IEC TS 62933-2-3:2025

IEC TS 62933-2-3 ®
Edition 1.0 2025-05
TECHNICAL
SPECIFICATION
Electric Energy Storage (EES) Systems –
Part 2-3: Unit parameters and testing methods – Performance assessment test
during site operation
ICS 13.020.30  ISBN 978-2-8327-0392-2

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– 2 – IEC TS 62933-2-3:2025 © IEC 2025
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms, definitions, abbreviated terms and symbols . 7
3.1 Terms and definitions . 7
3.2 Abbreviated terms . 7
3.3 Symbols . 7
4 General requirements of the operational performance testing . 9
4.1 Purpose . 9
4.2 Stakeholders . 10
4.3 Principles of assessment intervals and data sampling . 10
4.4 Data requirements . 10
4.5 Operational performance testing methods . 11
5 Operational performance testing items . 12
5.1 General . 12
5.2 General operational performance testing items . 12
5.2.1 Actual energy storage capacity . 12
5.2.2 Input and output power rating . 12
5.2.3 Roundtrip efficiency . 12
5.2.4 Auxiliary power consumption . 12
5.3 Energy-related operational performance testing items . 12
5.3.1 Available hours of an EES system . 12
5.3.2 Equivalent operation factor of an EES system . 12
5.4 Efficiency-related operational performance testing items . 13
5.4.1 Comprehensive efficiency of an EES system . 13
5.4.2 Energy loss rate of an EES system . 13
5.4.3 Duty cycle roundtrip efficiency . 13
5.5 Reliability-related operational performance testing items. 13
5.5.1 Planned outage factor of an EES system . 13
5.5.2 Unplanned outage factor of an EES system . 13
5.5.3 Availability factor of an EES system . 13
5.6 Grid-connected operational performance testing items . 13
5.6.1 Impact of grid conditions on the performance . 13
5.6.2 Active power control test. 14
5.6.3 Reactive power control test. 14
5.6.4 Primary frequency regulation test . 14
5.6.5 Secondary frequency regulation test . 14
5.6.6 Grid-connected operation power quality test . 14
5.7 Operational performance testing items in specific applications . 15
5.7.1 Classification and summary of EES system applications . 15
5.7.2 Frequency regulation mode . 15
5.7.3 Fluctuation reduction mode . 15
5.7.4 Reactive power voltage control mode . 15
5.7.5 Power quality management mode . 15
5.7.6 Peak shaving/peak shifting mode . 16
5.7.7 Renewable firming mode . 16
5.7.8 Islanded grid mode . 16

5.7.9 Backup power mode . 16
6 Operational performance testing methods of EES systems . 16
6.1 General . 16
6.2 General operational performance testing method . 17
6.2.1 Genaral . 17
6.2.2 Actual energy storage capacity . 18
6.2.3 Input and output power rating . 18
6.2.4 Roundtrip efficiency . 18
6.2.5 Auxiliary power consumption . 19
6.3 Energy-related operational performance testing method . 20
6.3.1 General . 20
6.3.2 Available hours of an EES system . 20
6.3.3 Equivalent operation factor of an EES system . 21
6.4 Efficiency-related operational performance testing method . 22
6.4.1 General . 22
6.4.2 Comprehensive efficiency of an EES system . 22
6.4.3 Energy loss rate of an EES system . 23
6.4.4 Duty cycle roundtrip efficiency . 24
6.5 Reliability-related operational performance testing method . 25
6.5.1 General . 25
6.5.2 Planned outage factor of an EES system . 25
6.5.3 Unplanned outage factor of an EES system . 25
6.5.4 Availability factor of an EES system . 25
6.6 Grid-connected operational performance testing method . 26
6.6.1 General . 26
6.6.2 Active power control test. 26
6.6.3 Reactive power control test. 28
6.6.4 Primary frequency control . 30
6.6.5 Secondary frequency control . 32
6.6.6 Grid-connected operation power quality test . 32
6.7 Operational performance testing method in specific application . 32
6.7.1 Genaral . 32
6.7.2 Frequency regulation mode . 33
6.7.3 Fluctuation reduction mode . 33
6.7.4 Reactive power voltage control mode . 34
6.7.5 Power quality management mode . 34
6.7.6 Peak shaving/peak shifting mode . 34
6.7.7 Renewable firming mode . 35
6.7.8 Islanded grid mode . 36
6.7.9 Backup power mode . 36
Annex A (informative) performance indicators of primary concern to major
stakeholders . 37
Bibliography . 39

