To get EROI figures that I thought were representative, I reviewed as many studies as I could find on each energy source, going over at least several dozen studies. Then I chose a recent EROI estimate for each energy source that appeared typical or average, and appeared to be calculated in a way roughly comparable with the estimates for other energy sources. Here are the studies from which I chose to draw for the EROI values:
- Conventional oil: M. C. Guilford et al., “A New Long Term Assessment of Energy Return on Investment (EROI) for U.S. Oil and Gas Discovery and Production,” in Sustainability (2011) (link).
- Ethanol from sugarcane: I. C. Macedo et al., “Green house gases emissions in the production and use of ethanol from sugarcane in Brazil: The 2005/2006 averages and a prediction for 2020,” Biomass and Bioenergy (2008) (link).
- Biodiesel from soybeans: A. Pradhan et al.: “Energy Life-Cycle Assessment of Soybean Biodiesel Revisited,” Transactions of the American Society of Agricultural and Biological Engineers (2011) (pdf).
- Tar sands: There are no peer-reviewed, published estimates of the EROI for tar sands, as far as I could tell. So I drew on an unpublished paper sent by Adam Brandt of Stanford University (Adam Brandt et al., “The energy efficiency of oil sands extraction: Energy return ratios from 1970 to 2010,” under review by Energy). The paper reports various types of EROIs for tar sands, and I used the number for the external energy ratio for refined fuels.
- Heavy oil from California: Adam Brandt, “Oil Depletion and the Energy Efficiency of Oil Production: The Case of California,” Sustainability (2011)(link). Note that this is for California only; there is no accepted definition of “heavy oil,” and the value may be different for other countries (such as Venezuela), which may use different technologies for extraction and refining.
- Corn ethanol: There have been heated arguments over what the EROI is for corn ethanol, such as a 2006 exchange in Science. But all seem to agree that its EROI is less than 2—which puts it at the bottom of the heap for liquid fuels. I drew on a meta-analysis that averaged six different estimates, giving an EROI of 1.4. Hammerschlag, “Ethanol’s Energy Return on Investment: A Survey of the Literature 1990–Present,” Environmental Science & Technology (2006) (link).
For sources of electricity, I used EROI values that are for electricity produced by a particular source, rather than the EROI for the production of the raw fuel that can be used to make electricity. So in the case of coal, for example, the EROI for the coal itself would be roughly three times higher than the EROI for electricity from coal (because the typical efficiency of a coal-fired power plant is around 33 percent).
- Hydroelectric: There are a wide range of EROI values reported for hydroelectric dams, from around 40 to more than 250. To reflect this range, I reported the value as “40+”. See, for example, Gagnon et al., “Life-cycle assessment of electricity generation options: The status of research in year 2001,” Energy Policy (2002) (link).
- Wind: I used a meta-analysis of 50 studies, including 119 different wind farms or turbines. Kubiszewski et al., “Meta-analysis of net energy return for wind power systems,” Renewable Energy (2010) (link).
- Coal: Most studies on the EROI of coal report the value at the “minemouth,” for all the energy content in the coal. To make it comparable with other electricity sources, especially renewables, I used the EROI for electricity from coal. I drew on one particular study that had as its main focus solar power, but which compared it with fossil fuels: Raugei et al.,“The energy return on energy investment (EROI) of photovoltaics,” Energy Policy (2012) (link). The EROI figure there was consistent with what you would get from a back-of-the-envelope calculation, dividing the minemouth EROI for coal by three, to account for the losses of energy in a power plant (personal communication, Charles Hall of S.U.N.Y. Environmental Science and Forestry).
- Solar (PV): There are a wide variety of estimates of solar PV's EROI as well—in part because the technologies and production techniques are improving fast, a major reason for the large price reductions over the past decade. I used the most recent peer-reviewed study I could find (Raugei et al., 2012, cited above). Solar PV's EROI is almost certainly rising (Raugei et al., 2012; personal communication, Michael Dale of Stanford University). The latest data in Raugei's study was at least a couple of years old, so the EROI today is most likely higher than 6, the number cited in my article.
- Natural gas: It was difficult to find an EROI estimate for natural gas because data for natural gas is typically reported along with that of oil. For the EROI figure of 7, I used an alternative measure devised by Carey King of the University of Texas at Austin that he calls the “energy intensity ratio,” and which is comparable with the EROI. King's value for the energy intensity ratio of electricity from natural gas is also consistent with what a back-of-the-envelope calculation would give, using an EROI of oil and natural gas of 20 at the wellhead, and adjusted to take into account the typical efficiency of a natural gas power plant (around 40 percent to 45 percent). King, “Energy intensity ratios as net energy measures of United States energy production and expenditures,” Environmental Research Letters (2010) (link).
- Nuclear: As with hydroelectricity, the EROI estimates for nuclear power span a very large range. Some claim that the EROI is actually less than 1—which would mean that the whole process is not a source of energy, but rather a sink—whereas others (such as the World Nuclear Association, an industry group) estimate that the EROI is much higher than perhaps any other source of energy, around 40 to 60 when using centrifuge enrichment. I drew on a paper that reviewed many studies, and estimated the EROI to be 5. Lenzen, “Life cycle energy and greenhouse gas emissions of nuclear energy: A review,” Energy Conversion and Management (2008) (link).