Entry · Water
REE: Equivalent Energy Reservoir
How do planning models represent hydropower plants?
Acronyms: ONS, National Electric System Operator; CCEE, Electric Energy Trading Chamber; MME, Ministry of Mines and Energy; ANEEL, Brazilian Electricity Regulatory Agency; EPE, Energy Research Office; PLD, Settlement Price for Differences (spot price).
In one sentence
In plain words
In the official operation planning models, hydropower plants do not appear one by one: those of each group of river basins are grouped into a single equivalent energy reservoir. The topology the EPE describes has 12 REE, in use since the Monthly Operation Program of January 2018.
Source text
Aggregated representation of hydropower plants in the official operation planning models, organized according to the river basins where the plants are located.
Real example
On Sep 29, 2026, the ONS published the stored energy of 12 REENature of the data: Calculated. Among the 11 that have storage (REE Itaipu has no storage, and therefore no percentage), the EAR ranged from 26.1% of the maximum EAR in REE Madeira to 86.2% in REE SulNature of the data: Observed: each REE has its own perimeter and its own maximum EAR. The link opens the panel on the REE with the lowest EAR.
No Check this number record of its own in this example: the number comes from the panel's published database, which carries the source and the reference date.
Calculated: Deterministic transformation made by Scrutiniums, with a published formula. Observed: Value published directly by the official or primary source.
Why it matters
How it is measured
It is not a quantity: it is the unit in which the ONS publishes the daily EAR and ENA. EAR is the energy associated with the volume of water available in the reservoirs that can be converted into generation at the plant itself and at all plants downstream in the cascade. In the observatory, EAR by REE appears in MWmonth and as a percentage of the maximum EAR, each REE with its own perimeter and its own maximum EAR.
Not to be confused with
EAR is the energy associated with stored water; the REE is one of the cuts in which the ONS publishes it, alongside the subsystem, the basin and the reservoir. Each cut has its own perimeter and its own maximum EAR.
Subsystem
The topology described by the EPE has 4 interconnected subsystems and 12 REE: they are different cuts, and the REE aggregates hydropower plants according to the river basin.
What cannot be concluded
- None of the documents consulted gives a formal definition of REE: the description above is the EPE's, and the composition of each of the 12 REE in plants is not in this entry. Submodules 2.4, 4.3 and 4.5 of the Grid Procedures, read for the PLD module, do not contain the term; the other submodules and the ONS glossary were not read, and the data dictionary of the “EAR Diário por REE” dataset uses the term without defining it.
- The GT report describes the increase from 4 to 12 REE without dating it. In the ONS files, the panel detects the reconfiguration of Dec 29 and 30, 2017, from 9 to 12 REE, which changed the perimeter of some of them, and the EPE records topology G, with 12 REE, in use since the Monthly Operation Program of January 2018. Series by REE before this change are not comparable on the same perimeter.
Related entries
- EARStored energyThe ONS publishes stored energy by REE, each with its own perimeter and its own maximum EAR.
- ENANatural inflow energyThe ONS publishes natural inflow energy by REE.
- NEWAVEThe EPE records the 12-REE topology together with version 24 of the NEWAVE model, from the PMO of January 2018.
In the learning paths
See in the panel
Official source
In other words: in the official operation planning models, hydropower plants do not appear one by one, but grouped into equivalent energy reservoirs, according to the river basin where they are located.
Literal excerpt from the document
Portuguese only“devido à relevância das UHEs em termos de capacidade instalada total do SIN e também sua representação agregada nos modelos oficiais de planejamento da operação, isto é, através dos reservatórios equivalentes de energia de acordo com as bacias hidrográficas onde estas usinas hidrelétricas estão localizadas.”
EPEEPE Technical Note EPE/DEE/099/2025 (December 2025), model assumptions, page 11
In other words: the topology used has 4 subsystems and, within it, 12 REE, the so-called topology G, in use since the PMO of January 2018, with version 24 of NEWAVE.
Literal excerpt from the document
Portuguese only“4 subsistemas interligados [...] Topologia de Reservatórios Equivalentes de Energia (REE): topologia G (12 REEs), considerada a partir do PMO de 01/2018, juntamente com a versão 24 do modelo NEWAVE, conforme Despacho nº 4.166, de 11 de dezembro de 2017”
MME, Power Sector Modernization working group (GT, coordinated by the CCEE)Report of the Thematic Group on Price Formation Mechanisms, July 2019, section 5, improvement proposals, p. 38 of the document (p. 41 of the PDF)
In other words: the number of equivalent reservoirs in the models rose from 4 to 12.
Literal excerpt from the document
Portuguese only“o aumento do número de Reservatórios Equivalentes em Energia (REEs), passando de 4 REEs para 12 REEs”
MME, Power Sector Modernization working group (GT, coordinated by the CCEE)Report of the Thematic Group on Price Formation Mechanisms, July 2019, section 2.2.1, improvements under development in the current models, p. 9 of the document (p. 12 of the PDF)
In other words: among the model improvements under development in 2019, the report lists relating the inflows of the different REE month by month, instead of an annual relation, which would better represent the difference in behavior between the basins.
Literal excerpt from the document
Portuguese only“A utilização de correlação espacial de ENAs, nos modelos NEWAVE e GEVAZP, entre os reservatórios equivalentes de energia (REE), em base mensal, em substituição à anual. Essa representação é mais aderente à realidade, capturando a diversidade hidrológica entre as diferentes bacias.”
ONSOpen data portal, dataset “EAR Diário por REE - Reservatório Equivalente de Energia” (description)
Literal excerpt from the document
Portuguese only“Dados das grandezas de energia armazenada (EAR) em periodicidade diária por Reservatórios Equivalentes. A Energia Armazenada (EAR) representa a energia associada ao volume de água disponível nos reservatórios que pode ser convertido em geração na própria usina e em todas as usinas à jusante na cascata.”
ONSOpen data portal, dataset “ENA Diário por REE - Reservatório Equivalente de Energia” (description)
Literal excerpt from the document
Portuguese only“Dados das grandezas de energia natural afluente (ENA) dos reservatórios com periodicidade diária por Reservatórios Equivalentes.”
Documents accessed and text checked on Oct 7, 2026.