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Entry · Water

EAR: Stored energy

How much energy can the water stored in the reservoirs generate?

● checked against the primary source on Sep 28, 2026Source: ONS

Acronyms: REE, Equivalent Energy Reservoir; ONS, National Electric System Operator; SIN, National Interconnected System.

In one sentence

Energy associated with the volume of water in the reservoirs that can be converted into generation at the plant itself and at all plants downstream in the cascade.

Real example

SIN EAR

61.5%Portuguese onlyda EAR máximaNature of the data: Calculated

Sep 29, 2026 · National Interconnected System

Source: ONS, EAR Diário por Subsistema. Reference up to Sep 29, 2026. Captured on Sep 30, 2026.

How much of the storage capacity of the SIN reservoirs was occupied on the last published day: sum of the EAR of the subsystems divided by the sum of the maximum EAR.

For Sep 29, the median of the complete years since 2001 (2001 to 2025, 25 years) is 49.1% of the maximum EAR; the 10th to 90th percentile range goes from 25.3% to 67.2%, and the value of this day is within this range. The Overview shows 61.6% on Sep 28, 2026, 1 day earlier: it reads the hydrology database, whose capture of the ONS file is older, and this example uses a more recent capture, which already includes Sep 29 and gives 61.7% for Sep 28 itself, because the ONS revises published values.

See in the panel: Water and climate: reservoirs

Calculated: Deterministic transformation made by Scrutiniums, with a published formula.

Why it matters

It summarizes in one quantity, by subsystem, the energy that the water stored in the reservoirs can generate at the plant itself and at the downstream plants. As a percentage of the maximum EAR, it indicates how much of the storage capacity is occupied.

How it is measured

In MWmonth and as a percentage of the maximum EAR (the capacity if all reservoirs were full), by day and by subsystem, basin, REE or reservoir.

Unit: MWmonth (the energy the reservoirs would produce in one month at an average power of 1 MW) or percentage of the maximum EAR

Not to be confused with

  • ENA (Natural inflow energy)

    EAR is the energy associated with the water already stored in the reservoirs; ENA is the energy that the arriving natural flows would allow to be produced. An ENA above the MLT does not say, by itself, how much is stored.

  • Storage capacity (maximum EAR)

    EAR is how much is stored; the maximum EAR is how much would fit with all reservoirs full. The EAR percentage is the ratio between the two, and the denominator changes when reservoirs come in.

  • REE (Equivalent Energy Reservoir)

    The REE is one of the cuts in which the ONS publishes the EAR, alongside the subsystem, the basin and the reservoir; each cut has its own perimeter and its own maximum EAR.

What cannot be concluded

  • Percentage of the maximum EAR: the capacity changes with the entry of reservoirs, which affects long comparisons.
  • The ONS states that the data undergo recurring consistency checks and might be revised.

Related entries

In the learning paths

See in the panel

Official source

  • ONSOpen data portal, dataset “EAR Diário por Subsistema” (description and data dictionary) 

    Literal excerpt from the document
    Portuguese only“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. A grandeza de EAR leva em conta nível verificado nos reservatórios na data de referência. A grandeza de EAR máxima representa a capacidade de armazenamento caso todos os reservatórios do sistema estivessem cheios.”

Documents accessed and text checked on Sep 28, 2026.