“The EROEI of Photo-Voltaics can be whatever you like these days” a friend of mine said once after attending a bio-physical economics conference. It epitomises a growing problem in this field, while the broad concept of net energy or Energy Return on Energy Invested (EROEI) is broadly accepted there is no unified methodology for its calculation. Different researchers apply different methods producing markedly different results.

Photo-Voltaics (PV) is an energy source where this problem has been more acute. Equipment and installation prices collapsed four or five fold since 2010, but published EROEI studies have not converged; in fact it appears they dispersed even further.

Fortunately, another friend, Rembrandt Koppelaar, has recently finished a study assessing these assorted EROEI estimates, entitled Solar-PV energy payback and net energy: Meta-assessment of study quality, reproducibility, and results harmonization. As it turns out, low EROEI figures are largely an artefact of outdated data and double accounting. A message by Rembrandt summarising the results of his study is reproduced below.

  • I looked at 29 existing studies, and attempted to reproduce all of their results (I managed to do so on the information provided in about 50% of the studies), after this I harmonized the data by making the boundaries equal.
  • The Boundaries I took for the harmonization include from quartz mining to end-of-life, including inverters and other components, disposal, transport. Excluded are grid connections and balancing, and excluded are labour wages (not an energy input) and financial capital values (as this is double-counting the material costs) (including embodied). The solar radiation value taken for the harmonization is 1700 kWh/m2/year

I also looked at several study methodology aspects, of which the most important are 1) The old age of the used data (Science is lagging on average 7 years behind reality). 2) The impact of (not incorporating) technological change.

The study also includes a sensitivity analysis that looks at the impacts of different solar radiation in kWh/m2/year and its effects on Net Energy.

The key conclusions:

  1. The study harmonization yielded a mean Energy payback time for mono- and polysilicon solar-PV of 3.9 and 2.9 years, and a mean Net Energy Ratio (EROI) of 8.6 and 9.2 times, as expressed in solar energy output gain per unit of energy input, respectively.
  2. The average time between study publication and sourced data was established at 7 years within a 2–18 year range, due to which energy input costs are typically overestimated as recent technological improvements are not captured. When filtering for studies with manufacturing data collected after 2008, the harmonized average EPT for mono- and polysilicon was found to be approximately half (e.g. 2.0 instead of 3.9) and NER (EROI) double (e.g. 14.4 instead of 7).
  3. An input correction with recent technological improvements for all studies resulted for mono- and polysilicon solar-PV in an adjusted mean harmonized EPT of 3.5 and 2.4 years and NER (EROI) of 9.7 and 11.4 times, respectively.

EROEI studies are yet to fully capture the collapse in PV technology costs of recent years. But even then, with the methodological harmonisation introduced by Rembrandt this energy technology can be easily classified as fully mature. The fact that it provides electricity in various parts of Europe for prices lower than fossil fuels is not merely conjectural.

Rembrandt is now working on a follow up study where he will investigate the impact of intermittency on these EROEI estimates. This aspect was seldom considered in previous studies and shall be tackled from a systems perspective using simulation software.

An earlier version of this study is available from Rembrandt’s website.