From Grid-Following to Grid-Forming BESS: How Metering Boundaries and EMS Data Requirements Change

Introduction: BESS Is Becoming More Than an Energy-Shifting Asset

Battery energy storage systems have traditionally been evaluated around functions such as peak shaving, solar self-consumption, time shifting, backup power and energy arbitrage.

As storage projects take on broader grid-support roles, the system-design question changes.

A BESS may now be expected to support:

• Active-power control
• Reactive-power support
• Voltage regulation
• Frequency-related functions
• Rapid power response
• Weak-grid operation
• Islanded or microgrid operation
• Black-start strategies where specified
• Plant-level grid services

These functions increase the importance of measurement boundaries, data ownership, time alignment, validation and system responsibility.

Grid-forming control changes what the BESS is expected to do, but it does not eliminate the need for clear, independent and time-aligned electrical measurement.

Moving from grid-following to grid-forming operation does not necessarily relocate the physical measurement boundaries. It changes the purpose of those boundaries and increases the requirements for data resolution, time alignment, validation and source-of-truth definition.

The central project question is:

When a BESS becomes an active grid-support asset, which electrical values should be measured at the battery, PCS, auxiliary-load and point-of-interconnection boundaries?

1. Grid-Following and Grid-Forming: What Actually Changes?

Grid-following and grid-forming systems differ primarily in how the power-conversion system interacts with the surrounding electrical network.

Grid-Following Operation

A grid-following PCS or inverter typically:

• Synchronises to an existing grid-voltage waveform
• Uses the external system as its voltage and frequency reference
• Injects or absorbs active and reactive power according to control commands
• Depends on a sufficiently stable external grid reference for normal operation

Grid-Forming Operation

A grid-forming PCS or inverter may be designed to:

• Establish or support a voltage reference
• Establish or support a frequency reference
• Operate in weak-grid conditions
• Support islanded-system operation
• Contribute to fast voltage or frequency response
• Coordinate with plant-level or microgrid controls
• Support black-start or restoration functions where specified

Grid-forming capability is not created by the energy meter.

Grid-forming behaviour is implemented by the PCS, inverter controls and plant-level control architecture—not by the energy meter.

The meter provides electrical measurements and data outputs. It does not perform the control algorithms, protection logic or plant-level coordination that create grid-forming behaviour.

Grid-forming is not a single universal package of functions.

Weak-grid operation, islanded operation, black-start capability, fault ride-through, fast-frequency response, inertia-like response and plant-level voltage support should be specified, modelled and tested separately where required.

A product described as grid-forming should not automatically be assumed to provide every one of these capabilities. The applicable functions depend on the PCS design, control configuration, available battery energy, plant architecture, protection system and project requirements.

2. Why kWh-Only Monitoring Is Insufficient

Basic storage monitoring may focus on:

• Charge energy
• Discharge energy
• Daily energy throughput
• Number of cycles
• State of charge
• State of health

These values remain important, but they are not enough to describe a BESS performing active grid-support functions.

Additional data may include:

• Active power
• Reactive power
• Voltage
• Current
• Frequency
• Power factor
• Import and export direction
• Charge and discharge direction
• Ramp behaviour
• Response timing
• Point-of-interconnection net exchange
• Auxiliary consumption
• Breaker or connection status where available
• Device and communication status

The required parameters, recording intervals and verification methods depend on:

• The project use case
• Applicable grid requirements
• The service being provided
• The contract
• The plant-control architecture
• The performance-testing method

Not every energy meter is intended to record high-speed dynamic behaviour.

A standard energy meter may support steady-state values, interval data and cumulative energy, while grid-forming performance validation may require dedicated high-speed equipment.

3. Four Common BESS Measurement Boundaries

The most important metering decision in a BESS project is not simply which meter to select. It is where the measurement takes place.

These four boundaries are common reference points rather than a universal architecture. Multi-block, hybrid or utility-scale projects may require additional transformer, feeder, collector-system, station-service or revenue-metering boundaries.

3.1 Battery DC Boundary

The battery DC boundary may be used to observe:

• DC voltage
• DC current
• Battery-side power
• Charge energy
• Discharge energy
• Rack or string contribution where applicable
• Direction of DC power flow
• Battery-side energy throughput

This boundary helps describe what is happening at the battery side before PCS conversion losses and downstream auxiliary consumption.

