The potential of rooftop solar energy: 40% of total U.S. electricity generation is possible

The predicted ceiling for rooftop solar energy growth has nearly doubled. A new study [1] released from the National Renewable Energy Laboratory (NREL) has combined light detection technology, geospatial mapping, and large-scale simulations to understand just how much electricity can be generated across the United States using only rooftop photovoltaic (PV) technology. This updated characterization of distributed solar generation should reshape how policymakers think about incentivizing photovoltaic technology and how to tailor building spaces to take advantage of such an abundant resource.

The rooftop potential

Rooftop solar panels exemplify the potential of distributed electricity generation. As opposed to the hulking coal-fired power plants that provide power to thousands while huffing pollution from a central location, solar cells stuck to roofs humbly generate energy for local homes or businesses. For rooftop solar to take over the consistent, base-load duties of power plants, the electrical grid will need to be transformed into a more flexible, smarter version of its old self. While this metamorphosis takes place, however, it's important to understand the limits of rooftop PV to provide all of our electricity so that informed policies and economic decisions can continue to efficiently grow the solar energy market.

NREL researchers tackled this question by looking at the technical potential of rooftop PV. Resource potential is the ultimate constraint on any energy technology: the amount of energy stored in sun rays hitting the Earth is the ultimate upper limit for solar power. Beyond this natural constraint, technical potential describes how land-use, topographic constraints, and overall system performance affect the total capture of an energy resource. In the case of rooftop PV, this is calculated by combining information such as the total amount of sunlight hitting a certain area, the availability of roofs for PV installation, and per-capita energy consumption. This analysis gives an upper bound on potential deployment of the technology before considering the impact of policies, regulations, or markets.

Previous studies had suggested modest rooftop PV potential, limiting solar power to 664 GW annual energy generation to 800 TWh. These values amount to just under a quarter of total U.S. electricity generation. However, these numbers were derived from analyses that lacked high-resolution geographical data and less-sophisticated computer simulations. Using the latest technology in computer modeling and geospatial analysis, NREL has now provided much more optimistic numbers that indicate rooftop PV could account for nearly half of all electricity generation in some states.

A matter of lighting

To determine the technical potential of rooftop PV, NREL researchers first used light detection and ranging (lidar) data to determine the amount of sunlight hitting roofs across America. Lidar technology shoots light from an aircraft or satellite toward Earth's surface, measuring the distance traveled and the amount of light that reflects back to the source. The end result is a surface model of the planet with one-square-meter resolution! From this information, researchers could determine shading and roof tilt, both of which affect the available sunlight for solar power generation and building suitability.

Lidar data was then combined with geospatial analyses and simulations to determine the PV potential for small (residential) vs medium/large buildings (commercial) across 128 cities. Results from these cities were then extrapolated to calculate the rooftop PV potential across all areas of the continental U.S.

The study results are extremely promising. Across all building sizes, rooftop PV could provide 1.1 TW of electrical power and 1432 TWh of annual energy generation. That's 39% of total electricity sales in 2013! Two-thirds of this potential comes from smaller residential buildings as opposed to commercial sites, which means that personal consumer decisions to install solar panels can be a primary driver for the industry's growth.

Considerable variation in rooftop potential exists between states. For example, California could generate 3/4th of its electricity through rooftop solar, and New England or Florida could reach nearly half of all its generation. The reasons for these high capacities differ and also suggest ways that other states could improve their potential. California has a high density of small buildings compared to most other states, increasing its potential, whereas the lower-than-average per capita energy consumption in New England states drive their high potential. In contrast, Bible-Belt states and the Northwest could generate 25-35% of its electricity from rooftop PV.

Why do these new model results improve upon previous estimates of technical potential? Two primary reasons account for the difference. First, solar cell technology has improved and now boasts better power density now compared to several years ago, allowing for a larger share of power generation. Second, experts have an improved understanding of what types of buildings are suitable for rooftop PV, so the newer model has calculated a higher overall number of suitable buildings.

Room for improvement

Rooftop PV is moving toward the potential to account for nearly half of all electricity generation in the United States. This is tremendous evidence to support the transition to a smart grid and investing in distributed generation. But rooftop PV is only one way of generating solar energy. Centralized solar technology can add to this technical potential, like the thousands of mirrors reflecting sunlight to run steam turbines in the deserts of the Southwest. In addition, ground-mounted solar power is more flexible than rooftop PV and could significantly add to solar power generation.

Above all else, I believe this data shows how important it is to rethink building infrastructure to take into account rooftop solar technology. Even though residential buildings accounted for most of the technical potential, only 26% of rooftop area was suitable, indicating tremendous room for rooftop PV growth if we design building facades, canopies, or other structures that are designed with PV installation in mind.

It's important to note that these analyses only provide an upper limit based on technology and geographic constraints, and do not account for policies or market trends that could affect the industry's growth. Policymakers should take these results as substantial evidence that rooftop PV technology is still in its nascent stages of development, and that policies promoting its expansion could stimulate jobs in the U.S., reduce dependence on oil and natural gas, and move us towards a diversified energy economy.

References

[1] Gagnon P, et al. "Rooftop solar photovoltaic technical potential in the United States: a detailed assessment." National Renewable Energy Laboratory, accessed April 8, 2016.

Photo and Figure Credits

Photo of rooftop photovoltaic system courtesy of Ken Fields via Wikipedia

Figure of U.S. maps and technical potential courtesy of Reference [1]