Electricity generated on the rooftops of buildings in the U.S. has the potential to hit 39% of current electricity sales, nearly double the amount that was estimated in 2008.
In a report, the National Renewable Energy Laboratory said that the “technical potential” of equipping all suitable rooftops was 1,118 gigawatts of capacity and 1,432 terawatt hours of electricity annually. That’s compared with 664 GW and 800 TWh in an analysis eight years ago.
The authors of the study said that previous estimates “lacked a rigorous foundation in geospatial data and statistical analysis.” This time, researchers said that they used light detection and ranging (LiDAR) data and other sophisticated tools to calculate the suitability for PV of rooftops in 128 cities around the country, collectively representing 23% of all U.S. buildings. The results were extrapolated to cover the entire continental U.S.
The PV potential was greater in some states than in others. California, for example, has the greatest potential to offset electricity, according to the report: 74% of all the electricity sold by state utilities in 2013. Percentages also were very high, 55% or more, in several New England states; they had a lower solar potential, but also lower per-capita consumption. Florida and Michigan also had better-than-average potential.
Even Washington State, with the lowest solar resource in the country, could generate 27% of its electricity with rooftop PV systems.
Researchers developed two statistical models, one for buildings under 5,000 square feet in size, and the other for medium and large buildings. Only about 26% of the small buildings were considered suitable for PV, but the huge number of buildings in this size class gave them the greatest technical potential — 739 GW of capacity and 926 TWh of electricity annually.
Modules are getting more efficient
To represent the mix of technology in the marketplace, the analysis assumes that PV modules have an efficiency of 16%. If a module efficiency of 20% is used instead — an efficiency that “premium systems” are now capable of delivering — each of the technical potential estimates would go up by 25% over the values in the report, the authors said.
“Furthermore, our results are only estimates of the potential from existing suitable roof planes, and they do not consider the immense potential of ground-mounted PV,” the report adds. “Actual generation from PV in urban areas could exceed these estimates by installing systems on less suitable roof area, mounting PV on canopies over open spaces such as parking lots, or integrating PV into building facades.”
The estimates represent “technical potential,” smaller than the total “resource potential” but greater than the “economic potential,” which in turn is bigger than the “market potential.” The report defines it this way: “Technical potential, which is the topic of this report, estimates how much of that total resource could actually be captured, given physically available area and technology performance without considering economics.”
The technical potential does not, for example, take into account what would be required to get all the PV-generated electricity to end users.
“In practice, the integration of a significant quantity of rooftop PV into the national portfolio of generation capacity would require a flexible grid, supporting infrastructure, and a suite of enabling technologies,” the authors said.
Even so, the report says that rooftops provide “a large expanse of untapped area for solar energy generation, and onsite distributed generation could potentially reduce the costs and losses associated with the transmission and distribution of electricity.”