Recent trends in renewable energy sources for electric power generation in the United States
With the increasing emissions of greenhouse gasses and other air pollutants that lead to global warming, the demand for renewable energy sources has been growing.
Author: Insu Kim
As renewable energy sources contribute significantly few emissions of greenhouse gases and other pollutants into the atmosphere, more and more countries are developing technology that leads to their more efficient and reliable use (e.g. wind farms and solar photovoltaic (PV) systems) in electric power generation. During the past several decades, the United States (US) has been a leading innovator in the research and development of renewable energy technologies. This article reviews recent trends in sources of renewable energy, including hydroelectric, wind, biomass, solar PV, and geothermal energy sources for electricity generation in the US and their potential and predicts their success in the future.
With the increasing emissions of greenhouse gasses and other air pollutants that lead to global warming, the demand for renewable energy sources has been growing. In fact, in the past several decades, the global demand for renewable energy sources for electricity generation has dramatically outpaced that for conventional fossil fuel. Therefore, many countries have developed technology to more efficiently and reliably use such sources for electric power generation. The United States (US) has led such a rapid expansion of renewable energy resources, particularly in photovoltaic (PV) and wind energy. For example, in 2012, the US produced 140.9 terra watt hours (TWh) of electric energy from wind power plants, accounting for the largest percentage of the world production, or 26.4% . (In December 2014, the US was the second leading producer with a cumulative capacity of 65,879 megawatts (MW), after China, 114,763 MW ). In 2015, renewable energy sources for US electricity generation account for 13.19% of total net electricity production, or 4092.9 TWh (6.32% hydroelectric, 4.44% wind, 1.57% biomass, 0.45% PV, and 0.41% geothermal) . At such a low fraction (e.g. 13.19%), electricity generated from other renewable energy sources in the US, excluding hydropower, the area shaded in red in Fig 1, have exhibited relatively steady increase since the mid-1980s (Fig 1). In other words, the country is expected (a) to meet new demand by increasing renewable energy sources abundant throughout the country (e.g. wind, biomass, geothermal, and solar), (b) to cope with highly unstable prices of conventional fossil fuels, and (c) to reduce the release of greenhouse gases and other pollutants into the atmosphere (in accordance with the United Nations Framework Convention on Climate Change). Thus, the ratio of renewables, including hydropower, to total US electricity supply decreased by the end of 1900s but increased after that, as shown in Fig 1.
Fig 1: Electricity generated from renewable energy sources from 1950 to 2013 
To estimate the extent to which US renewables can increase, many studies have examined their future potential [5–10] and predicted that they will either slightly or significantly increase. For example, the reference scenario shows that renewable energy sources for US electricity generation could comprise as much as 18% of total electricity production in 2040 (909 TWh of 5056 TWh) ; and another study forecasted that by 2050, they could account for as much as 80% . Using commercially available techniques, both studies, while showing different growth rates, estimated that US renewables would continuously increase. Therefore, the objective of this article is to (a) examine the recent trends in renewable energy sources for electricity generation in the US and (b) discuss the projections of their future. For this purpose, this article initially examines the current activities devoted to US renewable energy sources in the following order: hydroelectric, wind, biomass, PV, and geothermal energy sources. Then, it compares several case studies that forecast their potential and success. This article is organised as follows: ‘Renewable energy resources in the US’ section introduces the current status and future potential of hydroelectric, wind, biomass, PV, and geothermal electricity generation. ‘Future renewable energy resources in the US’ section summarises the future potential and predicts the success of US renewables. Finally, ‘Conclusion’ section presents the major conclusions of this article.
Renewable energy resources in the US
Comprising the largest fraction of the US renewables, hydropower generates electricity from falling water stored in dams or reservoirs. In fact, in the 12 months prior to March 2015, US hydropower had produced 267.2 TWh, or 6.6% of total US electricity generation of 4079.1 TWh and 48.9% of total US renewable generation of 546 TWh. By the end of March 2015, it had reached a total installed capacity of 79.4 gigawatts (GW), or 7.4% of total US net generation capacity of 1070.6 GW, in at least 48 states . Despite the various advantages of hydropower (e.g. it uses water provided for free by nature, adjusts output from zero to maximum, or vice versa, within a few minutes, and has relatively low operation and maintenance costs), because of the sites already being developed and environmental concerns, the use of hydropower appears to have stabilised. Fig 2, which presents the number and the total capacity of surface reservoirs of the US and Puerto Rico before 1920–1988, shows that the number of new reservoirs decreased significantly. Trends in the generation of hydropower generation in the top five hydroelectricity-producing countries from 1980 to 2012 in Fig 3 show that the production of hydropower in the US remained relatively constant.
