Long-Duration Energy Storage Metering: What Should Be Measured Beyond MWh Capacity?

Introduction: LDES Performance Is More Than a Nameplate Energy Rating

Long-duration energy storage is moving from policy discussion toward project selection and delivery.

On 26 June 2026, Ofgem published its minded-to decisions for the first application window of Great Britain’s long-duration electricity storage cap-and-floor scheme. The proposed portfolio includes 16 projects, representing 7,645 MW of capacity, but remains subject to consultation and final determination.

This policy development highlights an important project question:

When an energy-storage system is expected to operate for several hours, what should be measured beyond its stated MW and MWh capacity?

Definitions of long-duration storage vary by jurisdiction and programme. Ofgem uses a duration of eight hours or more in the current Great Britain scheme, while other frameworks may apply different thresholds.

A project may be described as having a specific power rating, energy capacity and discharge duration. However, those nameplate values do not, by themselves, show:

• How much usable energy is available
• How much energy enters the storage system
• How much energy is later discharged
• How much reaches the grid or customer
• How much is consumed during operation or standby
• How much capacity remains after ageing or degradation
• Whether the asset remains fully or partially available
• Which measurement boundary is used for efficiency or performance claims
• Whether data remains complete throughout a multi-hour dispatch

Long-duration energy storage metering should therefore connect nameplate capacity with measured electrical performance, derived capability values, technology-specific operating states and clearly defined contractual boundaries.

The central principle is:

MW describes the rate of power delivery, MWh describes energy, and duration depends on usable energy, sustained power, operating limits, losses and the selected measurement boundary.

1. What Is Long-Duration Energy Storage?

Long-duration energy storage, or LDES, stores electricity for later use over periods longer than those commonly associated with short-duration balancing, peak shaving or brief reserve applications.

LDES may use different technologies, including:

• Battery energy storage
• Pumped-storage hydro
• Compressed-air energy storage
• Liquid-air energy storage
• Flow batteries
• Thermal storage coupled with electricity conversion
• Gravity or mechanical storage
• Other electrochemical, mechanical or hybrid technologies

Although the technologies differ, several electrical measurement principles remain broadly applicable:

• Define where charging energy is measured
• Define where discharge output is measured
• Measure imported and exported energy
• Account for auxiliary and station-service consumption
• Separate gross output from net delivery
• Align operating data with dispatch periods
• Preserve timestamps, direction conventions and data-quality context

The required internal sensors and process variables vary by technology.

A battery-based system may use DC voltage, current and battery-management data. Pumped-storage hydro may require reservoir level, flow and hydraulic-head measurements. Compressed-air or thermal systems may require pressure, temperature, mass flow or thermal-state measurements.

A standard energy meter does not replace these technology-specific instruments.

2. MW, MWh and Discharge Duration Are Different Measures

Power capacity, energy capacity and duration are related, but they do not mean the same thing.

Power Capacity

Power capacity is normally expressed in megawatts.

It describes the rate at which the system can charge or discharge under defined conditions.

For example:

Power rating: 100 MW

This indicates a potential power level, not how long that output can be sustained.

Energy Capacity

Energy capacity is normally expressed in megawatt-hours.

It may refer to:

• Nameplate capacity
• Gross capacity
• Usable capacity
• Dispatchable capacity
• Tested capacity
• Beginning-of-life capacity
• Guaranteed capacity
• DC-side energy
• AC-side deliverable energy
• POI-level net delivered energy

These values should not be treated as interchangeable.

Rated or Declared Duration

Rated or declared duration is commonly calculated by dividing usable energy at a defined measurement boundary by the sustained discharge power specified at the same boundary and under the same operating conditions.

Declared duration =
Usable energy at a defined boundary
÷
Sustained discharge power at the same boundary

For example:

A system with 800 MWh of usable energy at a defined boundary and a sustained discharge capability of 100 MW at that same boundary has a nominal duration of eight hours, subject to the specified starting state, ending state and operating conditions.

The calculation should also identify:

• Starting storage state
• Ending storage state
• Environmental conditions
• Power profile
• Auxiliary-load treatment
• Conversion-loss treatment
• Downstream electrical losses
• Whether interruptions are permitted
• Whether output must be continuously sustained

An “eight-hour” or “ten-hour” system should not automatically be assumed to export rated power continuously at the POI under every condition.

