To generate enough energy with rooftop solar panels to cover total energy consumption in the USA would require more than 6 times as much rooftop space as exists in the whole country.

In the previous two posts I have constructed “back-of-the-envelop” estimates of how much land you would have to use to satisfy the USA’s energy consumption with wind and solar power. I discovered that to do it with wind power would require at minimum wind farms occupying land the size of South Carolina. To do it with solar would require at minimum a solar farm at almost as big as the state of Texas.

What if solar was distributed around the country, on every building in the country? Obviously, not every building is suitable for solar power – they are shaded by trees or other buildings, they are oriented the wrong direction, or the slope of their roofs isn’t good for solar panels. Still, this is an interesting exercise to demonstrate the size of the requirement.

There are an estimated 113 million residential structures in the USA, totaling an estimated 180 billion square feet. There are an estimated 4.7 million commercial buildings totaling 68.5 billion square feet. Combined, they total 248.5 billion sq. ft.

https://www.aps.org/energyefficiencyreport/report/energy-bldgs.pdf.

The average new residence in the USA has 1.6 stories, and I will use that as my estimate for all housing. Thus, the average size of the roofs would be 180 billion / 1.6 = 112.5 billion = 112,500,000,000 sq.ft. This is probably an overestimate, because it does not account for multifamily buildings, but it will have to do.

https://www.census.gov/construction/chars/highlights.html.

I could find no data regarding the average number of stories for commercial buildings. However, there is data that buildings over 50,000 sq. ft. constitute half of the entire square footage, even though they represent only about 6% of all buildings. Obviously, some very large skyscrapers are going to account for a lot of internal square footage, but have comparatively small roofs. It’s just a guestimate, but I’m going to say that the square footage of commercial building roofs is only 1/4 that of their total square footage.

68.5 billion / 4 = 17.1 billion sq. ft. = 17,100,000,000 sq. ft.

https://www.eia.gov/consumption/commercial/reports/2012/buildstock/.

Thus, I estimate the total amount of roof space in the United States to be 112,500,000,000 + 17,125,000,000 = 129,625,000,000 sq.ft.

In 2014, total energy consumption in the United States was 98,385.2 trillion Btu. = 28,833,750,564 MWh.

http://www.eia.gov/state/seds/data.cfm?incfile=/state/seds/sep_sum/html/rank_use.html&sid=US.

The Desert Southwest has the strongest solar resource in the country, while northerly latitudes with lots of cloudy days have the weakest. The Wikipedia article on solar efficiency says that in Colorado, one could expect a solar panel to generate 440 kWh/sq.m./year, while in Michigan, one could expect 280 kWh/sq.m./year.

https://en.wikipedia.org/wiki/Solar_cell_efficiency.

The National Renewable Energy Laboratory provides a map of the solar resource across the country in kWh/sq.meter/day, but I could find no resource that gave a nationwide average. Since I am assuming solar panels installed on every building across the country, I must use a national average.

http://www.nrel.gov/gis/images/map_pv_national_lo-res.jpg.

“Eyeballing” the map, it is clear that Colorado does not represent the strongest solar resource in the country. On the other hand, the area that does have the strongest solar resource is relatively sparsely settled, meaning there are fewer buildings there than in, say, the Northeast. I will assume that these factors balance out, and that one may estimate the annual yield from solar panels by averaging the figures from Colorado and Michigan.

Thus, I estimate the average annual yield from a solar installations to be (440 + 280) / 2 = 360 kWh per square meter per year = 33.4 kWh per square foot per year.

Therefore, the total potential energy production that could be achieved by completely covering every roof in the country with solar panels would be 129,625,000,000 * 33.4 = 4,335,324,557,000 kWh, = 4,335,000,000 MWh.

The fraction of national consumption that could be met would be 28,833,750,564 / 4,335,324,557 = 15%. Put another way, we would need more than six times as much roof space as exists in the USA to meet our energy consumption using rooftop solar photovoltaic.

Some thoughts on where Missouri fits in all this and what it all means in the next post.