NREL Study Shows US Rooftop Solar Potential Has Been Underestimated

(3BL Media/Justmeans) - Good news, bad news. The more we learn about the world around us, the more we can improve the models we use to plan and to set rational policy from. Right about the same time that Stanford researchers, looking at economics, found that the true social cost of carbon emissions is somewhere around six times higher than previously thought, other researchers at National Renewable Energy Laboratory (NREL) were announcing a revised estimate of the potential for rooftop solar energy generation in the US. It turns out that using a variety of new sophisticated methods and tools, including LiDAR, they discovered that the actual potential is nearly twice that of previous estimates.

The study, described as a detailed assessment, looked at 128 cities around the US, adding a rigorous methodology based on geospatial data and statistical analysis that had been lacking in previous attempts to quantify solar potential at a regional level. The researchers combined the use of light detection and ranging (LiDAR) data, along with geographic information system (GIS) methods, and PV-generation modeling to determine the potential for rooftop PV in 128 cities nationwide, a sample representing roughly 23% of all buildings in the U.S.

The authors examined the data in order to determine the percentage of rooftops in each zip code that would be suitable for a solar PV installation, taking into account many factors, including roof angles and hourly shading. They further computed the potential percentage of total electricity sales that could be provided by rooftop solar on a state by state basis. The majority of states fell between 25-45%, though a few (most notably, California) could potentially provide more than 55%.

All told, 1,118 gigawatts (GW) of installed capacity and 1,432 terawatt-hours (TWh) of annual energy generation could potentially be deployed. That would be 39% of total national electric-sector sales, substantially more than a previous National Renewable Energy Laboratory estimate of 664 GW of installed capacity and 800 TWh of annual energy generation. That would constitute less than 22% of electricity sales. According to the report, “The difference can be attributed to increases in module power density, improved estimation of building suitability, higher estimates of the total number of buildings, and improvements in PV performance simulation tools that previously tended to underestimate production.”

The analysis assumes an average module efficiency of 16%, even though modules with efficiencies as high as 22% are already beginning to become available. Overall estimates would grow proportionally, so that if the average became 22%, one would expect that 53% of electric sales could be provided by rooftop solar, all other things being equal.

Another interesting finding was that small buildings (<5,000 sq. ft.) show considerably more potential than large or medium sized buildings, because there are so many more of them.

This is indeed encouraging news, which should add an optimistic note into future projections of reduced emissions. That is of course, assuming that robust investment levels continue, increasing the role of distributed generation, and remaining undeterred by the efforts of numerous states, to side with utilities by cutting back on incentives for solar and other renewables.

Image credit: Peter Howe: Flickr Creative Commons