3. Four Common LDES Measurement Boundaries

The physical architecture differs across electrochemical batteries, pumped-storage hydro, compressed-air storage, liquid-air storage, electricity-storage systems using thermal media and other LDES technologies.

The following boundaries are common reference categories rather than a universal architecture.

Large, hybrid or multi-block projects may require additional feeder, transformer, collector-system, station-service, process or revenue-metering boundaries.

3.1 Storage-Medium or Primary Conversion Boundary

For electrochemical storage, the relevant boundary may be the battery DC side and may include:

• DC voltage
• DC current
• DC power
• Charge and discharge energy
• Rack or string contribution where applicable
• Direction of DC power flow

Battery-management-system data may also include:

• State of charge
• State of health
• Temperature
• Alarm status
• Availability
• Cell, module or rack condition

SOC and SOH are estimated or calculated operating states. They should not automatically be treated as equivalent to independently measured electrical energy.

For pumped, compressed-air, liquid-air, thermal or mechanical storage technologies, the corresponding primary boundary may involve:

• Pump input
• Turbine or generator output
• Compressor input
• Expander output
• Thermal charging input
• Mechanical conversion equipment
• Generator terminals
• Other project-defined process boundaries

Relevant internal operating data may include:

• Pressure
• Temperature
• Fluid level
• Flow
• Hydraulic head
• Rotational or mechanical state
• Thermal-storage state
• Pump, compressor, turbine or expander status

These measurements describe storage-medium condition and conversion processes. They do not replace electrical energy measurement at the defined electrical input and output boundaries.

3.2 Conversion-System AC Boundary

The conversion-system AC boundary may refer to PCS AC terminals, generator terminals or another defined electrical-conversion point.

Relevant measurements may include:

• AC input energy
• AC output energy
• Active power
• Reactive power
• Voltage
• Current
• Frequency
• Power factor
• Import/export direction
• Conversion-side performance

Internal controller values may be useful for operation but should not automatically be treated as equivalent to independently metered values at a defined electrical boundary.

3.3 Auxiliary and Station-Service Boundary

LDES facilities may require substantial supporting systems.

Auxiliary or station-service loads may include:

• Cooling
• Heating
• Pumps
• Fans
• Compressors
• Control power
• Fire-safety systems
• Lighting
• Communications
• Battery heaters
• Water-treatment equipment
• Auxiliary or station-service transformers
• Process-support systems
• Standby equipment

These loads may operate during charging, discharging, standby or all three.

Losses in the main step-up transformer, collector system and export cables should be treated separately according to the defined performance boundary.

3.4 Point-of-Interconnection Boundary

The point of interconnection, or POI, may measure:

• Grid import energy
• Grid export energy
• Net active power
• Reactive-power exchange
• Voltage
• Frequency
• Power factor
• Demand
• Dispatch-period net energy
• Connection or breaker status where available

The POI may provide the main grid-facing performance view, but the POI, PCC, contractual-delivery point, revenue-metering point and formal performance-test boundary should not be assumed to be identical unless the project documents define them that way.

POI data shows net exchange with the external system. It does not by itself explain:

• Internal conversion losses
• Auxiliary consumption
• Storage-medium losses
• Technology-specific degradation
• Individual block contribution

4. Charge Energy, Discharge Energy and Net Delivered Energy

LDES projects should distinguish several energy quantities.

Charge Energy

Charge energy may refer to:

• Energy imported from the grid
• Energy supplied by on-site generation
• AC energy entering the conversion system
• DC energy entering a battery
• Electrical input to a pump, compressor or heating system
• Another project-defined charging boundary

Discharge Energy

Discharge energy may refer to:

• Energy leaving the storage medium
• PCS AC output
• Generator output
• Gross plant output
• Energy delivered before auxiliary consumption
• Energy delivered before transformer or cable losses

Net Delivered Energy

Net delivered energy usually refers to energy delivered at a defined external boundary after the treatment of applicable internal consumption and losses.

A simplified performance-accounting relationship may compare:

Electrical energy received at a defined charging boundary → gross discharge energy → separately measured auxiliary consumption → defined conversion and network losses → net energy delivered at the POI

This is an accounting relationship, not a universal physical sequence.

