Solar energy can be harnessed in two basic ways. First, solar thermal technologies utilize sunlight to heat water for domestic uses, warm building spaces, or heat fluids to drive electricity-generating turbines. Second, photovoltaics (PVs) are semiconductors that generate electrical current from sunlight. Only 2.3% of U.S. electricity was generated with solar technologies in 2020.1
Solar Resource and Potential
- On average, 1.73 x 105 terawatts (TW) of solar radiation continuously strike the Earth, while global electricity demand averages 2.7 TW.3,4
- Electricity demand peaks around mid-day, leading to energy surplus and deficits. Energy storage and demand forecasting will help to match PV generation with demand.5
- If co-located with load centers, solar PV can be used to reduce stress on electricity distribution networks, especially during peak demand.6
- PV conversion efficiency is the percentage of incident solar energy that is converted to electricity.7
- Though most commercial panels have efficiencies from 15% to 20%, researchers have developed PV cells with efficiencies approaching 50%.8,9
- Assuming intermediate efficiency, PV covering 0.6% of U.S. land area would generate enough electricity to meet national demand.10
- In 2011, the U.S. Department of Energy (DOE) announced the SunShot Initiative. Its aim was to reduce the cost of solar energy by 75%, making it cost competitive with other energy options. In 2017, DOE announced that the 2020 goal of utility-scale solar for $0.06/kWh had been achieved three years earlier than expected. The 2030 goal includes reducing utility-scale solar energy to $0.03/kWh, cheaper than electricity from fossil fuel energy resources.11
Annual Average Solar Radiation2
PV Technology and Impacts
- PV cells are made from semiconductor materials that eject electrons when light strikes the surface, producing an electrical current.15
- Most PV cells are small, rectangular, and produce a few watts of direct current (DC) electricity.16
- PV cells also include electrical contacts that allow electrons to flow to the load and surface coatings that reduce light reflection.15
- A variety of semiconductor materials can be used for PVs, including silicon, copper indium gallium diselenide (CIGS), cadmium telluride (CdTe), perovskites and even some organic compounds (OPV).15 Although PV conversion efficiency is an important metric, cost efficiency—the cost per watt of power—is more important for most applications.
PV TECHNOLOGY TYPES AND EFFICIENCIES9,12
PV Cell Diagram13
PV Modules and Balance of System (BOS)
- PV modules typically comprise a rectangular grid of 60 to 72 cells, connected in several parallel circuits and laminated between a transparent front surface and a structural back surface. They usually have metal frames and weigh 34 to 62 pounds.17
- A PV array is a group of modules, connected electrically and fastened to a rigid structure.18
- BOS components include any elements necessary in addition to the actual PV panels, such as wires that connect modules, junction boxes to merge the circuits, mounting hardware, and power electronics that manage the PV array’s output.18
- An inverter is a power electronic device that converts electricity generated by PV systems from DC to alternating current (AC).18
- A charge controller is a power electronic device used to manage energy storage in batteries, which themselves can be BOS components.18
- In contrast to a rack-mounted PV array, Building Integrated PV (BIPV) replaces building materials and improves PV aesthetics.19
- Some ground-mount PV arrays employ a solar tracker. This technology can increase energy output by up to 100%.20
Residential PV System14
PV Installation, Manufacturing, and Cost
- In 2019, global PV power capacity grew by over 115 GW and reached 633.7 GW. Solar PV capacity has grown by nearly 400 times since 2000.23
- Top installers in 2019 were China (30.1 GW), the U.S. (13.3 GW), and India (8.8 GW).23
- New PV installations grew by 13% in 2019 and accounted for 48% of global power plant capacity additions. Even with this significant growth, solar power only accounts for 2.6% of global power generation.23
- The cost of solar power has dropped nearly 89% since 2009. Various contracts have been signed around the world with solar power prices as low as 1-2¢/kWh; this is much cheaper than conventional power sources.23 In comparison, U.S. retail electricity averaged 10.66¢/kWh for all sectors and 13.20¢/kWh for residential consumers in 2019.1
- In 2019, global investment in solar power dropped to $131.1 billion. This is partially a result of declining capital costs of PV systems.24
- PV system/component manufacturing employed 34,000 people in the U.S. in 2018.18
World Cumulative Installed PV Capacity21
Median Installed Price, Residential, Commercial, and Utility-Scale PV Systems22
Energy Performance and Environmental Impacts
- Net energy ratio compares the life cycle energy output of a PV system to its life cycle primary energy input. One study showed that amorphous silicon PVs generate 3 to 6 times more energy than are required to produce them.25
- Reusing multi-crystalline cells can reduce manufacturing energy by over 50%.26
- Although pollutants and toxic substances are emitted during PV manufacturing, life cycle emissions are low. For example, the life cycle emissions of thin-film CdTe are roughly 14 g CO2e per kWh delivered, far below electricity sources such as coal (1,001 g CO2e/kWh).27,28
- PVs on average consume less water to generate electricity (26 gallons per MWh), compared to non-renewable technologies such as coal (687 gallons per MWh).29
Solutions and Sustainable Actions
Policies Promoting Renewables
- Consumers that do not have roof space for PV panels can join community solar programs, which are local solar projects that community members can share and receive credit on their electricity bills.30 Property assessed clean energy (PACE) programs allow property owners to finance the upfront costs of a solar installation through a voluntary assessment on annual property taxes.31 Green banks and other lending institutions are being developed to specifically fund and support clean energy projects on local, regional, and national scales.32
- Carbon cap-and-trade policies would work in favor of PVs by increasing the cost of fossil fuel energy generation.33
PV policy incentives include renewable portfolio standards (RPS), feed-in tariffs (FIT), capacity rebates, and net metering.
