19th-century solar panels were made of selenium. Today, photovoltaic (PV) panels use thin wafers of silicon crystal. As photons strike them, they knock electrons loose and produce an electrical circuit. These subatomic particles are the only moving parts in a solar panel, which requires no fuel and produces clean energy.
Small-scale solar systems, typically sited on rooftops, accounted for roughly 30 percent of PV capacity installed worldwide in 2015. In Germany, a leader in solar, rooftops boast 1.5 million systems. In Bangladesh, population 157 million, more than 3.6 million home solar systems have been installed.
Rooftop solar is spreading as the cost of panels falls, driven by incentives to accelerate growth, economies of scale in manufacturing, and advances in PV technology. Innovative end-user financing, such as third-party ownership arrangements, have helped mainstream its use. Yet, costs associated with acquisition and installation can be half the cost of a rooftop system and have not seen the same dip.
In grid-connected areas, rooftop panels can put electricity production in the hands of households. In rural parts of low-income countries, they can leapfrog the need for large-scale, centralized power grids, and accelerate access to affordable, clean electricity—becoming a powerful tool for eliminating poverty.
Charles Fritts…“photoelectric” modules: Perlin, John. Let It Shine: The 6,000-Year Story of Solar Energy. Novato, California: New World Library, 2013.
first [coal] plant…Thomas Edison: Schobert, Harold H. Energy and Society: An Introduction. Hoboken: CRC Press, 2014.
a billion people [without electricity]: IEA and World Bank. Sustainable Energy for All 2015—Progress Toward Sustainable Energy. Washington, D.C.: The World Bank, 2015.
sun’s light [vs.] world’s total [energy] use: NOAA. “Energy on a Sphere.” http://sos.noaa.gov/Datasets/dataset.php?id=579.
photovoltaics…2 percent of…electricity: IRENA. Letting in the Light: How Solar PV Will Revolutionize the Electricity System. Abu Dhabi: International Renewable Energy Agency, 2016; IEA. Technology Roadmap: Solar Photovoltaic Energy. Paris: International Energy Agency, 2014.
distributed systems…percent of [total]: IHS Technology. Top Solar Power Industry Trends in 2015. London, 2015.
Germany…1.5 million systems: IRENA, Solar PV.
Bangladesh…3.6 million home solar systems: IEA and World Bank, Sustainable; IRENA, Solar PV.
16 percent of Australian homes: REN21. Renewables 2016 Global Status Report. Paris: REN21 Secretariat, 2016.
manufacturing boom in China…inexpensive panels: Goodrich, Alan C., Douglas M. Powell, Ted L. James, Michael Woodhouse, and Tonio Buonassisi. “Assessing the Drivers of Regional Trends in Solar Photovoltaic Manufacturing.” Energy & Environmental Science 6, no. 10 (2013): 2811-2821; Fialka, John. “Why China Is Dominating the Solar Industry.” Scientific American. December 19, 2016.
soft costs of…a rooftop system: Ardani, Kristen, Galen Barbose, Robert Margolis, Ryan Wiser, David Feldman, and Sean Ong. Benchmarking Non-Hardware Balance of System (Soft) Costs for US Photovoltaic Systems Using a Data-Driven Analysis from PV Installer Survey Results. Golden, CO: National Renewable Energy Laboratory, 2012.
cheap[er] than…the grid in some [places]: IRENA, Solar PV; REN21, Renewables 2016.
financial benefit of rooftop PV: Hallock, Lindsey, and Rob Sargent. Shining Rewards: The Value of Rooftop Solar Power for Consumers and Society. Washington, D.C.: Environment America, 2015: Muro, Mark, and Devashree Saha. Rooftop Solar: Net Metering Is a Net Benefit, Washington, D.C.: Brookings Institution, 2016.
investment in distributed solar: REN21, Renewables 2016.
Bangladesh…115,000 direct jobs: IRENA, Solar PV.
Project Drawdown defines rooftop solar as: distributed solar photovoltaic (PV) systems, typically sited on rooftops, that include both residential solar PV and community-scale solar PV systems with under 1 megawatt of capacity. This solution replaces conventional electricity-generating technologies such as coal, oil, and natural gas power plants.
Solar cells are typically divided into three generations. First-generation solar cells, which capture the majority of the current market, are based on crystalline silicon (either single crystalline or multi-crystalline). Second-generation solar cells are thin-film solar PV, which mainly includes three main families: a) amorphous silicon and micromorph silicon; b) cadmium telluride; and c) copper-indium-selenide and copper-indium-gallium-diselenide. Third-generation solar cells, such as high concentration PV, dye sensitized solar cells, and organic solar cells, are still under development and are not yet widely commercialized.
Most adoption scenarios of this technology predict low, single-digit percentages of total electricity generated by solar PV by 2050; but some, such as the Greenpeace Energy [R]evolution scenarios (2015), envision PV holding a much larger share of future electricity generation (near 20 percent of the electricity generation mix). These projections are based on increases in solar cell efficiencies and rapid declines in costs for PV installations, making them competitive with conventional generating sources in many parts of the world.