The actual electrical path and auxiliary-supply arrangement should be confirmed from the project single-line diagram.

For example:

• Charging energy may come from the grid, PV or another on-site source.
• Transformer and cable losses may occur during both charging and discharging.
• Auxiliary loads may be supplied from the grid, station-service bus or storage system.
• Some auxiliaries may operate continuously rather than only during discharge.

The project should not use “discharged energy” and “net delivered energy” as interchangeable terms unless they are measured at the same boundary.

4.1 Usable Energy Capacity and Capacity Retention

Rated or nameplate energy capacity should not automatically be treated as the energy that can be dispatched or delivered at the POI.

An LDES project may need to distinguish:

• Gross or nameplate energy capacity
• Usable energy capacity
• Dispatchable energy within operating limits
• Deliverable energy at a defined electrical boundary
• Tested energy capacity under a specified discharge profile
• Retained capacity after ageing, cycling or storage-medium degradation

A capacity test should define:

• Test date
• Starting storage state
• Minimum ending storage state
• Charge and discharge power profile
• Environmental conditions
• Measurement boundary
• Auxiliary-load treatment
• Data interval
• Missing-data treatment
• Treatment of interruptions
• Required sustained or variable power profile

Capacity retention should be reported against a defined initial, tested or contractual reference rather than inferred only from a displayed storage-state percentage.

A system may continue to show 100% SOC while the absolute usable energy represented by that SOC has declined over time.

5. Standby Losses, Self-Discharge and Energy Retention

LDES performance is not defined only by how long the system can discharge.

It may also depend on how much energy remains available after several hours or days of storage.

Relevant metrics may include:

• Active standby consumption
• Idle or dormant standby consumption
• Self-discharge
• Storage-medium leakage or loss
• Thermal-maintenance energy
• Pressure-maintenance energy
• Readiness consumption
• Control and communication consumption
• Restart energy
• Energy retained after a defined storage period

The source of these losses depends on the technology.

For battery storage, losses may include:

• Battery self-discharge
• Thermal-management consumption
• Battery-heating consumption
• PCS standby consumption
• Control and communication loads

For other LDES technologies, losses may include:

• Pressure decay
• Thermal leakage
• Fluid circulation
• Pumping or compression support
• Mechanical friction
• Process-state maintenance

An energy-retention test should define:

• Initial stored-energy condition
• Storage duration
• Environmental conditions
• Standby operating mode
• Auxiliary-supply arrangement
• Final stored-energy condition
• Restart requirements
• Measurement boundary

A system that can deliver ten hours immediately after charging may not deliver the same energy after an extended standby period unless retention losses are accounted for.

6. Why Auxiliary Consumption Matters During Long Dispatches

Auxiliary consumption is relevant in all energy-storage projects, but its cumulative effect becomes especially visible during extended operation.

For example:

Average auxiliary load: 1.5 MW
Dispatch duration: 10 hours
Auxiliary energy: 15 MWh

That 15 MWh may materially affect:

• Net export
• Available duration
• Round-trip efficiency
• Dispatch economics
• Performance guarantees

The project should confirm whether auxiliary loads are:

• Supplied from the storage system
• Supplied from the grid
• Supplied from a separate station-service connection
• Included in the POI measurement
• Metered separately
• Included in capacity or efficiency guarantees

A project that reports only gross PCS or generator output may overstate the energy available to the grid or customer.

7. Availability, Storage State and Metered Energy Are Not the Same

These terms should be treated as separate but related data categories.

Availability

Availability describes whether the system can provide service according to defined technical or contractual requirements.

Availability should be defined over a specified reporting period and should explain how the following are treated:

• Planned outages
• Forced outages
• Partial derating
• External network constraints
• Energy limitations
• Environmental restrictions
• Telemetry and data unavailability
• Excluded events
• Contractual exceptions

Relevant reporting fields may include:

• Reporting period
• Available MW
• Available MWh
• Partial-derating level
• Planned-outage treatment
• Forced-outage treatment
• External-constraint treatment
• Excluded-event definition
• Time-weighted calculation
• Energy-weighted calculation where applicable

A system may be:

• Fully available
• Fully unavailable
• Available at reduced MW
• Available at rated MW but for reduced duration
• Technically available but externally constrained
• Energy-ready but lacking valid telemetry
• Connected but not dispatchable

Availability is not a single meter value.

