The main persistent conceptual issues in the EROI literature are: how to define the boundary of analysis (as shown in Figure 1 ), how to account for embodied energy inputs (i.e., all the energy that went into a process; this is different from embedded energy, which relates to the energy content of specific materials or infrastructures), how to deal with temporality and how to account for energy quality. These issues are still being identified in recent EROI publications [ 17 34 ], but are largely the same as those that Leach [ 20 ] identified and were discussed in a NEA workshop held in August 1975 at Stanford, California. We will discuss each of them in turn, providing our own conceptual choices for this specific definition ofand an explanation of the reasoning behind our choices (which were sometimes conceptual and sometimes practical). However, our choices are not necessarily intended to point towards final solutions to these methodological issues, but rather should be seen as contributing to the discussion of defining EROI at a national level.

2.3.1. Boundary of Analysis

There is a consensus around the accounting starting point for EROI in general, regardless of the type. EROI “assumes that the energy in the ground (or coming from the sun) is not to be counted as an input” [ 35 ]. Therefore, EROI accounts for energy inputs once they have been either extracted or harnessed for human purposes, but not the energy content of the resource that is being extracted/harnessed (note that this start point of accounting for energy contrasts with the approach of another assessment tool: Life Cycle Analysis—LCA. In LCA the energy that is present in the environment or the energy source is the start point for accounting in measures of, for instance, cumulative energy demand).

However, there are three main considerations when assessing boundaries for EROI. Firstly, how many energy processing and transformation stages to take into account: primary energy, final consumption (of energy carriers) or useful energy. Primary energy generally refers to the energy extracted or captured from the natural environment (e.g., crude oil, coal, hydropower, etc.) [ 36 ]. Final energy (also called secondary energy) generally refers to energy as it is delivered to the final economic consumer, after undergoing transportation and transformation processes (e.g., gasoline, diesel, electricity, etc.) [ 36 ]. At the point of use, final energy undergoes one last transformation process as it passes through an end-use conversion device, for example furnaces, electric appliances or light bulbs. End-use devices transform energy into a form that is useful for human purposes, hence the term “useful energy” as the outcome of this last conversion process. Secondly, a decision is required as to the inclusion of energy inputs at each of the energy stages under analysis, i.e., should these inputs include embodied energy in capital equipment, operation and maintenance energy, energy consumed by the labour force, etc.? Thirdly, a consideration is required as to the range of energy sources that will be analysed, the geographical limits to be applied and the time frame to be considered.

EROI nat establishes this boundary at the first stage of extraction/capture of energy sources. We have chosen this stage for practical reasons, as it provides a well-defined starting point for a novel methodology that can be further built upon. In terms of most energy reporting (e.g., International Energy Agency—IEA—Energy Balances), this means energy “production”. Energy “production” does not include energy imports but it does include energy exports. In other words, we are assessing the energy extracted/captured in a country (energy returned), regardless of whether or not is then exported and without accounting for energy imports (see In relation to the first consideration, how far to go along the energy chain in order to include more processing and transformation stages depends on the type of EROI (see Figure 1 ). Our definition ofestablishes this boundary at the first stage of extraction/capture of energy sources. We have chosen this stage for practical reasons, as it provides a well-defined starting point for a novel methodology that can be further built upon. In terms of most energy reporting (e.g., International Energy Agency—IEA—Energy Balances), this means energy “production”. Energy “production” does not include energy imports but it does include energy exports. In other words, we are assessing the energy extracted/captured in a country (energy returned), regardless of whether or not is then exported and without accounting for energy imports (see Figure 2 ). This means that a country that imports all of its primary energy will not have an EROI value when using this methodology.

In relation to the second consideration, on the extent of energy inputs included at each energy processing and transformation stage, it depends on the specific EROI study. Most EROI studies include the direct energy and material (as embodied energy) inputs as well as the indirect energy and material inputs, i.e., the inputs required to make the initial inputs. We have decided to adopt this commonly used boundary in the calculation of EROI nat in order to make our results comparable to other results found in the literature.

EROI ext for US oil using an expanded boundary for the inputs. However, we consider these expansions to be an area suitable for further research, as an Input-Output framework is ideally suited to overcoming a key hurdle in national-level EROI analysis: allocating indirect energy use from different stages of the supply chain to the energy producing sectors. Brandt et al. [ 37 ] have developed a framework for tracking direct energy inputs as well as different number of indirect energy inputs. Further expansion of the boundary that determines the energy inputs can be made. For example, indirect labour consumption can be included, as well as the consumption of auxiliary services and the environmental impacts of the production of direct and indirect energy and materials. Hall et al. [ 38 ] calculatefor US oil using an expanded boundary for the inputs. However, we consider these expansions to be an area suitable for further research, as an Input-Output framework is ideally suited to overcoming a key hurdle in national-level EROI analysis: allocating indirect energy use from different stages of the supply chain to the energy producing sectors.