What if each and every building in the United States had a solar roof?

The demand for rooftop solar panels for residential and commercial buildings is growing every year. Although solar energy provides a meager 0.5% of the electric power in the U.S., we can imagine a future when every single solar-ready roof will be transformed into a solar roof. This is not a far-fetched scenario: it took telephones about 65 years to reach 40% market penetration in the U.S.. For smartphones, one of the fastest spreading technologies in human history, it only took 10 years to do the same. Solar power seems to be on a similar trajectory.

solar roof in california neighborhood
A vision of the future? Sacramento neighborhood covered in solar roofs. (photo credit: energy.gov)

Determining the “technical potential” of rooftop solar power in the United States

Researchers from the National Renewable Energy Laboratory (NREL) tackled this exact question. In their latest study (2016),  NREL concluded that, if all of the residential and commercial rooftops suitable for solar panels are used, a grand total of 1,118 gigawatts (GW) of solar power capacity can be installed in the U.S. This corresponds to an annual energy production of about 1,432 terawatt-hours (TWh), representing 39 percent of the nation’s electricity sales. Thus, leaving aside utility-scale solar projects, rooftops alone have enough solar “juice” to power 2/5ths of the whole country. This new study doubles the previous NREL estimate for solar rooftop potential in the U.S.

The latest NREL analysis included a more detailed assessment of the suitability of rooftops for hosting solar photovoltaics (PV) in 128 cities nationwide (representing approximately 23% of U.S. buildings), using light detection and ranging (lidar) data, geographic information system (GIS) methods, and PV-generation modeling. One interesting finding was that small buildings actually provide the greatest promise, with the potential to accommodate up to 731 GW of PV capacity and generate 926 TWh per year of solar energy (approximately 65 percent of the country’s total rooftop technical potential). This is due to the great number of this class of small building rooftop. Combined, they can contribute considerably to the nation’s electricity needs.

The study assumed a solar panel efficiency of 16%, but noted that if an efficiency of 20% were assumed (which corresponds to currently available premium systems), then the technical potential estimates would increase by 25%. Moreover, the report only took into consideration roof-mounted systems and not the immense potential of ground-mounted systems.

Economic impact of a 100% solar roof scenario

Aside from the direct effects of environmentally-friendly electricity generation, the potential rooftop solar market creates millions of job opportunities in the U.S. According to material scientist Tao Zheng and venture capitalist David Anthony, assuming 100% residential market penetration by 2030, 5 million new jobs could be created.

Arguably, the greatest economic impact is felt by electricity consumers themselves. As we discussed in an earlier article, once the cost of solar electricity reaches or dips below the retail price of electricity (a.k.a grid parity), we see a greater shift towards solar. An NREL figure of the average cost of solar clearly shows the trend in decreasing solar module costs. The number of states where grid parity has been reached is increasing as a result of these decreasing costs, combined with incentive programs that further reduce the cost to consumers.

Solar Power Cost

Technology s-curve for solar panels

The adoption rate of many technologies follows a trend line that looks like the letter “S”. Over time, with increasing performance and decreasing unit cost, more and more people opt for a new technology. At the initial stages, there is a slow rate of adoption as the technology is undergoing intensive research and development (R&D). Great effort may be spent exploring different paths towards improvement and some of these will result in dead ends. Eventually, the improvements will accelerate and developers will shift out of the research stage to focus on commercialization. This is when the market expands and companies start working on new iterations of their products, as well as decreasing their prices to beat the competition, further spurring the technology’s adoption. As the technology matures and more and more people acquire it, diminishing returns set in and the rate of growth slows down. At this stage, the overall market for the technology reaches its peak.