Storage State

Storage state may be represented by:

• Battery SOC
• Reservoir level
• Pressure
• Temperature
• Stored thermal-energy estimate
• Material state
• Mechanical position
• Another technology-specific state variable

These values support dispatch planning but do not replace measured electrical input and output.

Metered Energy

Metered energy records electrical energy crossing a defined measurement point.

It may support:

• Operational monitoring
• Energy accounting
• Efficiency calculations
• Billing
• Settlement
• Contractual verification

A project may show a high storage-state value but still be unavailable due to a conversion-system fault.

8. Available Power and Available Energy Are Derived Capability Values

EMS and SCADA platforms may display:

• Controller-reported available power
• Controller-reported available energy
• Expected remaining duration
• Dispatch headroom
• State-dependent power limits

These are normally derived or declared capability values.

They may be calculated by:

• BMS
• PCS
• Process-control system
• EMS
• Plant controller
• Performance model

Available power and available energy are not normally direct outputs of a standard energy meter.

Their source, assumptions, update method and validation status should be documented separately from directly metered electrical values.

Relevant assumptions may include:

• Current storage state
• Temperature
• Operating limits
• Degradation
• Reserve requirement
• Ramp limits
• Conversion-system availability
• Auxiliary demand
• Contractual restrictions

The source-of-truth hierarchy should state which system provides each value.

9. Dispatch Periods and Interval Data

A long-duration dispatch may last several hours, but interval data remains important throughout the operating period.

A single start and end reading may show total energy but cannot explain:

• Ramp-up behaviour
• Power variation
• Partial derating
• Short interruptions
• Auxiliary-load changes
• Import/export reversals
• End-of-discharge derating
• Missing-data periods
• Recovery after an outage

Relevant intervals may include:

• Meter recording interval
• EMS polling interval
• SCADA reporting interval
• Contractual dispatch interval
• Billing or settlement interval
• Performance-test interval
• Internal process-control interval

These intervals may differ.

A declared ten-hour performance may mean:

• Continuous rated-power output for ten hours
• A variable dispatch profile over ten hours
• Delivery of a specified MWh amount within a ten-hour window
• Average power over the dispatch period
• Output with limited permitted interruptions

These interpretations are not interchangeable.

A performance claim should state:

• Whether output must be continuously sustained
• Whether interruptions are permitted
• Whether the output follows a defined profile
• Whether average or minimum power is assessed
• Start and end conditions
• Measurement interval
• Data-correction rules

A gateway polling every second does not automatically provide one-second validated contractual performance data.

10. What Data Should EMS and SCADA Collect?

EMS and SCADA requirements should be organised by purpose.

10.1 Electrical Energy Data

Relevant data may include:

• Import energy
• Export energy
• Charge energy
• Discharge energy
• Active power
• Reactive power
• Voltage
• Current
• Frequency
• Power factor
• Maximum demand
• Auxiliary consumption
• Gross output
• Net output

10.2 Storage-State and Capability Data

Depending on the technology, this may include:

• Battery SOC
• Battery SOH
• Reservoir level
• Pressure
• Temperature
• Thermal state
• Stored-energy estimate
• Controller- or model-reported available power
• Controller- or model-reported available energy
• Minimum and maximum operating limits
• Expected remaining duration

Available power and available energy should be clearly identified as derived capability values rather than directly metered electrical quantities.

10.3 Availability and Operating Status

Relevant data may include:

• Available
• Unavailable
• Partially derated
• Charging
• Discharging
• Standby
• Maintenance
• Fault
• Grid disconnected
• Energy limited
• Dispatch limited
• Telemetry and data availability

10.4 Data-Quality Context

Each record may also require:

• Timestamp
• Device identifier
• Measurement-point identifier
• Data source
• Unit
• Scaling
• Direction convention
• Validation status
• Missing-data indication
• Estimated or substituted-data indication
• Corrected-record reference
• Communication status
• Firmware version
• Register-map version
• Data-processing method

A reliable architecture identifies which system is the source of truth for each parameter.

11. How Should LDES Efficiency Be Reported?

Round-trip efficiency compares useful output energy with input energy, but the result depends on the selected boundaries.

