Emergy
Emergy is the amount of energy consumed in direct and indirect transformations to make a product or service. Emergy is a measure of quality differences between different forms of energy. Emergy is an expression of all the energy used in the work processes that generate a product or service in units of one type of energy. Emergy is measured in units of emjoules, a unit referring to the available energy consumed in transformations. Emergy accounts for different forms of energy and resources (e.g.
Emergy is the amount of energy consumed in direct and indirect transformations to make a product or service. Emergy is a measure of quality differences between different forms of energy. Emergy is an expression of all the energy used in the work processes that generate a product or service in units of one type of energy. Emergy is measured in units of emjoules, a unit referring to the available energy consumed in transformations. Emergy accounts for different forms of energy and resources (e.g. sunlight, water, fossil fuels, minerals, etc.) Each form is generated by transformation processes in nature and each has a different ability to support work in natural and in human systems. The recognition of these quality differences is a key concept.

History
The theoretical and conceptual basis for the emergy methodology is grounded in thermodynamics, general system theory and systems ecology. Evolution of the theory by Howard T. Odum over the first thirty years is reviewed in Environmental Accounting and in the volume edited by C. A. S. Hall titled Maximum Power.

Background
Beginning in the 1950s, Odum analyzed energy flow in ecosystems (e.g. Silver Springs, Florida; Enewetak atoll in the south Pacific; Galveston Bay, Texas and Puerto Rican rainforests, amongst others) where energies in various forms at various scales were observed. His analysis of energy flow in ecosystems, and the differences in the potential energy of sunlight, fresh water currents, wind and ocean currents led him to make the suggestion that when two or more different energy sources drive a system, they cannot be added without first converting them to a common measure that accounts for their differences in energy quality. This led him to introduce the concept of "energy of one kind" as a common denominator with the name "energy cost". He then expanded the analysis to model food production in the 1960s, and in the 1970s to fossil fuels.Odum's first formal statement of what would later be termed emergy was in 1973:

Energy is measured by calories, btu's, kilowatthours, and other intraconvertable units, but energy has a scale of quality which is not indicated by these measures.  The ability to do work for man depends on the energy quality and quantity and this is measurable by the amount of energy of a lower quality grade required to develop the higher grade. The scale of energy goes from dilute sunlight up to plant matter, to coal, from coal to oil, to electricity and up to the high quality efforts of computer and human information processing.
In 1975, he introduced a table of "Energy Quality Factors", kilocalories of sunlight energy required to make a kilocalorie of a higher quality energy, the first mention of the energy hierarchy principle which states that "energy quality is measured by the energy used in the transformations" from one type of energy to the next.
These energy quality factors, were placed on a fossil-fuel basis and called "Fossil Fuel Work Equivalents" (FFWE), and the quality of energies were measured based on a fossil fuel standard with rough equivalents of 1 kilocalorie of fossil fuel equal to 2000 kilocalories of sunlight. "Energy quality ratios" were computed by evaluating the quantity of energy in a transformation process to make a new form and were then used to convert different forms of energy to a common form, in this case fossil fuel equivalents. FFWE's were replaced with coal equivalents (CE) and by 1977, the system of evaluating quality was placed on a solar basis and termed solar equivalents (SE).

Embodied energy
The term "embodied energy" was used for a time in the early 1980s to refer to energy quality differences in terms of their costs of generation, and a ratio called a "quality factor" for the calories (or joules) of one kind of energy required to make those of another. However, since the term embodied energy was used by other groups who were evaluating the fossil fuel energy required to generate products and were not including all energies or using the concept to imply quality, embodied energy was dropped in favor of "embodied solar calories", and the quality factors became known as "transformation ratios".

Introduction of the term "emergy"
Use of the term "embodied energy" for this concept was modified in 1986 when David Scienceman, a visiting scholar at the University of Florida from Australia, suggested the term "emergy" and "emjoule" or "emcalorie" as the unit of measure to distinguish emergy units from units of available energy. The term transformation ratio was shortened to transformity in about the same time. It is important to note that throughout these twenty years, the baseline or the basis for evaluating forms of energy and resources shifted from organic matter to fossil fuels and finally to solar energy.
After 1986, the emergy methodology continued to develop as the community of scientists expanded and as new applied research into combined systems of humans and nature presented new conceptual and theoretical questions. The maturing of the emergy methodology resulted in more rigorous definitions of terms and nomenclature and refinement of the methods of calculating transformities. The International Society for the Advancement of Emergy Research Archived 2016-05-13 at the Wayback Machine and a biennial International Conference at the University of Florida support this research.