Battery-management-system data may also include:

• State of charge
• State of health
• Cell voltage
• Temperature
• Alarm status
• Availability
• Protection status

However, SOC and SOH are battery-management estimates or status values. They are not equivalent to independent energy measurement.

The battery BMS remains the primary source for battery condition and safety information, while a DC energy meter may provide an independent electrical-measurement layer where required by the project.

3.2 PCS AC Boundary

The PCS AC boundary may be used to observe:

• PCS AC input or output
• Active power
• Reactive power
• AC energy
• Operating direction
• Voltage
• Current
• Frequency
• Power factor
• Conversion-side comparison

This boundary helps project teams evaluate how battery-side DC energy is converted into AC electrical output.

It may also support comparison between:

• Battery DC power
• PCS AC power
• PCS-reported internal values
• Metered AC values
• Plant-level energy records

PCS controller data and external energy-meter data may not be identical because they can use different:

• Measurement points
• Sampling methods
• Scaling
• Time bases
• Filtering
• Internal calculations
• Update rates

Internal PCS values should not automatically be treated as equivalent to independently metered values at a defined electrical boundary.

The system architecture should define which data source is used for control, monitoring, performance analysis and billing or settlement where applicable.

3.3 Auxiliary-Load Boundary

A BESS includes more than battery modules and a PCS.

Auxiliary loads may include:

• HVAC
• Cooling systems
• Pumps
• Fans
• Control power
• Fire-safety systems
• Lighting
• Monitoring systems
• Communication equipment
• Auxiliary or station-service transformers
• Station-service loads
• Container or enclosure systems

These loads affect the difference between gross storage output and net exported energy.

Gross discharge energy and net exported energy are not necessarily the same because auxiliary loads consume part of the output.

Separate auxiliary-load measurement may help quantify:

• Parasitic consumption
• Standby consumption
• Cooling energy
• Control-system energy
• Net plant efficiency
• Energy available at the point of interconnection

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

Without this boundary, a project may overstate the usable output or round-trip performance of the complete storage plant.

3.4 Point-of-Interconnection Boundary

The point of interconnection, or POI, represents a grid-facing boundary of the BESS or site.

Relevant measurements may include:

• Grid import energy
• Grid export energy
• Net active power
• Reactive-power exchange
• Voltage
• Frequency
• Power factor
• Demand
• Ramp behaviour
• Site net exchange
• Breaker or connection status where available

The POI may also serve as:

• A utility-facing measurement point
• A contractual delivery boundary
• A billing or settlement boundary where applicable
• A performance-verification boundary
• A grid-code assessment boundary

The POI, PCC, POM, contractual delivery point, revenue-metering point and grid-code performance-measurement point may coincide in some projects, but they should not be assumed to be identical unless the project documents define them that way.

The POI meter shows the net electrical exchange with the grid.

It does not, by itself, explain how that result was created inside the plant.

For example, the same POI export value could result from:

• Battery discharge
• PV generation
• Reduced site load
• Backup generation
• A combination of several resources

Internal measurements are needed to explain the source of the net exchange.

4. PCS Data, Energy-Meter Data and POI Data Are Not the Same

A BESS project often combines data from several devices and systems.

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

For example:

• SOC may come from the battery BMS
• PCS operating mode may come from the PCS controller
• Grid import and export may come from the POI meter
• Auxiliary consumption may come from a dedicated meter
• Plant commands may come from the PPC
• Aggregated operating data may be presented by SCADA or EMS

The project should document:

• Data source
• Measurement point
• Intended use
• Unit
• Scaling
• Direction convention
• Update rate
• Timestamp source
• Validity status
• Register-map or firmware version

5. What Data Should EMS and SCADA Collect?

EMS and SCADA platforms should collect data according to the operating purpose of the project.

