When exploring possible futures, it’s helpful to have a lot of “what if?” to ask. For example, what if we could install solar panels on any suitable roof in the United States? How much electricity would they generate?
Much research has followed this line of thought, although much of it has necessarily focused on working out the details for individual cities or regions. But with enough of these studies in the pipeline, a group of researchers at the US National Renewable Energy Laboratory decided to strike another blow against a national estimate.
There are many things you need to know to do this: the number of buildings, the size of the roofs, the direction of the roofs, the strength of the sunlight, the number of sunny days, and so on. So first, the researchers used a Department of Homeland Security program that laser maps buildings that now covers nearly a quarter of buildings in the US. From this it is possible to get the roof area, the roof slope, the roof direction and whether the roof is shaded by trees. Roofs were discarded if they were too small, too steep, or north-facing, or otherwise lose more than 20 percent of their possible solar output, but most roofs were suitable.
To estimate the rest of the country, the researchers calculated statistics for the area covered and then used things like census data to scale them for each different zip code area.
The researchers then calculated the average amount of sunlight in a year for each location. Using the average efficiency of rooftop solar panels installed in 2015, they combined everything to create a map of the maximum possible rooftop solar production.
In total, they estimate that there are just over 8 billion square feet of suitable roofs in the US. Top that with solar panels, and you’d be producing about 1,400 terawatt hours of electricity each year — about two-thirds of which would come from small residential buildings. Total production is equivalent to nearly 40 percent of the total electricity currently sold by utilities in the US.
A simpler 2008 National Renewable Energy Laboratory estimate came in at just 22 percent of electricity — the new estimate shows a higher percentage, in part because the efficiency of solar panels has improved, but also because new data sources allowed for a more accurate estimate.
Aside from the large numbers, there are some interesting details at the state or local level. States with strong sunlight and lots of roofs clearly have the most potential — California, for example, could provide 74 percent of its total electricity consumption by covering its buildings with solar panels, while Wyoming could only achieve about 14 percent.
But that is partly due to other electricity to use. New England doesn’t have the sunniest skies, but the limited need for air conditioning in the summer helps keep electricity consumption low. As a result, that region could produce about half of its total electricity from rooftop solar. And if you look at residential buildings separately, they can produce about as much electricity as people use at home.
Overall, the all-inclusive scenario of slapping solar panels on every single building wouldn’t be enough to replace all of our power plants, but 40 percent isn’t bad. More plausible (and less Fiddler-on-the-Roof-If-I-Were-a-Rich-Man) scenarios would obviously stay south of that number. Still, the “what if?” is educational.
Letters for environmental research2018. DOI: 10.1088/1748-9326/aaa554 (About DOIs).