Figure 1 – Example of classification of EES systems . 10
Figure 2 – The prioritization and test procedures of the test methods . 11
Figure 3 – EES system architecture with one POC type . 17
Figure 4 – EES system architecture with two POC types . 17

– 4 – IEC TS 62933-2-3:2025 © IEC 2025
Figure 5 – Example of active power test curve in charging state of an EES system . 27
Figure 6 – Active power control response time of an EES system . 28

Table 1 – Example of typical and not exclusive applications classification . 15
Table 2 – Qualified frequency range of islanded mode . 36
Table A.1 – Performance indicators of primary concern to major stakeholders . 37

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL ENERGY STORAGE (EES) SYSTEMS –

Part 2-3: Unit parameters and testing methods –
Performance assessment tests during site operation

FOREWORD
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IEC 62933-2-3 has been prepared by IEC technical committee TC 120: Electrical Energy
Storage (EES) systems. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
120/392/DTS 120/413/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this Technical Specification is English.

– 6 – IEC TS 62933-2-3:2025 © IEC 2025
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 62933 series, published under the general title Electrical energy
storage (EES) systems, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
ELECTRICAL ENERGY STORAGE (EES) SYSTEMS –

Part 2-3: Unit parameters and testing methods –
Performance assessment tests during site operation

1 Scope
This part of IEC 62933 specifies the unit parameters and testing methods that validate and
assess the performance of EES systems after commissioning.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61000-4-30, Electromagnetic compatibility (EMC) – Part 4-30: Testing and measurement
techniques – Power quality measurement methods
IEC 62933-2-1:2017, Electrical energy storage (EES) systems – Part 2-1: Unit parameters and
testing methods – General specification
3 Terms, definitions, abbreviated terms and symbols
3.1 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.2 Abbreviated terms
AC Alternating current
EES Electrical energy storage
EESS Electrical energy storage system
POC Point of connection
SOE State of energy
3.3 Symbols
E actual energy storage capacity of an EES system
act_c
η roundtrip efficiency of an EES system
rt
E total output energy measured at (primary) POC
oδ
E total input energy measured at (primary) POC
iδ
– 8 – IEC TS 62933-2-3:2025 © IEC 2025
E
auxiliary power consumption during output operation
aux _ oδ
E
auxiliary power consumption during input operation
aux _iδ
E
auxiliary power consumption under reference environmental conditions during
ref_aux _ oδ
output operation
E
auxiliary power consumption under reference environmental conditions during
ref_aux _iδ
input operation
P rated power of an EES system
N
th
P rated power of i EESS unit
Ni
AH available hours of an EES system
AH charging hours of an EES system
c
AH discharging hours of an EES system
d
AH idle hours of an EES system
s
EOF equivalent operation factor of an EES system
th
EOF equivalent operation factor of i EESS unit
i
PH counted hours
η comprehensive efficiency of an EES system
EES
E output energy of an EES system during the assessment period
o
E input energy of an EES system during the assessment period
i
E
energy consumption of the auxiliary subsystem measured at the auxiliary
aux_o
POC during the output operation
E energy consumption of the auxiliary subsystem measured at the auxiliary
aux_i
POC during the input operation
R energy loss rate of an EES system
EES
η duty cycle roundtrip efficiency of an EES system
DCRT
E
output energy of an EES system during the duty cycle
DC_o
E
compensated output energy of an EES system during the duty cycle
com_DC_o
E
input energy of an EES system during the duty cycle
DC_i
E
compensated input energy of an EES system during the duty cycle
com_DC_i
E
energy consumption of the auxiliary subsystem during the duty cycle in output
aux_DC_o
stage
E energy consumption of the auxiliary subsystem during the duty cycle in input
aux_DC_i
stage
POF planned outage factor
POH planned outage hours
UOF unplanned outage factor
UOH unplanned outage hours
AF availability factor
A control accuracy of active power control
active
a
T response time of active power control
sr
A control accuracy of reactive power control
reactive
v regulation rate of the primary/secondary frequency regulation
EES
ΔP amount of deviation of the primary/secondary frequency regulation
EES
t
response time of the primary/secondary frequency regulation
f
S
maximum power sag of an integrated system
max_PS
U voltage deviation of POC of the reactive power voltage control
dev
D absolute peak-to-valley difference of peak shaving/peak shifting
EES
F voltage fluctuation rate of an EES system in islanded grid mode
v
4 General requirements of the operational performance testing
4.1 Purpose
The operational performance of an EES system can deteriorate or change throughout its entire
life cycle. Periodic testing and assessment of performance can reflect the health and utilization
status of an EES system during site operation, which allow the stakeholders to recognize the
EES project achievements, identify techniques and approaches that worked, and devise steps
to ensure they are used in the future.
The purpose of this document is to provide methods for testing and assessing the performance
of EES systems data during site operation. It specifies the requirements for data collection,
defines key performance indicators, and proposes methods for testing and assessment.
It should be noted that operational performance changes of an EES system are due to different
reasons. Some changes are caused by degradation of the EES system itself, and some are
caused by the operating environment or the operating conditions of the power grid to which the
EES system is connected.
The performance indicator test methods provided by this document are not for laboratory testing.
Each test includes varying boundary conditions, influenced by operational conditions and
environmental factors. The performance assessment results obtained from operational data
reflect the combined effects of various factors after the EES system is connected to the grid. It
provides stakeholders with a relative quantitative assessment for reference, not for precise
quantitative evaluation. If accurate performance indicator results are required, testing can be
conducted according to the methods described in IEC 62933-2-1.
This document considers all storage technologies that are capable of storing and releasing
electrical energy (from electricity to electricity), including the energy storage form shown in
Figure 1.
– 10 – IEC TS 62933-2-3:2025 © IEC 2025