Chronology
Definitions and examples
Emergy— amount of energy of one form that is used in transformations directly and indirectly to make a product or service. The unit of emergy is the emjoule or emergy joule. Using emergy, sunlight, fuel, electricity, and human service can be put on a common basis by expressing each of them in the emjoules of solar energy that is required to produce them. If solar emergy is the baseline, then the results are solar emjoules (abbreviated seJ). Although other baselines have been used, such as coal emjoules or electrical emjoules, in most cases emergy data are given in solar emjoules.
Unit Emergy Values (UEVs) — the emergy required to generate one unit of output. Types of UEVs:

Transformity — emergy input per unit of available energy output. For example, if 10,000 solar emjoules are required to generate a joule of wood, then the solar transformity of that wood is 10,000 solar emjoules per joule (abbreviated seJ/J). The solar transformity of the sunlight absorbed by the earth is 1.0 by definition.Specific emergy — emergy per unit mass output. Specific emergy is usually expressed as solar emergy per gram (seJ/g). Because energy is required to concentrate materials, the unit emergy value of any substance increases with concentration. Elements and compounds not abundant in nature therefore have higher emergy/mass ratios when found in concentrated form since more environmental work is required to concentrate them, both spatially and chemically.Emergy per unit money — the emergy supporting the generation of one unit of economic product (expressed in monetary terms). It is used to convert money into emergy units. Since money is paid for goods and services, but not to the environment, the contribution to a process represented by monetary payments is the emergy that money purchases. The amount of resources that money buys depends on the amount of emergy supporting the economy and the amount of money circulating. An average emergy/money ratio in solar emjoules/$ can be calculated by dividing the total emergy use of a state or nation by its gross economic product. It varies by country and has been shown to decrease each year, which is one index of inflation. This emergy/money ratio is useful for evaluating service inputs given in money units where an average wage rate is appropriate.Emergy per unit labor — the emergy supporting one unit of direct labor applied to a process. Workers apply their efforts to a process and in so doing they indirectly invest in it the emergy that made their labor possible (food, training, transport, etc). This emergy intensity is generally expressed as emergy per time (seJ/yr; seJ/hr), but emergy per money earned (seJ/$) is also used. Indirect labor required to make and supply the inputs to a process is generally measured with the dollar cost of services, so that its emergy intensity is calculated as seJ/$.Empower — a flow of emergy (i.e., emergy per unit time).

Accounting method
Emergy accounting converts the thermodynamic basis of all forms of energy, resources and human services into equivalents of a single form of energy, usually solar. To evaluate a system, a system diagram organizes the evaluation and account for energy inputs and outflows. A table of the flows of resources, labor and energy is constructed from the diagram and all flows are evaluated. The final step involves interpreting the results.

Purpose
In some cases, an evaluation is done to determine the fit of a development proposal within its environment. It also allows comparison of alternatives. Another purpose is to seek the best use of resources to maximize economic vitality.

Systems diagram
System diagrams show the inputs that are evaluated and summed to obtain the emergy of a flow. A diagram of a city and its regional support area is shown in Figure 1.

Evaluation table
A table (see example below) of resource flows, labor and energy is constructed from the diagram. Raw data on inflows that cross the boundary are converted into emergy units, and then summed to obtain total emergy supporting the system. Energy flows per unit time (usually per year) are presented in the table as separate line items.

LegendColumn #1 is the line item number, which is also the number of the footnote found below the table where raw data sources are cited and calculations are shown.
Column # 2 is the name of the item, which is also shown on the aggregated diagram.
Column # 3 is the raw data in joules, grams, dollars or other units.
Column # 4 shows the units for each raw data item.
Column # 5 is the unit emergy value, expressed in solar emergy joules per unit. Sometimes, inputs are expressed in grams, hours, or dollars, therefore an appropriate UEV is used (sej/hr; sej/g; sej/$).
Column # 6 is the solar emergy of a given flow, calculated as the raw input times the UEV (Column 3 times Column 5).All tables are followed by footnotes that show citations for data and calculations.