Fig 2: Number and total capacity of large reservoirs in the US 
Fig 3: Top five hydroelectricity producing countries 
Future status of hydropower generation
To project the long-term future of US renewables, including hydropower, the US Energy Information Administration modelled the national future energy portfolio of a reference in six cases, shown in Fig 4: low-economic growth, high-economic growth, low-oil price, high-oil price, and high-oil and gas price cases. The figure shows that using the projection model, the model estimates that in the reference case, US hydropower will produce 297 TWh in 2040, 5.9% of total electric power generation in the US in 2040 (e.g. 5056 TWh), and 32.7% of total renewable power generation (e.g. 909.1 TWh). Their net generation capacity will be 80.4 GW, 6.9% of total net generation capacity (e.g. 1170 GW), and 29.2% of total renewable generation capacity (e.g. 275.2 GW) . Using the Regional Energy Deployment System (i.e. an economic dispatch model developed by the National Renewable Energy Laboratory), another study projects that if renewable energy sources for electricity generation could supply about 80% of total US electricity generation (referred to as the 80% renewable scenario) in 2050, a cumulative installed capacity of US hydro power plants could increase to between 81 and 170 GW, which corresponds to 8.3–16% of the overall renewable electricity generation capacity of the US projected in 2050 .
Fig 4: Projection of renewable energy sources for electric power generation in TWh in 2040 
Owing to its geographical- and water-dependence, US hydropower production appears to remain relatively constant or increase slightly during several decades, as shown in Fig 4. Note that in all six future cases, production remains relatively constant. Therefore, the following strategies could be effective:
- Small-scale hydroelectric plants: Although constructing large reservoirs across the country would be difficult, building small-scale hydroelectric plants for local communities, allowing each community to generate their own electricity without conventional fossil-fuel generation plants, would be feasible. Hydroelectric plants and reservoirs could prove beneficial to the environment [e.g. hydrological effects (water supply and irrigation) and social effects (recreational opportunities, boating, fishing, and swimming for local community members)].
- Pumped-storage hydroelectric plants for spinning reserve: Pumped-storage hydroelectric plants could be ideal spinning reserves because they can increase or decrease their output quickly  (e.g. within tens of seconds to few minutes). Small pumped-storage hydroelectric plants could be built on streams or rivers to reduce peak load provided by energy sources that burn the most expensive fuel.
Invented by Professor James Blyth of the Royal College of Science and Technology (now the University of Strathclyde) in July 1887 , wind power generation converts wind to electrical energy by a turbine on a tower or on top of a building. By the end of March 2015, the US had more than 48,000 wind turbines operating in at least 39 states with a total installed capacity of 66,008 MW , accounting for 6.2% of total US net electricity generation capacity, or 1070.6 GW . Wind power, comprising the second largest fraction of renewable energy sources after hydropower, is the most cost-effective renewable energy source (as a result of the continuous improvement in wind turbine technology) . In fact, it is a mainstream energy source of new electric power generation, as shown in Fig 5 . For example, between 2005 and 2012, it grew 800% at an average annual growth rate of 31% , and in the 12 months prior to 31 March 2015, it had produced 177.6 TWh of electric energy, 4.4% of total US electricity generation, or 4079.1 TWh, and 32.5% of US total renewable generation, or 546 TWh .
Fig 5: Projection of renewable energy sources for electric power generation in TWh (in the reference scenario) 
Unlike in previous years, the US wind industry experienced a busy year in 2015. In the first quarter of 2015, the country installed 68 wind turbines with a capacity of 131–110 MW in Texas alone. On a national scale, wind power plants with a capacity of more than 13,600 MW were under construction in 23 states: more than 7800 MW in Texas, 890 MW in Oklahoma, 870 MW in Kansas, 680 MW in New Mexico, and at least 500 MW in Illinois, Iowa, and North Dakota . In addition, total power purchase agreements (PPAs) signed by utilities and their customers during 2013–2014 were <11,300 MW . New PPAs of 750 MW were signed in early 2015  and previously signed PPAs of 5000 MW will begin construction in 2015 or 2016 . These PPAs include long-term contracts of Dow Chemical (200 MW), Walmart (50 MW), Google Energy (43 MW), and Kaiser Permanente (43 MW) for wind turbines in their data centres .