Figure 1 – Example of classification of EES systems
4.2 Stakeholders
The stakeholders are as follows:
a) Primary stakeholders: EES system operators, grid operators, EES system investors,
laboratory researchers, and field-testing personnel seeking testing guidelines to
characterize EES systems and verify technical specifications. Annex A provides
performance indicators that the main primary stakeholders might be more interested in. This
is a recommendation, and the indicators to be tested depend on the specific assessment
needs of the stakeholders.
b) Secondary stakeholders: The suppliers and integrators of EES systems seeking guidance
on how to characterize technical performance and functional capabilities.
NOTE The EES system operators are entities responsible for the operation, maintenance, and optimization of EES
systems to ensure they meet expected performance and lifecycle targets. The grid operators are organizations
responsible for the real-time operation of the electrical power system, ensuring power supply and demand balance,
and maintaining system stability and reliability.
4.3 Principles of assessment intervals and data sampling
The assessment intervals can be tailored based on the assessment requirements, spanning
options such as daily, weekly, monthly, quarterly, or annually. However, these intervals should
be sufficiently frequent to capture significant changes of the EES system's performance.
Data sampling should adhere to principles of randomness, uniformity, and representativeness
to ensure the objectivity and impartiality of the test results. As a general recommendation, at
least three samples should be collected.
NOTE The sample refers to the event selected for assessing the performance items.
4.4 Data requirements
To ensure the validity and representativeness of the operational data used for assessing the
performance of an EES system, preliminary data processing is essential. This preprocessing
involves handling missing values, anomalies, and data noise. Furthermore, a time-series
analysis of the data should be conducted to identify trends and seasonal characteristics, which
will serve as a foundational reference for subsequent data sampling.