Solar power is only at the very early stages of its own “S-curve”. Decreasing installation costs, new financing options, and increased reliability of products are creating a very favorable environment for increased adoption of solar panels by homeowners. In order to determine where solar energy is headed, researchers from the Stern School of Business at New York University (Melissa A. Schilling and Melissa Esmundo) compared the trajectory of solar energy with other forms of energy generation (including other renewables, as well as fossil fuels). They concluded that, while fossil fuel technology has reached maturation, with little noticeable gains in kWh (energy) generated per dollar from further investment in research and development, solar photovoltaics are just starting to demonstrate remarkable improvements in performance. The figure below shows how the R&D expenditure directly correlates to a faster growth in solar panel performance.

S-Curve Photovoltaics
R&D spending vs. performance increase (source: M. A. Schilling and M. Esmundo)

Our verdict

Rooftop solar panels have a bright future ahead. Cross checking various metrics suggest that we are currently at around the 1% point of technology adoption in the United States. Technology forecasting is a very tricky business, so we won’t even attempt to declare when solar will truly go mainstream. However, it is clear that if the underlying cost and performance trends continue to support the growth of this business, you’ll be hearing more and more of your neighbors boasting about the beauty of solar (remember the day your friend showed you a gizmo called the “iPhone”?).

If you haven’t yet started investigating solar panels for your rooftop, you haven’t missed the boat. As the market matures, consumers will have more options to benefit from solar energy. But don’t forget that the federal solar Investment Tax Credit (ITC) of 30% is set to decrease at the end of 2019, and will be phased out completely after 2022. As part of your research, why not run the numbers for your home and see if solar energy makes sense for you? When you feel ready, we can help you find highly-qualified installers where you live and get you quotes.

Updated version of an article originally published 8 October 2014

6 Replies to “What if each and every building in the United States had a solar roof?”

  1. This seems to be a very promising achievement in the eyes of Elon Musk. I have no doubt that the possibility of this happening within our lifetime is very doable.

  2. Grid demand is not uniform. At night, solar panels will provide precisely zero kW of power. Even based on the numbers used in this calculation, average solar power output would be about 14% of the peak listed. A more reliable supply will need to ramp up during this time. While some may read this article as a sign that we will get the vast majority of our power from solar in the future… it actually confirms the opposite. Even assuming every single household has a solar roof as they do in this calculation, there is still not nearly enough power to cover grid demand. Never put all eggs in one or two baskets. A regional hurricane could cause electricity production to plummet across perhaps a quarter of the country if the grid relied heavily on solar power. Wind turbines would automatically engage brakes in areas of high wind as well. It all sounds so perfect though when you don’t know any technical details.

  3. It’s actually much better than 40%. The nreal study only considered 25% pf the total roofs as suitable and limited the amount of solar per roof to 5 KW*. They did not allow smaller roofs, or roofs broken up into may small surfaces. They eliminated roofs too steep, to flat, facing the wrong direction they assumed a 15% panels when 22% are available competitively, They assumed only Colorado insolation, which is not good.

    As they state, the parking lots , road cover and road sides are far more area than their calculated roof area.

    *they limited the max size in previous studies but did not list it here. They did list this “5The technical potential of rooftop PV is the amount of energy that could be generated by installing PV modules on
    all the roof area suitable for development. In contrast, resource potential would include all solar energy falling on rooftops, economic potential would include all energy generated if all economically viable sites were developed, and market potential would estimate energy generation considering factors such as maximum system sizes set by utilities.”

    Still it’s twice the last NREL political study.

    As for complaints about backup, the grid already has reserve generators and spinning reserve and has no problem with solar and wind intermittency. Germany and Denmark have near or more than 100% peaks of renewable and they are the most reliable grids on earth. The whole ” the sun doesn’t shine at night” thing, it getting boring.

  4. Also, utility-scale battery storage units are coming online right now, which will solve the “no sun at night” issue. With Tesla’s battery Gigafactory operating now, its battery costs have dropped to under $200 per Kilowatt-hour. That’s less than 1/5 the $1000 price from just 10 years ago. Elon Musk expects to halve the cost again to $100 per Kilowatt-hour by 2019. At that cost, solar panels plus battery storage will be the cheapest new generating capacity available to many utilities.

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