The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) said it has used detailed lidar data for 128 cities nationwide, along with improved data analysis methods and simulation tools, to update its estimate of total U.S. technical potential for rooftop photovoltaic (PV) systems. The analysis reveals a technical potential of 1,118 GW of capacity and 1432 TWh of annual energy generation, equivalent to 39 percent of the nation's electricity sales. This estimate is significantly greater than that of a previous NREL analysis, which estimated 664 GW of installed capacity and 800 TWh of annual energy generation. Analysts attribute the new findings 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. Within the 128 cities studied, the researchers found that 83 percent of small buildings have a suitable location for PV installation, but only 26 percent of those buildings' total rooftop area is suitable for development. Because of the sheer number of this class of building across the country, however, small buildings actually provide the greatest combined technical potential. Altogether small building rooftops could accommodate up to 731 GW of PV capacity and generate 926 TWh per year of PV energy-approximately 65 percent of the country's total rooftop technical potential. Medium and large buildings have a total installed capacity potential of 386 GW and energy generation potential of 506 TWh per year, approximately 35 percent of the total technical potential of rooftop PV. Technical potential is an established reference point for renewable technologies. It quantifies the amount of energy that can be captured from a particular resource, considering resource availability and quality, technical system performance, and the physical availability of suitable area for development-without consideration of economic factors like return on investment or market factors such as policies, competition with other technologies and rate of adoption.