The basic information and operational data of an EES system mainly include:
a) Rated power, rated energy storage capacity and system structure of the EES system, etc.
b) Voltage level, main electrical connection, main equipment type and parameters, etc.
c) The structure and main monitoring items of the energy management system of the EES
system.
d) Operational data including power, voltage, current, electric quantity of EES units and the
EES system.
e) Fault, maintenance records of EES units and the EES system.
f) Grid dispatch logs of the EES system.
g) Other.
4.5 Operational performance testing methods
The operational performance testing methods of an EES system include reviewing existing test
reports, operational data analysis and on-site testing. In accordance with the recommended
prioritization, the optional methods for assessing operational performance items are as follows:
a) Review the most recent periodic test reports, the most recent maintenance test reports, and
other relevant documents of an EES system to obtain test results of the performance item.
b) If the test results for the performance item in the testing period are not available, investigate
the historical data recorded by the monitoring system during the testing period. Read the
required data of the performance item and record this event as a sample event. Record
several sample events according to the characteristics of the performance item and
assessment objectives.
c) If there is neither corresponding report nor qualified historical data, and the on-site testing
is necessary, the performance item should be tested under operating field conditions of the
EES system. The performance item test of an EES system should be assessed in
accordance with the corresponding methods.
The prioritization and test procedures for the above optional test methods are shown in Figure 2.

Figure 2 – The prioritization and test procedures of the test methods

– 12 – IEC TS 62933-2-3:2025 © IEC 2025
NOTE For large-scale EES systems, test items involving frequent charging and discharging, and other operations
mainly rely on historical data for assessment, and on-site testing is just a optional step with the user selecting test
methods according to the actual situation.
5 Operational performance testing items
5.1 General
The operational performance testing of EES systems includes the following main aspects:
• general operational performance testing;
• energy-related operational performance testing;
• efficiency-related operational performance testing;
• reliability-related operational performance testing;
• grid-connected operational performance testing;
• operational performance testing in specific application.
5.2 General operational performance testing items
5.2.1 Actual energy storage capacity
Actual energy storage capacity shall be defined as energy storage capacity of an EES system
at a given time of the service life as a result of several factors (mainly degradation and operating
stresses). Here the actual energy storage capacity is linked to output energy storage capacity.
5.2.2 Input and output power rating
Input and output power rating shall be defined as the value of power that an EES system can
absorb or provide for a specified period at the POC.
5.2.3 Roundtrip efficiency
Roundtrip efficiency shall be defined as discharged energy measured at the primary POC
divided by the energy absorbed, as a sum of what is measured at all the POCs (primary and
auxiliary), over one EESS predetermined charging-discharging cycle in a specified operating
mode at continuous operating conditions.
5.2.4 Auxiliary power consumption
Auxiliary power consumption shall be defined as average active power requested by the
auxiliary subsystems of an EES system in a specified time and in a specified operating mode
at continuous operating conditions.
Reference auxiliary power consumption shall be defined as the power necessary to operate the
auxiliary subsystem under standard testing conditions defined in IEC 62933-2-1:2017, 5.1.3.
5.3 Energy-related operational performance testing items
5.3.1 Available hours of an EES system
Available hours of an EES system shall be defined as the weighted average available hours of
each EESS unit based on their rated power.
5.3.2 Equivalent operation factor of an EES system
Equivalent operation factor of an EES system shall be defined as the weighted average of each
EESS unit based on the equivalent operation factor and their rated power.