Possible expressions include:

• DC round-trip efficiency at a defined battery or conversion boundary
• AC round-trip efficiency at defined conversion-system terminals
• POI-to-POI round-trip efficiency
• Gross plant efficiency
• Net plant efficiency including defined auxiliary consumption
• Dispatch-period efficiency
• Annual or seasonal operational efficiency

A performance statement should identify:

• Charging boundary
• Discharging boundary
• Input energy
• Output energy
• Start and end storage state
• Auxiliary-load treatment
• Transformer- and cable-loss treatment
• Standby-loss treatment
• Environmental conditions
• Dispatch power
• Dispatch duration
• Data interval
• Missing-data treatment

Two efficiency values should not be compared unless their boundaries and conditions are equivalent.

12. Technology Changes, but the Measurement Principles Remain

LDES technologies use different internal sensing and control systems.

Battery Storage

May require:

• DC voltage and current
• AC energy metering
• SOC and SOH
• Thermal data
• Rack or string status

Pumped-Storage Hydro

May require:

• Pump input energy
• Generator output energy
• Reservoir level
• Water flow
• Hydraulic head
• Turbine and pump status

Compressed-Air or Liquid-Air Storage

May require:

• Compressor input energy
• Generator or expander output energy
• Pressure
• Temperature
• Mass flow
• Thermal-system status

Thermal or Process-Based Storage

May require:

• Electrical charging energy
• Thermal-state measurements
• Temperature
• Flow
• Conversion output
• Process losses

Despite these differences, the common questions remain:

1. Where does charging energy enter?
2. Where does output energy leave?
3. Which auxiliary loads are included?
4. Which losses occur between internal systems and the POI?
5. Which system reports storage state?
6. Which meter records external electrical energy?
7. How is usable capacity determined?
8. How is standby loss measured?
9. What interval and timestamp are used?
10. How are missing and corrected records handled?

13. Operational Metering Is Not Complete Performance Validation

Standard energy meters may support:

• Cumulative energy
• Interval energy
• Active power
• Reactive power
• Voltage
• Current
• Frequency
• Power factor
• Import/export tracking
• EMS and SCADA data acquisition

A complete LDES performance assessment may also require:

• Capacity testing
• Capacity-retention testing
• Self-discharge testing
• Energy-retention testing
• Technology-specific state measurement
• Degradation analysis
• Thermal-performance testing
• Pressure or flow testing
• Mechanical-performance testing
• Dynamic-response testing
• Protection and fault-event records
• Availability and outage analysis
• Contract-specific calculations

Routine electrical metering and specialised storage-performance testing are complementary, not interchangeable.

The test conditions, instruments, sampling rates and acceptance criteria should be defined by the storage-technology provider, project engineer, test organisation, contract or applicable standard.

14. Common LDES Measurement Risks

Common project risks include:

1. Treating MW and MWh as interchangeable
2. Calculating duration from gross capacity rather than usable energy
3. Using power and energy values from different boundaries
4. Reporting gross output as net delivered energy
5. Ignoring auxiliary consumption
6. Ignoring standby or readiness consumption
7. Failing to measure energy retention
8. Using SOC as a substitute for metered energy
9. Treating availability as a simple status flag
10. Ignoring partial derating
11. Comparing efficiency values from different boundaries
12. Omitting transformer or collector-system losses
13. Reversing charge/discharge or import/export direction
14. Applying incorrect CT, shunt or scaling values
15. Mixing EMS polling frequency with validated interval data
16. Using inconsistent timestamps
17. Failing to distinguish actual, estimated and corrected data
18. Treating derived available energy as a direct meter value
19. Reporting duration without defining the discharge profile
20. Using an energy meter as the only evidence for capacity retention or final acceptance

15. LDES Metering Architecture Checklist

The measurement architecture should be defined before the meter and data interfaces are selected.

16. How YTL Can Support Initial Meter Evaluation

Zhejiang Yongtailong Electronic Co., Ltd. (YTL) can support the initial evaluation of selected AC energy meters, DC energy meters, CT-operated meters, panel meters and communication-enabled metering products for energy-storage and power-distribution applications.

