PV on rooftops could offset nearly 40% of U.S. electricity sales

Percentage of small buildings suitable for PV in the United States. (Graphic from the report of the National Renewable Energy Laboratory)
Percentage of small buildings suitable for PV in the United States. (Graphic from the report of the National Renewable Energy Laboratory)

How much energy could be generated if all suitable roof areas had photovoltaic systems? A new study asked just that and its extensive analysis shows that building rooftops in the United States of America have a potential PV capacity of 1,118 GW.

With an annual electricity generation of 1,432 TWh, those rooftops could generate 39% of the U.S. electricity sales. The new estimate by analysts at the Energy Department's National Renewable Energy Laboratory (NREL) is significantly higher than the one in their previous study. In 2012 they had estimated the capacity for PV rooftop systems to be around 664 GW and 800 TWh of annual energy generation.

There could be several reasons for the difference in the two predictions for energy generation with PV. These include a higher estimate of the total number of buildings, improved estimation of building suitability, better PV performance and simulation tools, and of course technically advanced PV modules with a higher efficiency. For their latest study the NREL scientists used LiDAR (light detection and ranging) data for 128 cities nationwide (approximately 23% of U.S. buildings) and published results for the PV generation capacity of 48 cities and a detailed analysis for 11 cities.

With the help of this technique they could determine shading, tilt, and azimuth of each rooftop at a horizontal resolution of 1 m2 and then establish whether a roof was suitable for PV.

Every little (building) helps

“This report is the culmination of a three-year research effort and represents a significant advancement in our understanding of the potential for rooftop PV to contribute to meeting U.S. electricity demand,” said Robert Margolis, NREL Senior Energy Analyst and co-author of the report.

Within the 128 analysed cities, about 83% of small buildings are suitable for a PV installation – but only for 26% of those buildings the total rooftop area can be used, due to the diverse architecture among small buildings. Still, if all those small buildings are combined, their total technical potential is huge: 65% of the country’s total technical potential or 731 GW of installed PV capacity and an electricity generation of 926 TWh per year. Another 386 GW of PV could be installed on large and medium buildings, which could then produce about 506 TWh of solar power per year.

Due to different irradiation levels the different states have also different potentials, when it comes to PV yields. Sunny California has the greatest potential to offset electricity use with the help of photovoltaic systems. The percentage of electricity a state could offset with the help of PV also depends on the electricity consumption in said state. Wyoming for example has the lowest potential for offsetting its electricity sales with rooftop PV systems. Only 14% of the sales could be produced using PV since this state has the highest electricity sales in the U.S., which are 250% above the national average due to a very high electricity use in the industrial sector.

With ground-mounted PV the potential would be even higher

“It is important to note that this report only estimates the potential from existing, suitable rooftops, and does not consider the immense potential of ground-mounted PV,” added Margolis. “Actual generation from PV in urban areas could exceed these estimates by installing systems on less suitable roof space, by mounting PV on canopies over open spaces such as parking lots, or by integrating PV into building facades. Further, the results are sensitive to assumptions about module performance, which are expected to continue improving over time.”

The researchers hope that with the help of their data PV research and policymaking will be made easier. You can download the full report here: "Rooftop Solar Photovoltaic Technical Potential in the United States: A Detailed Assessment."

Tanja Peschel

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