Future status of wind power generation
Wind energy in the US is a policy-driven industry. For example, various policies such as the production tax credit (PTC), the investment tax credit (ITC), and a renewable electricity standard, also known as the renewable portfolio standard, have played an important role in the continual growth of the US wind power industry . Therefore, since 2000, a number of studies have predicted that the US wind energy industry will continue to grow for several decades [5–7,16]. One study estimates that US wind power plants will produce 319.3 TWh of electric energy in the reference case in 2040, which corresponds to 6.3% of total electric power generation of the nation in 2040 (e.g. 6.3% of 5056 TWh) and 35.1% of total renewable power generation (e.g. 35.1% of 909.1 TWh), depicted in Fig 4 . Their net generation capacity will be 109.7 GW, 9.4% of total net generation capacity (e.g. 1170 GW), and 39.9% of total renewable generation capacity (e.g. 275.2 GW) . Another study estimates that in the 80% renewable scenario, the cumulative installed capacity of US wind power plants will be between 390 and 560 GW, which corresponds to 32–43% of overall US renewable electricity generation capacity in 2050 . The other study estimates that US wind power plants will contribute about 35% of US electricity generation in 2050 [7,8]. Therefore, from these feasibility studies, it can be expected that wind power in the US will be a mainstream source of renewable energy for electricity generation, contributing as much as one-tenth to one-third of the US total electricity capacity in 2040–2050.
The third-largest renewable energy source in the US after hydroelectricity and wind, biomass is a biological material derived available from living or recently living organisms. US biomass power generation uses biomass material processed from industrial sources, mainly pulp and paper. In the 12 months prior to 31 March 2015, US biomass plants produced 64.2 TWh of electric energy, accounting for 1.6% of total US electricity generation, or 4079.1 TWh, and 11.8% of total US renewable generation, or 546 TWh. By the end of March 2015, they had reached a total installed capacity of 13501.7 MW, 1.3% of total US net generation capacity, or 1070.6 GW in at least 48 states , and California, the industry of which has historically relied on forest-derived biomass, led in biomass power generation with an installed capacity of 1327.3 MW, followed by Florida (1189.5 MW), Virginia (881.5 MW), Georgia (707.2 MW), and Maine (652.3 MW) . Fig. 6 shows a historical growth of the US biomass power industry for installed capacity (in GW) and generation (in TWh). Unlike wind energy, which has continuously grown in capacity, biomass begins to stabilise in the 1990s. Fig 7 indicates an increase in the amount of biomass energy consumed in the US from 2002 to 2013, by type of material. The increase in the generation of biomass energy is due to the increase in the production of biofuel, mainly ethanol, most of which is used for transportation, not electric power generation.
Fig 6: Total capacity in GW and electric energy generation in TWh of the US biomass power industry from 1981 to 2010 
Fig 7: Biomass energy consumed from 2002 to 2013, by type of material 
Future status of biomass power generation
The steady growth of the US biomass power industry, including the rapid growth until the early 1990s, is the result of the following: (a) biomass power generation can be used for base load, dispatchable power, or cogeneration; (b) biomass fuel (e.g. agricultural residues, forest residues, wood waste, mill residues, and energy crops) is abundant in many areas of the US, particularly the midwestern states; and (c) the costs of generating electricity from biomass material are relatively low compared with the costs of generating other renewable energy sources such as geothermal, PV, fuel cell, and microturbine sources . Therefore, many studies have predicted that biomass power generation in the US will increase in the next few decades. For example, one study estimates that US biomass power generation will produce 113.1 TWh in the reference case in 2040, which corresponds to 2.2% of total electric power generation of the country in 2040 (e.g. 5056 TWh) and 12.4% of total renewable power generation (e.g. 909.1 TWh). In 2040, its net generation capacity will be 15.4 GW, accounting for 1.3% of total net generation capacity (e.g. 1170 GW) and 5.6% of total renewable generation capacity (e.g. 275.2 GW) . Another study estimates that in the 80% renewable scenario, the US biomass power industry will have a cumulative installed capacity of 52–98 GW, which corresponds to 7.9–15% of overall US renewable electricity generation capacity in 2050 . Therefore, US biomass will be a steady source of electricity generation (e.g. at an average growth rate of 3.1% in generated energy [in TWh] and 1.1% in capacity [in GW] between 2013 and 2040 ).