5.4 Efficiency-related operational performance testing items
5.4.1 Comprehensive efficiency of an EES system
Comprehensive efficiency of an EES system shall be defined as the ratio of output energy to
input energy during the assessment period, including non-operational states.
5.4.2 Energy loss rate of an EES system
Energy loss rate for an EES system shall be defined as the ratio of the total electricity loss in
the charging and discharging process to the input energy during the assessment period.
5.4.3 Duty cycle roundtrip efficiency
Duty cycle roundtrip efficiency shall be defined as energy discharged measured at the primary
POC divided by the energy absorbed by the EES system, as a sum of what is measured at all
the POCs (primary and auxiliary), during duty cycles in a specified operating mode at continuous
operating conditions with the same final state of energy as the initial state of energy.
5.5 Reliability-related operational performance testing items
5.5.1 Planned outage factor of an EES system
Planned outage factor of an EES system shall be defined as the ratio of planned outage hours
to the counted hours during the assessment period. Planned outage hours are the total outage
time scheduled in advance measured in hours.
5.5.2 Unplanned outage factor of an EES system
Unplanned outage factor of an EES system shall be defined as the ratio of unplanned outage
hours to the counted hours during the assessment period. Unplanned outage hours are the total
outage time that has not been scheduled in advance measured in hours.
5.5.3 Availability factor of an EES system
Availability factor of an EES system shall be defined as the ratio of available hours to the
counted hours during the assessment period. Available hours are the total available time
measured in hours.
5.6 Grid-connected operational performance testing items
5.6.1 Impact of grid conditions on the performance
The grid-connected operational performance indicators, including active or reactive power
control, frequency or voltage regulation, and power quality, are shaped both by the
characteristics of an EES system and the conditions of the power grid. The operational data at
the POC only partially reflects the actual performance of an EES system after being connected
to the power grid.
To determine if a particular grid-connected performance indicator stems from the characteristics
of an EES system or from the influences of the grid, one method is to compare the performances
before and after integration of an EES system to the grid. Performance tests shall be conducted
in accordance with the methods provided in IEC 62933-2-1.

– 14 – IEC TS 62933-2-3:2025 © IEC 2025
5.6.2 Active power control test
Active power control test includes the active power steady-state control assessment and the
active power dynamic-state control assessment.
The active power steady-state control assessment aims to assess the relative active power
deviation between the measured active power value at the POC and the set value of the grid
dispatch after the completion of the active power regulation of an EES system.
The active power dynamic-state control assessment aims to assess the response time of an
EES system during the execution of active power commands as a dynamic response indicator.
5.6.3 Reactive power control test
Reactive power control test includes the reactive power steady-state control assessment and
the reactive power dynamic-state control assessment.
The reactive power steady-state control assessment aims to assess the relative reactive power
deviation between the measured reactive power value at the POC and the set value of the grid
dispatch after the completion of reactive power regulation in an EES system.
The reactive power dynamic-state control assessment aims to assess the response time of an
EES system during the execution of reactive power commands as a dynamic response indicator.
5.6.4 Primary frequency regulation test
An EES system can be used to stabilize the electric power system frequency on a steady state
value through the capability to respond to a measured frequency deviation.
The primary frequency regulation is activated by an autonomous primary control system with a
dead time of less than a few seconds from the measured frequency deviation and fully activated
according to a given ramp rate.
The primary frequency regulation test aims to assess indicators including the regulation rate,
regulation accuracy, and response time.
5.6.5 Secondary frequency regulation test
An EES system can be used to restore system frequency to the nominal system frequency
usually following a primary frequency regulation.
The secondary frequency regulation is manually or automatically activated usually following a
primary frequency regulation.
The secondary frequency regulation test aims to assess indicators including the
regulation rate, regulation accuracy, and response time.
5.6.6 Grid-connected operation power quality test
Grid-connected operation power quality test includes measurement of harmonics,
interharmonics, voltage fluctuation, flicker, voltage unbalance test.
NOTE The current set of power quality indicators are of more concern during operation. If necessary,
IEC 61000-4-30 can be referred to to test other power quality indicators.

