Rooftop solar systems have the potential to play a significant role in producing electricity in the United States. In a January 2016 report the National Renewable Energy Laboratory (NREL) has estimated a total potential for US rooftop photovoltaic power generation at 1,118 GW of installed capacity and 1,432 terawatt-hours (TWh) of annual energy generation. This equates to approximately 39% of total national electricity sales.
These results represent a combination of commercial (see Figure 1) and residential (see Figure 2) building potential that takes into consideration average building rooftop footprint, local climate conditions, average module efficiencies, and population densities. California has the highest technical potential with as much as 74% of electricity needs coming from rooftop solar. This is due to its mix of high population (i.e., lots of rooftops) and relatively good solar resource. Figure 3 shows how potential varies by state across the US.
Total US electrical generation capacity at the end of 2014 was approximately 1,200 GW capacity and approximately 4,000 TWh of annual generation. Of this rooftop solar represented approximately 7 GW of capacity. The above numbers indicate that rooftop solar, if all potential was utilized, could provide about one-quarter of total current US usage. However, it also shows that rooftop solar has a long way to go to become a significant contributor to the country's energy capacity, as it currently only supplies about 1% of its potential. Clearly there is a huge, relatively untapped potential to meet the US renewable energy goals with rooftop solar.