5.1 Operational Energy Data

Operational energy data may include:

• Import energy
• Export energy
• Charge energy
• Discharge energy
• Active power
• Reactive power
• Maximum demand
• Auxiliary consumption
• Daily and cumulative energy
• Direction of power flow

5.2 Grid-Interface Data

Grid-interface data may include:

• POI voltage
• Frequency
• Active-power exchange
• Reactive-power exchange
• Power factor
• Import and export direction
• Demand
• Breaker status where available
• Connection status where available

5.3 Asset-Status Data

Asset-status data may include:

• PCS operating mode
• Grid-following or grid-forming mode where exposed by the controller
• Battery BMS status
• SOC
• SOH
• Alarm status
• Availability
• Thermal status
• Protection status
• Maintenance state

5.4 Data-Quality Context

Each record may also require contextual information such as:

• Timestamp
• Device identifier
• Measurement-point identifier
• Unit
• Scaling factor
• Direction convention
• Data origin
• Validity status
• Missing-data status
• Communication status
• Firmware version
• Register-map version
• Data-source priority

This context is necessary when the EMS combines data from meters, PCS controllers, battery BMS devices, PPC systems and protection equipment.

6. Operational Monitoring Is Not the Same as Dynamic Performance Testing

Standard EMS metering and grid-forming performance testing serve different purposes.

Standard EMS Metering May Support

• Steady-state active power
• Steady-state reactive power
• Voltage
• Current
• Frequency
• Power factor
• Interval power values
• Cumulative energy
• Import and export tracking
• Routine EMS data collection

Grid-Forming Performance Testing May Require

• High-resolution waveform or event data at sampling rates defined by the applicable test plan
• Voltage-step and phase-jump response
• Frequency-step or frequency-disturbance response
• Fault ride-through and current-limiting behaviour
• Control-loop and system-strength stability assessment
• Fast-frequency, inertial or inertia-like response assessment where specified
• Black-start sequencing where the capability is required
• Grid-connected-to-islanded transition records
• Harmonic, transient and disturbance analysis
• Protection-system coordination and sequence-of-events evidence

These functions may require:

• Power-quality analysers
• Disturbance recorders
• Protection relays and event records
• Oscillography
• PCS high-speed logs
• Specialised test equipment
• Engineering simulation models

The required instruments, sampling rates, test conditions and acceptance metrics should be defined by the grid operator, test specification, interconnection agreement or project test plan.

Standard EMS metering and high-speed grid-forming performance testing are complementary, not interchangeable.

A routine energy meter should not be presented as a substitute for dedicated dynamic-performance testing equipment.

7. Time Synchronisation and Data Alignment

Time alignment becomes critical when several systems record the same event.

A project may use:

• Meter internal timestamps
• PCS controller timestamps
• Battery BMS timestamps
• Gateway timestamps
• EMS timestamps
• SCADA timestamps
• Protection-relay event times
• Central plant-clock time

These timestamps may differ because of:

• Polling delay
• Communication latency
• Clock drift
• Register-refresh timing
• Data buffering
• Gateway processing
• Different time zones
• Different synchronisation sources

A typical issue may look like this:

• The PCS reports a response beginning at 14:00:02
• The POI meter records the corresponding power change at 14:00:04
• SCADA displays the event at 14:00:06

The project must determine whether the difference is caused by:

• Actual system response
• Measurement location
• Meter refresh timing
• Communication delay
• Timestamp assignment
• Data processing

Relevant design items may include:

• NTP
• PTP where required
• GPS-based time sources where applicable
• Central SCADA clock
• Meter clock synchronisation
• Maximum permitted clock drift
• Event-sequence requirements
• Missing-data recovery
• Delayed-record handling

The required time accuracy should be defined by the project, grid requirement, service type and testing method.

No single time-accuracy requirement applies to every BESS project.

8. How to Calculate BESS Energy Performance Without Mixing Boundaries

BESS performance depends on the selected calculation boundary.

A typical energy path is:

Battery DC discharge → PCS AC output → transformer and cable losses → auxiliary consumption → POI net export

These are not interchangeable values.

Battery DC Boundary

This boundary may be used to calculate battery-side energy throughput.

PCS AC Boundary

This boundary may be used to evaluate conversion output before downstream plant losses.

Auxiliary Boundary

This boundary quantifies station-service consumption.

POI Boundary

This boundary shows the net amount delivered to or taken from the grid.

Round-trip efficiency must therefore state the measurement boundary.

