 percent of its capacity (real-world numbers are between 25 and 50 percent) we still are generating over 102 million megawatts -- approximately ninety times the current US generating capacity. Note that doesn't include existing hydropower and nuclear plants, and I reasonably assume we'd keep most of them for their useful lives.
I haven't even mentioned that we could also put wind turbine on the land that we use to grow cows. There are approximately 770 million acres of rangeland in the United States distinct from the farmland mentioned previously.
This of course assumes that we'd restructure our grid to distribute that energy efficiently. Given that our power grid is over a century old it is time for a remodel anyway and we might as well get it right.
So using 4th-grade math I've shown that we could get there with wind power and some slight remodeling of farmland. This is more rigorous work than I've ever seen from any of the naysayers who just say how it is impossible. The physics and the engineering work, and given the
recent cost figures showing that on-shore wind is by far the cheapest source of electricity today I think it a safe bet that the economics work too. Notice that I left out offshore wind power too.
Now, back to the topic.
I'll do a similar calculation for solar power. Let us assume that we decide to put sunshades over all of the parking spaces in the United States. There are estimated to be between 800 million and 2 billion parking spaces in the United States. Pretty standard size for parking spaces is ten by twenty feet, so you can pack 225 parking spaces in an acre (I don't count the lanes you need to drive the parking space, and also ignore that a lot of parking spaces are compact and smaller -- for a Fermi number those two probably cancel out).
I'm also going to ignore multi-floor parking garages. My argument would be that if I use the lower number and make some reasonable assumptions about the prevalence of multi-floor parking garages the numbers again about work out.
That works out to about 3.6 million acres used for parking in the United States if you use the lower number.
Now generation per acre for PV is all over the place, with estimates from 0.1Mw/acre to 10Mw/acre -- I'd guess that is because they are including capacity factors in their figure, and capacity factors will vary by location far more for PV than for wind. Just to make this easy I am going to split the baby and use 1Mw/acre.
So you lead the conclusion that PV panels covering all parking in the United States would generate
THREE TIMES (THREE TIMES!) the amount of electricity we currently generate.
My conclusion is that not only can we go to 100 percent renewables and generate just as much electricity, we'd have an insane amount of growth capacity to power all of those new electric cars we'd need too. And probably have a few centuries' of headroom for generating more electricity if we decide we need it. All at far lower costs than our current electricity generation systems.
Again, with 4th-grade math this is a no-brainer.