Using semiconductors with the PV effect, PV systems convert solar radiation energy into direct current (DC) electric energy. US solar PV systems can be divided into residential and commercial roof-mounted systems with up to 5 MW in capacity (typically connected to the distribution side) and utility ground-mounted PV farms >5 MW (typically connected to the transmission side) . The US solar industry experienced a banner year in 2014. In fact, the newly installed capacity of US PV plants amounted to 6201 MW in DC capacity, shown in Fig 8 , and by the end of the year, it had reached a cumulative capacity of 20 GW . In the 12 months prior to 31 March 2015, the country had produced 20.23 TWh, <0.5% of total US electricity generation and 3.7% of total US renewable generation .
Fig 8: Annual PV capacity of newly installed systems in the US 
As shown in Fig 8 , the US solar PV industry has grown dramatically during the past decade, even during difficult economic times. The reasons for such rapid growth include the following:
- Falling PV module prices: The prices of PV modules decreased by about 20% whenever cumulative PV module production doubled, shown in the learning curves regarding PV modules during the past three decades . In addition, in 2014, total system prices decreased by as much as 10% as a result of a decrease in the prices of balance-of-systems .
- Financial solutions and regulations: Many solar PPAs, which are contracts between producers or sellers of electricity and buyers regulated by the Federal Energy Regulatory Commission, are now being signed in many states. For example, California, a cumulative capacity of which is 10,695 MW (in June 2015), ranking first in the country, is now successfully utilising the benefits of PPA . For example, the Multifamily Affordable Solar Housing Program (started by the California Solar Initiative) interconnected solar PV systems with a capacity of 20.5 MW to multi-family homes in 323 statewide projects . In addition, users of residential and commercial solar systems can receive a 30% solar ITC, a federal policy that has been largely responsible for the expansion of the US solar PV industry. In fact, since the ITC was initiated in 2006, the US PV industry has experienced an approximate sixteen-fold increase in solar installations annually .
Future status of PV power generation
Within only few decades, solar power in the US could represent the second largest source of renewable electricity after wind, excluding hydropower. One study predicts that the US solar power industry will produce 110.1 TWh in the reference case in 2040, 2.2% of total electric power generation of the nation in 2040 (e.g. 5056 TWh), or 12.1% of total renewable power generation (e.g. 909.1 TWh). In 2040, its net generation capacity will be 60.6 GW, accounting for 5.2% of total net generation capacity (e.g. 1170 GW) and 22% of total renewable generation capacity (e.g. 275.2 GW) . Another study estimates that in the 80% renewable scenario, a cumulative installed capacity of US solar power plants will range between 91 and 330 GW, which corresponds to 2.9–22% of overall US renewable electricity generation capacity in 2050 . Furthermore, the SunShot Initiative, initiated by the US Department of Energy, plans to increase US cumulative capacity to 330 GW by 2030 and 715 GW by 2050 . According to these studies, the US solar industry, expected to grow continuously and rapidly for several decades, will probably become the second largest source of renewable energy after wind, excluding hydropower.
Geothermal power generation converts hydrothermal fluids (e.g. steam, water, and their heat or thermal production) generated or stored in the Earth into electricity. By the end of 2014, the US had reached a nameplate capacity of <3.5 GW and a net capacity of 2.7 GW . In the 12 months prior to 31 March 2015, it had produced 16.76 TWh, <0.4% of total US electricity generation, and 3.1% of total US renewable generation, ranking fifth in the renewable energy sources of the country . Fig 9, which shows the US geothermal nameplate and net capacity in MW during the past three decades, indicates that it has exhibited steady growth. However, in 2014, its growth was impeded by the following:
Low demand for new electric power generation: The growth rate of the demand for new electric power generation remained relatively low and even declined after 2000, as shown in Fig 10.
- Uncertainty in legislation: In 2014, after the PTC debates, the US Congress extended the PTC, a corporate tax credit for the operation of renewable energy facilities for the first 10 years (e.g. 2.3 ¢ per kilowatt-hour for geothermal [ 28 ]) only until the end of 2014, so it has expired. Only the 10% ITC has been extended to the end of 2016 [ 29 ]. Therefore, the US geothermal industry, waiting for further acts of Congress that will determine the future of renewable energy policy, is reluctant to invest in, secure financing for, or develop new projects.
Fig 10: US geothermal capacity in MW 
Future status of geothermal power generation
As the generation of geothermal power remained relatively constant in 2014, a number of studies have predicted that it will grow more slowly than that of the other renewables for the following reasons:
- Costs: Costs for new technology (e.g. for enhanced geothermal plants) are higher than those for conventional geothermal plants . In addition, the costs of generating electricity from geothermal renewable energy are still higher than those of generating electricity from wind, biomass, and utility-scale solar energy .
