The Azimuth Project
Energy return on energy invested (Rev #7)



The term EROEI denotes Energy Returned On Energy Invested, a quantity used as part of analysing the viability of an “energy production” scheme. Intuitively, since most “energy production” involves some “energy use” it is simply the ratio of “useful energy acquired” to “useful energy expended”. The difficult and often heated debate arises in how one decides which inputs and outputs count as “useful”.

Is EROEI the same as Energy Yield Ratio? — Graham Jones

Good question: quick googling results below — David Tweed

I also googled, and added another para below - I think the 2 paras should be merged. Also added more redirects — Graham Jones

There is an essentially identical concept Energy Yield Ratio, defined as the ratio of energy produced by a piece of apparatus over its entire operational lifetime to the amount of energy both used to construct the entity and any other energy inputs over its entire lifetime. It appears that EROEI tends to be used for processes where the apparatus has no typical lifetime (thus often allowing neglecting the energy used in construction) and significant ongoing energy inputs, whilst Energy Yield Ratio tends to be used for finite lifetime apparatus (often with no ongoing “useful” inputs, eg, photovoltaic cells).

There is various nomenclature that describes this concept. Energy profit ratio, surplus energy, energy gain, EROI, and ERoEI all represent virtually the same relationship of how much energy we receive, relative to an energy input(dollars do not factor in). (See Oil Drum ref.)


The definition of EROEI for a process of “extracting energy” is the useful acquired energy divided by the useful energy expended. The “useful” tag denotes energy which is usable by human beings now. There are often inputs and outputs which could not be used for other purposes, eg, the use of “energy” by ancient stars generating uranium by nucleosynthesis has already occurred so that it makes no sense to include it in the EROEI inputs. In practice, one uses EROEI even talking about inputs and outputs which aren’t strictly “energies” but “substances from which energy can be extracted”, eg, one could look at the EROEI of growing trees for fuel, where the wood produced is counted as an output according to the energy extractable by burning.

Clearly in general the higher the EROEI value the “better” a process is. In particular, once the EROEI drops below 1 more energy is being used in the extraction process than is being output at the end. Because it only considers energy issues (and not resource scarcity, scalability, pollution, etc) it is only one input into the decision process of selecting technologies and actions.

The key difficulty in using EROEI lies in determining which inputs and outputs should be included in the ratio, particularly since this generally involves considering which other competing processes are genuinely viable.

A further complication arises because, whilst various forms of energy can generally be converted to each other, this will incur loses due to conversion efficiency. Thus, one cannot generally look at two schemes with the same useful energy inputs which produce different kinds of energy (eg, electricity and heat) and declare the one with the higher EROEI as more suitable.


Let’s consider a fictional situation of growing of grass X crop and then fermenting it to produce a liquid fuel to see some of the difficulties in calculating an EROEI. The most obvious inputs and outputs are:

“Energy” outputs

  1. The liquid fuel itself. This is unarguably useful output “energy”.

  2. There may be excess heat produced by the fermentation process. Whether this is useful is debatable since the energy is of high entropy and produced at plants located away from energy consumers.

  3. The remaining biomass may be suitable for burning. Again usefulness is debatable since the biomass may be better used for fertilising the fields used to grow the crop; even if this isn’t the case, the biomass may require yet more energy to collect into a dry, burnable state.

“Energy” inputs

  1. Sunlight. Except for exceptional circumstances, there is no other use for sunlight falling on fields so this does not count as a useful input.

  2. Artificial fertilizer. This requires energy to produce and could be used for growing food or other crops, so it definitely counts as a useful energy input.

  3. Energy used by motorised vehicles, both during farming and transportation to the biomass plant. For the same reasons as fertilizer, this counts as a useful energy input.

  4. Mechanical energy used to extract liquid fuel after fermentation and clear waste products from the apparatus. Again a useful energy input.

Thus one computation of EROEI would count outputs 1 and inputs 2, 3 and 4. However, suppose that the grass crop is genuinely being grown anyway (eg, as part of a crop rotation scheme) and the plant is sufficiently small that the excess heat can be used fully by the plant for staff heating. Then one could argue that the EROEI should count outputs 1 and 2 whilst counting inputs 3 and 4. Thus the determination of the meaningful EROEI depends upon determining which alternative uses are genuinely viable.

Note also how this EROEI calculation is purely about energy and does not reflect issues such as whether the land usage is sustainable, possible soil depletion/erosion, scarcity of mineral inputs for artificial fertilizer, etc.


Discussion – to be deleted

  • Energy for workers. The janitor’s refrigerator uses energy. Actually all the money paid to all the workers will be spent in ways that cause energy to be consumed. If this is included in EROEI calculation then we can see what happens when there is declining EROEI. The EI can be reduced by reducing wages (in real terms). This can only happen if all wages in the community drop, otherwise the energy business can’t hire. So this is the last resort way of reducing the EI and improving the EROEI.
  • Oil is an energy carrier. In the proposal for a Hydrogen Economy the Hydrogen is just an energy carrier. Similarly oil and its distillates are energy carriers, particularly useful for the transport industry. Recently oil’s price has decoupled from other energy sources (electricity and natural gas). The extra price represents the value of oil as an energy carrier. So it is difficult to use EROEI calculations directly on oil. Ultimately we will still be producing a lot of oil when the EROEI is less than 1, using up non-oil energy in the process.
  • Energy’s reign. It is possible to view the production of goods and services as requiring energy and (skill-weighted) workers. When we spend money, then the services we get use up energy and worker time. The recipients of the money spend it and use up more energy and worker time. Ultimately the money’s circulation uses up some of both. The two have to come into balance. The industrial revolution meant that energy was plentiful and skilled workers were scarce. So energy prices were driven to the floor and wages rose creating the middle class. If energy is in short supply then energy prices will get off the floor and wages will be driven down to restore balance. All of which amounts to an argument that cost/price is the best measure of total energy in or out at any point in time. EROEI calculations that exclude pay and dividends are the way to look at the crucial limit case. There is a need to assess this argument against the more common one, which says that energy is a small part of our current costs, and so it doesn’t matter much if we replace carbon-emitting energy with more expensive energy sources.