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Water, operation and price

How does the water that reaches the reservoirs show up in the short-term price?

6 steps · 18 entries · body, dataset and date of each number in the step's record

Acronyms: ENA, Natural Inflow Energy; EAR, Stored Energy; MWmed, average megawatt; CMO, Marginal Operating Cost; PLD, Settlement Price for Differences (spot price).

Short answer: water reaches the price in stages that the observatory shows in six steps. Inflow and storage feed the models with which the ONS plans operation. These models estimate the CMO. The CCEE starts from the CMO to calculate the PLD, and it is at the PLD that the market values the differences between contracted and verified energy. Between water and price there are operating decisions and market rules: no step alone determines the next one.

  1. Step 1 of 6

    The water that arrives

    ENA is the energy that the natural flows reaching the reservoirs would allow to be produced, calculated by the ONS with the productivities of the plants at 65% of the useful volumes. The ONS also publishes it as a percentage of the MLT, the long-term average: 100% is the value of the MLT itself, and above 100% the inflow exceeded the average.

    The number in the observatory

    30-day gross ENA of the SIN

    172.3% of the MLTNature of the data: Calculated

    Aug 31, 2026 to Sep 29, 2026 · National Interconnected System

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

    Gross ENA of the SIN over the last 30 days, as the sum of ENA over the sum of the MLT of the four subsystems.

    See in the panel: Water and climate: inflow

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

    Physical flowWhat stays in the reservoirs is measured by the EAR, the energy associated with the volume of stored water.

    Physical flow: energy, water or fuel passes from one link to the other.

  2. Step 2 of 6

    The stored water

    EAR is the energy associated with the volume of water in the reservoirs that can become generation at the plant itself and at those downstream. As a percentage of the maximum EAR, it says how much of the possible storage is occupied. The ONS also publishes it by equivalent energy reservoir (REE), the grouping of hydropower plants by basin that the planning models use, a different cut from that of the basins the ONS also publishes.

    The number in the observatory

    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.

    SIN EAR: sum of the EAR of the subsystems divided by the sum of the maximum EAR.

    See in the panel: Water and climate: reservoirs

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

    Operating decisionStorage and inflows enter as inputs into the models with which the ONS plans operation. On their own, they do not determine the price.

    Operating decision: the ONS uses the information to schedule and dispatch the system.

  3. Step 3 of 6

    Operation decides who generates

    The ONS decides which plants generate, considering cost, security and grid limits. For thermal plants, the unit variable cost (CVU) of each plant is considered in the Monthly Operation Program and in the models; the panel separates thermal generation by the dispatch reason the ONS itself reports. Wind generation can also be reduced by order of the ONS, for a reason outside the plant: this is constrained-off. The number below is an average power over 12 months, in MWavg, and not total energy; the weight of the source in the total is in the panel.

    The number in the observatory

    Thermal generation dispatched by the ONS, last 12 complete months

    8,033 MWavgNature of the data: Calculated

    Sep 2025 to Aug 2026 · SIN

    Source: ONS, Geração Térmica por Motivo de Despacho. Reference up to Aug 2026.

    Thermal and nuclear generation verified over the last 12 complete months, as average power.

    See in the panel: Generation: thermal dispatch

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

    Operating decisionThe ONS operation models estimate the CMO: DECOMP by operating week and load level, DESSEM by half hour.

  4. Step 4 of 6

    The cost of one more unit of load

    The CMO is the cost, per unit of energy produced, of serving one more unit of load in the interconnected system. It is an estimate of the models, by subsystem.

    The number in the observatory

    Average DESSEM CMO in the operating week from 19/09/2026 to 25/09/2026

    R$ 63.31/MWhNature of the data: Calculated

    Sep 19, 2026, 00:00 to Sep 25, 2026, 23:30 · Southeast/Center-West

    Source: ONS, CMO Semi-Horário (DESSEM). Reference up to Sep 25, 2026. Captured on Sep 30, 2026.

    Average, over the half hours of the operating week, of the CMO that DESSEM estimates for the Southeast/Center-West, side by side with the PLD of the same week in the panel.