This analysis models distributed solar PV systems, including both residential and community-scale systems, with under 1 megawatt of capacity.
Total Addressable Market 
The total addressable market for rooftop solar is based on projected global electricity generation in terawatt-hours from 2020-2050, with current adoption  estimated at only 0.33 percent of generation (IRENA, 2016). With no definitive estimate of the type of future solar PV adoption, it is assumed that rooftop installations represent approximately 40 percent of the market, with utility-scale solar PV (i.e. solar farms) capture the remaining 60 percent (US DOE, 2012; IEA, 2014; SEIA, 2014).
Adoption Scenarios 
Impacts of increased adoption of rooftop solar from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to a Reference Scenario where the solution’s market share was fixed at the current levels.
- Plausible Scenario: This scenario is based on the evaluation of five optimistic scenarios from the EU project AMPERE (2014),  the 2°C Scenario of the International Energy Agency’s Energy Technology Perspectives (2016), and the Greenpeace Energy [R]evolution Scenario (2015) using a high growth trajectory.
- Drawdown Scenario: This scenario is aligned with the Greenpeace Advanced Energy [R]evolution Scenario. 
- Optimum Scenario: Like the Drawdown Scenario, this scenario is aligned with the Greenpeace Advanced Energy [R]evolution Scenario.
To capture the rapid decrease in costs seen in recent years, the low boundary of data collected on installation costs is assumed, which results in a total first cost of US$1,884 per kilowatt.  A customized learning rate of 19.66 percent was developed, accounting for independent impact on PV modules and balance of systems; this has the effect of reducing the installation cost to US$901 per kilowatt in 2030 and US$628 per kilowatt in 2050, compared to US$1,923 per kilowatt for the conventional technologies (i.e. coal, natural gas, and oil power plants). An average capacity factor of 20 percent is used for the solution, compared to 55 percent for conventional technologies.
Through the process of integrating rooftop solar with other solutions, the total addressable market for electricity generation technologies was adjusted to account for reduced demand resulting from the growth of more energy-efficient technologies,  as well as increased electrification from other solutions like electric vehicles and high-speed rail. Grid emissions factors were calculated based on the annual mix of different electricity-generating technologies over time. Emissions factors for each technology were determined through a meta-analysis of multiple sources, accounting for direct and indirect emissions.
The Plausible Scenario projects 6.88 percent of total electricity generation worldwide from rooftop solar by 2050 (i.e. 3,578 terawatt-hours). In the Drawdown and Optimum Scenarios, the market share reaches 10.1 percent and 10.5 percent, respectively.
The climate and financial impacts for the accelerated adoption of rooftop solar are both significant. The Plausible Scenario results in the avoidance of 24.6 gigatons of carbon dioxide-equivalent greenhouse gas emissions from 2020-2050, with US$453.14 billion in associated net costs. Nearly US$3.5 trillion of net operating savings are projected over the same period. Both the Drawdown and Optimum Scenarios are more ambitious in the growth of rooftop solar technologies, with impacts on greenhouse gas emission reductions over 2020-2050 of 43.1 gigatons and 40.34 gigatons, respectively.
Solar has an incredibly promising long-term potential, as solar resources are plentiful and widespread and future advances in both battery and PV technologies should continue to drive the adoption of this technology, even in a world without specific policy interventions. Based on the financial impacts alone, it is clear that global adoption of rooftop solar is economically viable and will provide a significant return on investment. Rapid adoption will also contribute substantially to global greenhouse gas abatement.
Nevertheless, the massive adoption of rooftop solar requires several issues to be contended with. It must be noted though that sunlight is intermittent, and electricity profiles from solar PV do not always match well with the typical demand profile of electricity consumers. This means that PV often must be installed alongside dispatchable sources such as coal and natural gas. Alternatively, solar PV can be installed with an energy storage system so that solar electricity generated during the day can be stored for use during the hours when the sun is not shining. Also, there will need to be more demand flexibility, to change the demand profile to better match the generation profile. There may also be materials constraints on the expansion of production capacity for current PV technology, for several critical materials are only mined as by-products of other metals and could be limited in their ability to meet the levels of production needed for significant global adoption. More research into materials reduction in PV systems design will help address this issue.
 For more about the Total Addressable Market for the Energy Sector, click the Sector Summary: Energy link below.
 Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
 To learn more about Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Energy Sector-specific scenarios, click the Sector Summary: Energy link.
 The adoption from the 450 scenarios of three AMPERE models (GEM E3, MESSAGE, and IMAGE) was used.
 It represents an ambitious pathway towards a fully decarbonized energy system in 2050, with significant additional efforts compared to the Energy [R]evolution Scenario. The Advanced Energy [R]evolution Scenario needs strong efforts to transform the energy systems of all world regions towards a 100 percent renewable energy supply.
 All monetary values are presented in US2014$.
 For more on Project Drawdown’s Energy Sector integration model, click the Sector Summary: Energy link below.
 For example: LED lighting and heat pumps.