- An RPS requires electricity providers to obtain a minimum fraction of energy from renewable resources.34
- A FIT sets a minimum per kWh price that retail electricity providers must pay renewable electricity generators.35
- Capacity rebates are one-time, up-front payments for building renewable energy projects, based on installed capacity (in watts).35
- With net metering, PV owners get credit from the utility (up to their annual energy use) for energy returned to the grid.35
What You Can Do
- “Green pricing” allows customers to pay a premium for electricity that supports investment in renewable technologies. Renewable Energy Certificates (RECs) can be purchased to “offset” commodity electricity usage and help renewable energy become more competitive.36
- Emerging PV technologies include perovskites, bifiacial PV modules and concentrator PV (CPV) technology. Perovskite solar cells have a high conversion efficiency (over 25%) and low production cost. Bifacial modules are able to collect light on both sides of the PV cells. CPV utilizes low-cost optics to concentrate light onto a small solar cell.37,38, 39
- Designing for end-of-life could improve the current 10% rate of PV module recycling.40
A watt is a unit of power, or a rate of energy flow. 1 TW = 1,000 GW = 1,000,000 MW = 1,000,000,000 kW.
A kilowatt-hour is a unit of energy. 1 kWh is the electricity energy required to light a 100 watt light bulb for 10 hours.
- U.S. Energy Information Administration (EIA) (2021) Monthly Energy Review April 2021.
- U.S. Department of Energy (DOE), National Renewable Energy Lab (NREL) (2018) U.S. Annual Solar GHI map.
- National Oceanic and Atmospheric Administration (2017) “Energy on a Sphere.”
- U.S. EIA (2021) International Energy Statistics.
- U.S. DOE, Energy Efficiency and Renewable Energy (EERE) (2017) “Confronting the Duck Curve: How to Address Over-Generation of Solar Energy.”
- America’s Energy Future Panel on Electricity from Renewable Resources, National Research Council (2010) Electricity from Renewable Resources: Status, Prospects, and Impediments.
- U.S. DOE, EERE (2020) “Solar Energy Glossary.”
- Energy Sage (2021) “Most efficient solar panels: solar panel cell efficiency explained.”
- NREL (2021) Best Research-Cell Efficiencies.
- NREL (2012) SunShot Vision Study.
- U.S. DOE (2021) “The SunShot Initiative.”
- NREL (2021) Champion Module Efficiencies.
- Adapted from NASA Science (2008) “How Do Photovoltaics Work?”
- Photo courtesy of National Renewable Energy Laboratory, NREL-45218.
- U.S. DOE, EERE (2021) “Solar Photovoltaic Cell Basics.”
- U.S. DOE, EERE (2021) “Solar Photovoltaic Technology Basics.”
- Platzer, M. (2015) U.S. Solar Photovoltaic Manufacturing: Industry Trends, Global Competition, Federal Support. Congressional Research Service.
- Congressional Research Service (2020) Solar Energy: Frequently Asked Questions.
- Barbose, G., et al (2014) Tracking the Sun VI: An Historical Summary of the Installed Price of Photovoltaics in the United States from 1998 to 2012. Lawrence Berkeley National Laboratory, LBNL-6350E.2013.2017.
- Mousazadeh, H., et al. (2009) “A review of principle and sun-tracking methods for maximizing solar systems output.” Renewable and Sustainable Energy Reviews, 13:1800-1818.
- International Energy Agency (2020) Trends 2020 in Photovoltaic Applications Survey Report of Selected IEA Countries between 1992 and 2019.
- NREL (2021) U.S. Solar Photovoltaic System Cost Benchmark Q1 2020.
- Solar Power Europe (2020) Global Market Outlook For Solar Power 2020-2024.
- Frankfurt School, United Nations Environment Programme, Bloomberg New Energy Finance (2020) Global Trends in Renewable Energy Investment 2020.
- Pacca, S., et al. (2007) “Parameters affecting life cycle performance of PV technologies and systems.” Energy Policy, 35:3316–3326.
- Muller, A., et al. (2006) “Life cycle analysis of solar module recycling process.” Materials Research Society Symposium Proceedings, 895.
- Kim, H., et al (2012) “Life cycle greenhouse gas emissions of thin-film photovoltaic electricity generation.” Journal of Industrial Ecology, 16: S110-S121.
- Whitaker, M., et al. (2012) “Life cycle greenhouse gas emissions of coal-fired electricity generation.” Journal of Industrial Ecology, 16: S53-S72.
- NREL (2011) Review of Operational Water Consumption and Withdrawal Factors for Electricity Generating Technologies.
- Solar Energy Industries Association (SEIA) (2021) “Community Solar.”
- U.S. DOE, EERE (2021) “Property Assessed Clean Energy Programs.”
- Clean Energy Credit Union (2020) “Our Story.”
- Bird, L., et al. (2008) “Implications of carbon cap-and-trade for U.S. voluntary renewable energy markets.” Energy Policy, 36(6): 2063-2073.
- U.S. EPA (2021) “State Renewable Energy Resources.”
- U.S. DOE, EERE (2011) Solar Powering Your Community: A Guide for Local Governments.
- U.S. Environmental Protection Agency (2021) “Green Power Supply Options.”
- U.S. DOE EERE (2021) “Perovskite Solar Cells.”
- NREL (2016) Evaluation and Field Assessment of Bifacial Photovoltaic Module Power Rating Methodologies.
- NREL (2017) Current Status of Concentrator Photovoltaic Technology.
- NREL (2021) Solar Photovoltaic Module Recycling: A Survey of U.S. Policies and Initiatives.