- Constraints of location and construction time: Geothermal plants, most located in the western US, have geographical constraints that require not only accessible and sufficiently large deposits of high-temperature groundwater but also accessible transmission lines. Thus, the estimated time necessary for constructing a geothermal plant is longer than that of other renewable energy plants .
As a result of cost, location, and time constraints, studies have predicted that the growth of the US geothermal power generation industry will be slow. For example, one study estimates that US geothermal power plants will produce 69.6 TWh in the reference case in 2040, 1.4% of total electric power generation in 2040 (e.g. 5056 TWh), and 7.7% of total renewable power generation (e.g. 909.1 TWh). Their net cumulative generation capacity will be 9.1 GW, 0.8% of total net generation capacity (e.g. 1170 GW) and 3.3% of total renewable generation capacity (e.g. 275.2 GW) . Another study estimates that in the 80% renewable scenario, the cumulative installed capacity of US geothermal power plants will range from 12 to 25 GW, which corresponds to 2.1–4.2% of overall US renewable electricity generation capacity in 2050 . According to these studies, the US geothermal industry will continue to grow for several decades but more slowly than other renewables such as wind, solar PV, and biomass energy.
Future renewable energy resources in the US
Table 1 summarises current and future capacities of US renewables. In 2014, hydropower led in total energy production, followed by wind, biomass, solar, and geothermal energy. However, because of the geographical constraints of hydropower, it will probably remain relatively constant for the next several decades. With relatively fewer constraints, wind and solar will become the dominant renewable energy sources, especially in a scenario of the high penetration of renewables (e.g. 80%). In fact, they may account for 61.9% (e.g. 170.3 GW) of total renewable generation capacity (e.g. 275.2 GW) in the reference scenario presented in the Annual Energy Outlook in Table 1. Finally, because of the cost, location, and construction time constraints, geothermal generation will probably increase only slightly, occupying fifth place in US future generation by renewables. Note that solar plants with a capacity of <1 MW are excluded in Table 1. Therefore, the solar generation capacity of 10.67 GW presented in Table 1  differs from that of 20 GW in .
|ELECTRIC POWER MONTHLY ||ANNUAL ENERGY OUTLOOK (REFERENCE SCENARIO) ||NREL (80% RENEWABLE SCENARIO) |
|31 MARCH 2015||12 MONTHS ENDING IN 31 MARCH 2015||2040||2050|
|total all fuels||1070.57||4079.13||1170.0||5056.0||1086–1284|
Table 1: Current and future capacities of US renewables
The objective of this article is to review the recent trends in renewable energy sources for electric power generation in the US and anticipate their future potential and success. For this purpose, this article has examined the current activities of renewable energy sources: hydroelectric, wind, biomass, PV, and geothermal energy. This article has determined that wind and solar will be the mainstream renewable energy sources for the next several decades, especially in light of the high penetration of renewables. In addition, because of the geographical and fuel constraints of geothermal power generation, it is expected to increase more slowly than wind and PV generation in the next few decades. Owing to its geographical- and water-dependence, US hydropower generation is expected to remain relatively constant or increase only slightly in the next few decades. Finally, if legislative uncertainty regarding renewables (e.g. PTC and ITC) is removed, the US renewable industry could outpace current predictions.
This article reviewed recent trends and the future potential of only hydroelectric, wind, biomass, PV, and geothermal renewable energy sources. It did not examine combined and heat power systems, microturbines, fuel cells, energy storage systems, or other renewables (e.g. tidal and wave energies). Examining these topics in a future study should provide a more comprehensive and realistic understanding of US renewables and their future.
- Observ'ER: ‘Worldwide electricity production from renewable energy sources’. Technical Report , Paris, France, 2013.
- Global Win Energy Council: ‘Global wind statistics 2014’. Technical Report , Brussels, Belgium, 2015.
- U.S. Energy Information Administration: ‘Monthly energy review April 2015’. Technical Report, DOE/EIA-0035(2015/04), Washington DC, USA, 2015.
- Plazak: (6 November 2013). Electricity generated in the US from renewable sources 1950-2012. Available at https://www.en.wikipedia.org/wiki/Renewable_energy_in_the_United_States#/media/File:USRenewableElectricity.jpg .
- Mai T.Wiser R. Sandor D. T. et al.: ‘Renewable electricity futures study volume 1: exploration of high-penetration renewable electricity futures’. Technical Report, NREL/TP-6A20-52409-1, National Renewable Energy Laboratory, Golden, CO, USA, 2012.