Possible definitions may include:

• DC round-trip efficiency at a defined battery or PCS DC boundary
• AC round-trip efficiency at the PCS AC terminals
• POI-to-POI round-trip efficiency
• Gross plant efficiency
• Net plant efficiency including defined auxiliary consumption

A single percentage without a defined boundary can be misleading.

For example:

• Battery-level efficiency may exclude PCS losses
• PCS-level efficiency may exclude transformer losses
• POI-level efficiency may include auxiliary consumption
• Site-level efficiency may also be affected by PV, local loads or other assets

Project reports should identify:

• Start boundary
• End boundary
• Import or charging energy
• Export or discharging energy
• Auxiliary-load treatment
• Transformer-loss treatment
• Data interval
• Direction convention
• Missing-data treatment

9. Grid-Forming BESS in C&I, Microgrid and Data-Centre Applications

9.1 C&I Sites

C&I storage may support:

• Peak-demand reduction
• Backup power
• PV self-consumption
• Energy arbitrage
• Power-quality support
• Grid-support services where permitted

Relevant measurements may include:

• Site import and export
• BESS charge and discharge
• Auxiliary consumption
• Production-load demand
• POI net exchange
• PCS active and reactive power

9.2 Microgrids

Microgrids may operate in both grid-connected and islanded modes.

Relevant considerations may include:

• POI measurement
• Internal feeder measurement
• Generation contribution
• BESS charging and discharging
• Priority loads
• Grid-connected versus islanded data
• Transition records
• Coordination of voltage and frequency references among multiple grid-forming resources, synchronous machines and plant controllers
• Coordination between PCS, PPC, EMS and protection systems

Where several grid-forming resources are present, additional design issues may include:

• Droop or other power-sharing strategies
• Synchronisation
• Current limiting
• Reference coordination
• Mode transition
• Protection coordination

9.3 Data Centres

Data-centre applications may involve:

• Rapid load changes
• UPS and BESS interaction
• IT and cooling-load separation
• Grid and behind-the-meter power flows
• Backup generation
• POI demand
• EMS, facility BMS and DCIM integration
• Battery-BMS data where applicable

In this section:

• Battery BMS means Battery Management System
• Facility or building BMS means Building Management System

In this context, the BESS may support continuity, demand management or grid interaction.

However, data-centre system design must still distinguish:

• UPS energy
• BESS energy
• IT load
• Cooling load
• Auxiliary consumption
• Grid-facing net power

10. Common BESS Integration Risks

Common project risks include:

1. Treating PCS data as independent settlement data without confirmation
2. Mixing DC, PCS AC and POI measurement boundaries
3. Ignoring auxiliary consumption
4. Reversing import/export or charge/discharge direction
5. Applying an incorrect CT ratio or scaling factor
6. Using inconsistent timestamps across meter, PCS and SCADA
7. Failing to document the source of truth for each parameter
8. Using operational data as high-speed performance evidence
9. Mixing gross energy with net energy
10. Using different units across devices
11. Losing data during communication interruptions
12. Changing register maps after firmware updates
13. Assuming Modbus support guarantees EMS compatibility
14. Confusing meter refresh rate with dynamic-test sampling rate
15. Treating BMS estimates as automatically equivalent to independent metering
16. Omitting transformer and cable losses from performance calculations
17. Using one efficiency value without defining the calculation boundary
18. Failing to confirm grid-code and contractual requirements
19. Assuming every energy meter can capture grid-forming dynamics
20. Failing to complete meter–gateway–EMS–PCS pilot validation

11. BESS Metering Architecture Checklist

The architecture should be defined before the meter is selected.

12. How YTL Can Support Initial Meter Evaluation

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

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

• Initial meter-model selection
• Voltage and current-range review
• Initial technical discussion of customer-proposed AC and DC measurement points and intended boundaries
• Review of customer-proposed CT ratios, shunt inputs or compatible sensor arrangements
• 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
• Initial technical review of customer-proposed measurement points

Product capabilities vary by meter model, hardware, firmware, current-sensing arrangement, measurement interval, communication interface, register-map version and certification scope.

Communication functions, measurement intervals, accuracy, certification scope and platform compatibility must be confirmed for the selected model and project requirements.

YTL does not define the project POI, PCC, POM, revenue-metering boundary, settlement boundary, source-of-truth hierarchy or dynamic-performance test plan.