Calculating unit values
The table allows a unit emergy value to be calculated. The final, output row (row “O” in the example table above) is evaluated first in units of energy or mass. Then the input emergy is summed and the unit emergy value is calculated by dividing the emergy by the units of the output.

Performance indicators
Figure 2 shows non-renewable environmental contributions (N) as an emergy storage of materials, renewable environmental inputs (R), and inputs from the economy as purchased (F) goods and services. Purchased inputs are needed for the process to take place and include human service and purchased non-renewable energy and material brought in from elsewhere (fuels, minerals, electricity, machinery, fertilizer, etc.). Several ratios, or indices are given in Figure 2 that assess the global performance of a process.

Emergy Yield Ratio (EYR) — Emergy released (used up) per unit invested. The ratio is a measure of how much an investment enables a process to exploit local resources.
Environmental Loading Ratio (ELR) — The ratio of nonrenewable and imported emergy use to renewable emergy use. It is an indicator of the pressure a transformation process exerts on the environment and can be considered a measure of ecosystem stress due to a production (transformation activity).
Emergy Sustainability Index (ESI) — The ratio of EYR to ELR. It measures the contribution of a resource or process to the economy per unit of environmental loading.
Areal Empower Intensity — The ratio of emergy use in the economy of a region to its area. Renewable and nonrenewable emergy density are calculated separately by dividing the total renewable emergy by area and the total nonrenewable emergy by area, respectively.Other ratios are useful depending on the type and scale of the system under evaluation.

Percent Renewable Emergy (%Ren) — The ratio of renewable emergy to total emergy use. In the long run, only processes with high %Ren are sustainable.
Emprice. The emprice of a commodity is the emergy one receives for the money spent in sej/$.
Emergy Exchange Ratio (EER) — The ratio of emergy exchanged in a trade or purchase (what is received to what is given). The ratio is always expressed relative to a trading partner and is a measure of the relative trade advantage of one partner over the other.
Emergy per capita — The ratio of emergy use of a region or nation to the population. Emergy per capita can be used as a measure of potential, average standard of living of the population.
Emergy-based energy return on investment was introduced as a way to bridge and improve the concept of Energy returned on energy invested to also include environmental impacts.

Uses
The recognition of the relevance of energy to the growth and dynamics of complex systems has resulted in increased emphasis on  environmental evaluation methods that can account for and interpret the effects of matter and energy flows at all scales in systems of humanity and nature. The following table lists some general areas in which the emergy methodology has been employed.

Controversies
The concept of emergy has been controversial within academe including ecology, thermodynamics and economy. Emergy theory has been criticized for allegedly offering an energy theory of value to replace other theories of value. The stated goal of emergy evaluations is to provide an "ecocentric" valuation of systems, processes. Thus it does not purport to replace economic values but to provide additional information, from a different point of view.The idea that a calorie of sunlight is not equivalent to a calorie of fossil fuel or electricity strikes many as absurd, based on the 1st Law definition of energy units as measures of heat (i.e. Joule's mechanical equivalent of heat). Others have rejected the concept as impractical since from their perspective it is impossible to objectively quantify the amount of sunlight that is required to produce a quantity of oil. In combining systems of humanity and nature and evaluating environmental input to economies, mainstream economists criticize the emergy methodology for disregarding market values.

See also
Notes
References for Table 4

External links
Emergy Systems - University of Florida where publications, systems symbols and diagrams, templates, powerpoint lectures, etc. can be downloaded
Paper by H.T. Odum describing emergy (1998)
Environment, Power, and Society for the Twenty-First Century: The Hierarchy of Energy
Hall Maximum Power - The Ideas and Applications of H.T. Odum. University Press of Colorado, Niwot, 454 pp, C. A. S., ed., 1995
Odum H.T. and E.C. Odum, 2001 - A Prosperous Way Down: Principles and Policies. University Press of Colorado]
Marvuglia, Benetto, Rios, Rugani, 2013 - SCALE: Software for CALculating Emergy Based on Life Cycle Inventories