Depending on the selected model and customer requirements, YTL can support:

• Initial meter-model selection
• Voltage and current-range review
• Initial technical discussion of customer-proposed electrical measurement points and intended boundaries
• Review of customer-proposed CT ratios, shunt inputs and meter-side compatible sensor requirements
• Import/export and charge/discharge direction review
• RS485 and Modbus option confirmation
• Register-map, unit, scaling and data-format review
• Sample testing support
• Review of the proposed meter-to-gateway or meter-to-EMS data interface

Measurement intervals, accuracy, communication functions, certification scope and platform compatibility must be confirmed for the selected model and project.

YTL supports the field-level electrical-measurement and data-output layer.

YTL does not define or guarantee:

• Gross storage capacity
• Usable storage capacity
• Deliverable storage capacity
• Availability methodology
• Storage-medium condition
• Capacity retention
• Self-discharge performance
• Energy-retention performance
• Dispatch capability
• Performance-test procedures
• Cap-and-floor eligibility
• Final project acceptance

These matters remain the responsibility of the storage-technology supplier, PCS or conversion-system provider, EMS provider, system integrator, project engineer, owner, test organisation and relevant regulatory or contractual parties.

17. Frequently Asked Questions

What is the difference between MW, MWh and discharge duration?

MW measures power, or the rate of energy transfer. MWh measures energy. Duration describes how long a system can maintain a defined output using usable energy measured at the same boundary.

Does an 800 MWh, 100 MW system always deliver for eight hours?

Not necessarily. The calculation is valid only when usable energy and sustained power refer to the same boundary and operating conditions. Actual duration may be affected by losses, auxiliaries, reserve limits and degradation.

What is usable energy capacity?

Usable energy capacity is the energy available within defined operating limits. It may be lower than gross or nameplate capacity.

What is capacity retention?

Capacity retention compares tested usable or deliverable capacity with a defined initial or contractual reference after ageing, cycling or degradation.

How should net energy from LDES be measured?

Net energy should be measured at a clearly defined delivery boundary, such as the POI, with auxiliary consumption and defined losses treated according to the project methodology.

Does SOC replace an energy meter?

No. SOC is an estimated storage state. An energy meter records electrical energy crossing a defined boundary.

Why do standby losses matter?

A system may lose energy while waiting for dispatch because of self-discharge, thermal maintenance, pressure maintenance or readiness consumption.

How should availability be reported?

Availability should use a defined reporting period and explain partial derating, planned outages, forced outages, external constraints and energy limitations.

Are available power and available energy meter values?

Usually not. They are derived capability values reported by a controller, BMS, process system, EMS or model.

Can a standard energy meter validate every LDES performance claim?

No. A standard meter can measure supported electrical values, but capacity retention, self-discharge, internal storage state and technology-specific performance require additional systems and test methods.

18. Conclusion

Long-duration energy storage cannot be evaluated from MWh capacity alone.

A complete measurement and performance architecture should distinguish:

• Power capacity
• Gross energy capacity
• Usable energy capacity
• Deliverable energy
• Capacity retention
• Rated duration
• Actual delivered duration
• Charge energy
• Discharge energy
• Net delivered energy
• Auxiliary consumption
• Standby losses
• Energy retention
• Storage state
• Available power
• Available energy
• Partial derating
• Availability
• Gross and net efficiency

The central design principle is:

Every performance claim should identify the measurement boundary, operating conditions, reporting period, data source and treatment of losses, auxiliary consumption and degradation.

Meters provide the electrical foundation. Controllers, BMS platforms, process systems, EMS, SCADA and specialised test equipment provide the additional state, capability and validation information needed for a complete LDES assessment.

References

1. Ofgem, “Ofgem boosts long duration storage to secure more homegrown energy for customers,” 26 June 2026.
2. Ofgem, “Long duration electricity storage window 1: minded-to decisions,” consultation published 26 June 2026.
3. Ofgem and the UK Government, Long Duration Electricity Storage: Technical Decision, 2025.
4. Sandia National Laboratories, Protocol for Uniformly Measuring and Expressing the Performance of Energy Storage Systems.
5. S. Department of Energy Office of Electricity, Energy Storage Handbook, Chapter 16: Energy Storage Performance Testing.
6. National Laboratory of the Rockies, Performance and Health Test Procedure for Grid Energy Storage Systems.
7. IEEE 762-2023, Standard Definitions for Use in Reporting Electric Generating and Storage Unit Reliability, Availability, and Productivity.