YTL does not provide grid-forming PCS controls, PPC control algorithms, BESS dispatch platforms, protection coordination, dynamic system modelling, grid-code compliance approval, high-speed dynamic-performance validation or ancillary-service qualification and market approval.

Grid-forming control, plant-level coordination, protection design, dynamic modelling, performance testing and grid-code compliance remain the responsibilities of the PCS supplier, plant-controller provider, protection engineer, system integrator, consultant, utility and other project stakeholders.

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

13. Frequently Asked Questions

What is the main difference between grid-following and grid-forming BESS?

A grid-following BESS normally synchronises to an existing grid-voltage and frequency reference. A grid-forming BESS can establish or support voltage and frequency references through its PCS and control architecture.

Does grid-forming automatically include black start, islanding and inertia-like response?

No. These functions should be specified, modelled and tested individually. Their availability depends on the PCS design, battery energy, protection system, plant controls and project requirements.

Does an energy meter make a BESS grid-forming?

No. Grid-forming behaviour is implemented by the PCS, inverter controls and plant-level control architecture. The meter measures and outputs electrical data.

What are the common BESS measurement boundaries?

Common boundaries include the battery DC side, PCS AC side, auxiliary-load circuits and point of interconnection. Larger or hybrid projects may require additional transformer, feeder, collector-system or revenue-metering boundaries.

Are POI, PCC, POM and revenue-metering points always the same?

No. They may coincide in some projects, but they should not be assumed to be identical unless the project documents define them that way.

Is PCS data the same as independent meter data?

Not necessarily. PCS data may be based on internal controller measurements and calculations. External energy meters may provide independent values at a defined electrical boundary.

Why should auxiliary consumption be measured?

Auxiliary loads reduce the difference between gross BESS output and net exported energy. Separate measurement helps quantify net plant performance.

What should be measured at the POI?

Depending on the project, relevant values may include import/export energy, active power, reactive power, voltage, frequency, power factor and net site exchange.

Can a standard energy meter verify grid-forming performance?

A standard energy meter may support operational monitoring and cumulative energy measurement. High-speed grid-forming performance testing usually requires specialist equipment and a project-defined test plan.

Why is time synchronisation important?

Different devices may record the same response at different times because of clock drift, polling, buffering or communication latency. Time alignment is necessary for reliable event reconstruction.

How should BESS round-trip efficiency be calculated?

The calculation must define the start and end measurement boundaries and explain whether PCS losses, transformer losses and auxiliary consumption are included.

Can a Modbus energy meter integrate with a BESS EMS?

Potentially. Integration depends on the physical interface, protocol implementation, register map, scaling, refresh rate and EMS architecture.

14. Conclusion

Grid-forming capability changes the control role of the BESS, but it does not remove the need for structured electrical measurement.

A reliable BESS data architecture should clearly distinguish:

• Battery DC measurement
• PCS AC measurement
• Auxiliary-load measurement
• POI measurement
• Battery BMS status data
• PCS control data
• PPC commands
• EMS and SCADA aggregation
• High-speed testing data

The central design principle is:

Each parameter should have a clearly defined measurement boundary, source of truth, timestamp, direction convention and intended use.

Standard energy meters support the operational measurement layer. They remain complementary to battery BMS data, PCS controls, protection systems and specialist dynamic-performance equipment.

For grid-forming BESS projects, accurate data depends not on a single device, but on consistent coordination between measurement boundaries, communication systems and plant-control architecture.

References

1. National Renewable Energy Laboratory, Introduction to Grid Forming Inverters: A Key to Transforming Our Power Grid, 2024.
2. Energy Systems Integration Group, Grid-Forming Technology in Energy Systems Integration, 2022.
3. Energy Systems Integration Group, Testing the Performance of Grid-Forming Resources: Test Methods and Performance Metrics for Evaluating the Voltage Source Behavior of Grid-Forming Resources, 2025.
4. North American Electric Reliability Corporation, Performance, Modeling, and Simulations of BPS-Connected Battery Energy Storage Systems and Hybrid Power Plants, 2023.
5. National Renewable Energy Laboratory and project partners, Microgrid Black Start Challenges: The Role of Grid-Forming Inverters, 2025.