5.7 Operational performance testing items in specific applications
5.7.1 Classification and summary of EES system applications
The application of an EES system can be classified into three classes, and the representative
applications are described in Table 1.
a) Class A applications (short-duration application/power intensive application): EES system
application demanding in terms of step response performance and with frequent charge and
discharge phase transitions or with reactive power exchange with the electric power system.
b) Class B applications (long-duration application/energy intensive application): EES system
application with long charge and discharge phases at variable powers.
c) Class C applications (hybrid and emergency application): EES system application
demanding in terms of step response performance but with frequent and long discharge
phases at variable discharge power.
Table 1 – Example of typical and not exclusive applications classification
Classifications Class A Class B Class C
1. Frequency regulation
1. Peak shaving/ peak
2. Fluctuation reduction
shifting
Typical applications 1. Backup power
3. Reactive power voltage
2. Renewable firming
control
3. Islanded grid
4. Power quality management
5.7.2 Frequency regulation mode
An EES system used for the stabilization of the electric power system frequency through active
power exchange.
Frequency regulation mode includes primary frequency regulation test and secondary frequency
regulation test.
5.7.3 Fluctuation reduction mode
An operational mode of an EES system used to compensate the power fluctuation of power
generation units (especially renewable energy sources) at the POC by absorbing active power
at times of high generation and by feeding in additional active power at times of low generation.
The indicators of fluctuation reduction mode include smoothness and maximum power sag.
5.7.4 Reactive power voltage control mode
An EES system used for the stabilization of the voltage at the primary POC or neighbouring
nodes through active or reactive power exchange.
Reactive power voltage control mode includes voltage deviation of POC test.
5.7.5 Power quality management mode
The power quality management mode is a short-term application of an EES system that
mitigates power quality issues at the POC through the exchange of active or reactive power
between the EES system and the power grid.
The indicators of power quality control mode, including the voltage unbalance ratio and the total
harmonic distortion (THD) of voltage or current, among others, should be determined based on
the specific power quality issues that should be addressed.

– 16 – IEC TS 62933-2-3:2025 © IEC 2025
5.7.6 Peak shaving/peak shifting mode
An EES system discharges stored energy into the grid upon an excess or peak of demand or
absorbs excess energy, available in the grid, for storage. With this balancing a time shift
between power generation and electricity usage is achieved.
The indicator of this mode includes peak shaving/peak shifting power.
5.7.7 Renewable firming mode
An EES system used to provide energy to supplement renewable power generation such that
their combination produces steady power output over a desired time window. More precisely,
the purpose of renewable firming is to provide energy (or conversely, to absorb energy) when
renewable generation falls below some threshold (or conversely, exceeds this threshold).
The indicator of renewable firming mode includes tracking plan deviation.
5.7.8 Islanded grid mode
Island resulting from planned action(s) of automatic protections, or from deliberate action by
the responsible network operator, or both, in order to keep supplying electrical energy to a
section of an electric power system.
The indicators of islanded grid mode include voltage fluctuation and frequency stability.
5.7.9 Backup power mode
A hybrid application of an EES system used to provide electrical energy during a specified time
and for a pre-defined maximum power, during which the main electrical energy supply is not
available at the primary POC.
The indicators of backup power mode include output voltage unbalance and harmonics.
6 Operational performance testing methods of EES systems
6.1 General
The testing methods are required to assess the operational performance of EES systems to
comply with operational performance testing items.
The typical architecture of an EES system is shown in Figure 3. The boundary between an EES
system and the electrical power system is defined as POC. Each parameter defined in this
document shall be measured at the POC.

Figure 3 – EES system architecture with one POC type
If the auxiliary subsystem is fed from another feeder, the optional architecture of the EES
system is shown in Figure 4.
Figure 4 – EES system architecture with two POC types
6.2 General operational performance testing method
6.2.1 Genaral
The following parameters should be specified as the main electrical parameters to assess EES
system capability and performance:
– actual energy storage capacity (Wh);
– input and output power rating (W);
– roundtrip efficiency (%);
– auxiliary power consumption (W).

– 18 – IEC TS 62933-2-3:2025 © IEC 2025
6.2.2 Actual energy storage capacity
a) Refer to step a) in 4.5, and obtain the test results of the actual energy storage capacity
(output energy storage capacity) of an EES system in the most recent periodic/maintenance
test reports. If no relevant reports or information are available, perform step b).
b) Refer to step b) in 4.5. The required data for the actual energy storage capacity is the
capacity discharged at the POC from its full state to empty state, usually at rated output
power. Select at least several data samples (e.g., at least three) from the latest operational
data during the testing period, and the average value of the records is the actual energy
storage capacity of an EES system. If the auxiliary subsystem is fed from another feeder as
shown in Figure 4, additional auxiliary system losses should be subtracted from the total
output energy. If there is neither correspon
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