    In the same cut (Southeast/Center-West, same operating week), the average PLD was R$ 124.09/MWh, above the average DESSEM CMO. The PLD starts from the CMO, but it is not the CMO: it is calculated by the CCEE by hour and submarket and applies the regulatory limits.

    See in the panel: PLD: CMO and price formation

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

    Market ruleThe CCEE calculates the PLD for each hour of the following day based on the CMO, applying the floor and ceilings in force.

    Market rule: a trading rule turns a quantity into an amount to be settled.

  5. Step 5 of 6

    The short-term price

    The PLD is the price of the Short-Term Market, calculated by hour and by submarket. The calculation applies a floor and two ceilings that ANEEL sets each year; that is why the PLD might differ from the CMO of the same hour. The number below is the average Southeast/Center-West PLD in the same operating week as the CMO of the previous step, so that the two sit side by side.

    The number in the observatory

    Average PLD of the operating week from 19/09/2026 to 25/09/2026

    R$ 124.09/MWhNature of the data: Calculated

    Sep 19, 2026, 00:00 to Sep 25, 2026, 23:00 · Southeast/Center-West

    Source: CCEE, hourly PLD by submarket. Reference up to Sep 25, 2026. Captured on Sep 29, 2026. Dataset code at the source: PLD_HORARIO.

    Average, over the hours of the operating week, of the Southeast/Center-West PLD: the price the CCEE calculates for each hour from the CMO, applying the floor and the ceilings. The DESSEM CMO of the same week is in the DESSEM entry.

    In 2026, the PLD floor is R$ 57.31/MWh and the hourly ceiling, R$ 1,611.04/MWh. In the same cut, the average CMO that DESSEM estimates was R$ 63.31/MWh, below the average PLD.

    See in the panel: PLD: CMO and price formation

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

    Market ruleThe CCEE settles at the PLD the differences between contracted and verified energy. The PLD is not the tariff of the consumer served by the distribution utility.

Try it with hypothetical values

Synthetic exampleHypothetical values, chosen for teaching. They are not observatory data and do not enter any indicator.

From the CMO to the PLD and the settlement of the difference

An agent contracted 100 MWh for one hour and consumed 110 MWh. The difference, 10 MWh, is settled at the hourly price, which starts from the CMO and respects the floor and the ceiling. Move the CMO above R$ 1,600/MWh or below R$ 60/MWh to see the price stop following it.

Hourly price · synthetic

R$ 250/MWh

Within the floor and the ceiling, the price follows the hypothetical CMO.

Difference bought at the hourly price · synthetic

pays R$ 2,500.00

10 MWh × R$ 250/MWh. It consumed more than it contracted.

Teaching simplification. The floor (R$ 60/MWh) and the hourly ceiling (R$ 1,600/MWh) are hypothetical and are not the ones in force, which are in the PLD limits entry; the structural ceiling, which limits the daily average, is excluded. The actual balance of each agent profile follows the CCEE Trading Rules, not checked in this phase: here only the idea of settling the difference between contracted and verified energy at the hourly price appears.

  1. Step 6 of 6

    Where the price becomes a result

    In the MCP, the CCEE determines the energy balance and the result of each agent profile by submarket and hour. The MRE hydropower plants share hydrological risk: when, together, they generate less than the physical guarantee, the MRE adjustment factor (GSF) falls below 100%.

    The number in the observatory

    Number read from the published database

    In Aug 2026, the MRE plants generated, together, 40,397 MWavgNature of the data: Calculated, against 51,185 MWavgNature of the data: Calculated of physical guarantee modulated and adjusted by the availability factor. The mechanism is this group that shares the water risk; the ratio between the two numbers is the GSF (see the GSF entry).

    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.

    See in the panel

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

    The ratio between generation and the physical guarantee modulated and adjusted by the availability factor is the GSF of Aug 2026: 78.9%.

What this learning path does not allow you to conclude

  • Rainfall does not directly determine the price: between one and the other there are operation, models and market rules.
  • The PLD is not a tariff: it is the price of the differences settled in the short-term market.
  • The MRE adjustment factor does not say how much each plant gained or lost or how the result is settled: the CCEE Trading Rules were not read.