- U.S. Energy Information Administration: ‘Annual energy outlook 2015 with projections to 2040’. Technical Report, DOE/EIA-0383(2015) , Washington DC, USA, April 2015.
- Global Wind Energy Council: ‘Global wind report annual market update 2014’. Technical Report , Brussels, Belgium, 2015.
- American Wind Energy Association: (6 May 2014). WINDPOWER 2014 inaugurates vision of a far larger industry. Available at http://www.awea.org/MediaCenter/pressrelease.aspx?ItemNumber=6436 .
- National Renewable Energy Laboratory: ‘SunShot vision study February 2012’. Technical Report, DOE/GO-102012-3037 , U.S. Department of Energy, 2012.
- Matek B.: ‘ 2015 Annual U.S. & global geothermal power production report’, Geothermal Energy Association, Washington, DC, USA, February 2015.
- U.S. Energy Information Administration: ‘Electric power monthly with data for March 2015’. Technical Report , Washington DC, USA, May 2015.
- Ruddy B. C. Hitt K. J.: ‘Summary of selected characteristics of large reservoirs in the United States and Puerto Rico’. Technical Report, USGS Open-file 90-163 , U.S. Geological Survey, 1990, p. 295.
- Plazak: (26 November 2013). Trends in the top five hydroelectricity-producing countries. Available at http://www.commons.wikimedia.org/wiki/File:Top_5_Hydropower-Producing_Countries.png .
- Wood A. J. Wollenberg B. F. Sheblé G. B.: ‘Power generation, operation, and control’ (John Wiley & Sons, New York, 2013).
- Boyle G.: ‘Renewable energy: power for a sustainable future’ (Oxford University Press, 2012).
- American Wind Energy Association: ‘U.S. wind industry fourth quarter 2015 market report’. Technical Report , Washington DC, USA, 29 April 2015.
- Lazard: ‘Levelized cost of energy analysis – version 8.0’. Technical Report , New Orleans, LA, USA, 18 September 2014.
- Wiser R. Bolinger M.: ‘2013 wind technologies market report’. Technical Report, U.S. Department of Energy, August 2014.
- Augustine C. Bain R. Chapman J. et al.: ‘Renewable electricity futures study volume 2: renewable electricity generation and storage technologies’. Technical Report, NREL/TP-6A20-52409-2 , National Renewable Energy Laboratory, Golden, CO, USA, 2012.
- Joyce M.: (18 March 2014). Biofuels production drives growth in overall biomass energy use over past decade. Available at http://www.eia.gov/todayinenergy/detail.cfm?id=15451.
- Barbose G. Weaver S. Darghouth N.: ‘Tracking the Sun VII’. Technical Report , September Lawrence Berkeley National Laboratory, Berkeley, CA, 2014.
- Kann S. Shiao M. J. Honeyman C. et al.: ‘U.S. solar market insight report 2014 year in review and executive summary’. Technical Report, Solar Energy Industries Association, 2015.
- Begovic M .M. Kim I. Novose lD. et al.: ‘ Integration of photovoltaic distributed generation in the power distribution grid’. Hawaii Int. Conf. System Science, Maui, HI, USA, 4–7 January 2012.
- Solar Energy Industries Association: (1 June 2015). California Solar. Available at http://www.seia.org/state-solar-policy/california.
- California Solar Initiative: (4 May 2015). CSI Multifamily Affordable Solar Housing Program. Available at http://www.cpuc.ca.gov/PUC/energy/Solar/mash.htm.
- Solar Energy Industries Association: (1 June 2015). Solar Investment Tax Credit (ITC). Solar Energy Industries Association. Available at http://www.seia.org/policy/finance-tax/solar-investment-tax-credit .
- Adam Sieminski: ‘U.S. electricity use and GDP percent growth’, Washington DC, USA, 14 April 2015.
- U.S. Department of Energy: (1 June 2015). Renewable Electricity Production Tax Credit (PTC). Available at http://www.energy.gov/savings/renewable-electricity-production-tax-credit-ptc.
- Renewable Energy Insights: ‘Without tax credit extensions, returns or PPAs for renewables will have to give, GAO Says’ (Renewable Energy Insights, 2015).
- U.S. Energy Information Administration: (18 November 2011). U.S. has large geothermal resources, but recent growth is slower than wind or solar. Available at http://www.eia.gov/todayinenergy/detail.cfm?id=3970 .