In this article
This energy storage glossary helps you understand a technical proposal, equipment data sheet and project description consistently. Abbreviations can stand for different things: the installation itself, its components, the current state of the battery or an identifier for accounting. Look up a term in the index, read the explanation, and check how it's used in your facility's documentation. The same abbreviation does not guarantee the same measurement conditions.
System and units of measurement
Energy storage system and battery storage
ESS stands for energy storage system. It stores energy for further use. BESS means battery energy storage system. These concepts are related, but the word "battery" does not describe the entire composition of the installation: it also requires conversion, control, protection and auxiliary systems for operation.
In the offer, specify whether it is a separate battery module, a ready-made cabinet or a complete complex on the site. This determines the scope of supply and the additional work required. The general principle of operation and composition is explained in the article what is ESS.
Power and capacity: kW and kWh
kW characterizes power, that is, the rate of energy transfer. kWh characterizes the amount of energy. The power answers the question about the load that the system can support, and the energy capacity helps to estimate the duration of such work.
Illustrative example without losses: 100 kWh of available energy would be enough for two hours with a constant load of 50 kW. The actual result depends on the allowable charge range, losses, auxiliary consumption and equipment limitations. Therefore, the nameplate capacity of the battery is not automatically equal to the energy that the consumer will receive. Calculations are analyzed separately in the material on power and capacity of ESS.
Alternating and direct current
AC, or Alternating Current, means alternating current; DC, or Direct Current, means direct current. The battery works on the DC side, and connecting to a regular power grid requires a suitable conversion. The measurement side designation is important for comparing power, energy and losses.
If one quote shows the energy at the battery terminals and the other at the output of the entire installation, the numbers cannot be directly compared. First, you need to agree on the boundaries of the system. Just specifying "where measured" is often more useful than an extra digit after the comma.
Control, conversion and supervision
Battery Management System
BMS, or Battery Management System, monitors battery health. It receives data on voltage, current and temperature and maintains operation within acceptable limits. Depending on the design, its functions may include state of charge assessment, cell balancing, and protection signal transmission.
BMS should not be taken as universal protection against any malfunction. The result depends on the sensors, actuators, settings and coordination of the components. When evaluating equipment, ask what parameters are monitored, what events are logged, and who has access to the log. The presence of the name BMS in the equipment list does not replace the description of these functions.
Energy Management System
EMS, or Energy Management System, coordinates energy operating modes: when to charge the battery, when to discharge energy and what limitations to consider. Input data can be consumption, generation, available charge, set priorities and signals from other systems.
For example, an enterprise may require to keep part of the energy for backup. The normal discharge plan must then take this reserve into account. EMS does not create additional capacity or remove battery limits. For a specific project, you need to agree on the available modes, the procedure for changing settings and the behavior in case of loss of communication.
Power conversion system
PCS, or Power Conversion System, provides the conversion of electrical power between the battery side and the AC side. Appropriate bidirectional operation is required for charging and discharging the battery ESS. Specific functions are determined by the converter design and settings.
Not every device with the name "inverter" performs all the functions that a storage device needs. Compatibility, voltage ranges, power in both directions and intended modes are checked. On the Sungrow equipment page, you can explore the relevant product ranges, but compatibility is confirmed by the documentation of a specific configuration, and not by the name of the manufacturer.
Supervisory control and data acquisition
SCADA, or Supervisory Control and Data Acquisition, stands for supervisory control and data acquisition system. It helps to see equipment parameters, signals, events and facility status. Depending on the implementation, control commands are also transmitted through it.
The roles of SCADA and EMS can partially overlap in the same software product, so an agreed list of functions is important. Find out what data is stored, how long history is available, who can change modes, and how such changes are recorded. A good monitoring screen alone does not confirm the completeness of management.
Power plant controller
PPC, or Power Plant Controller, coordinates the execution of the specified modes and the adjustment of parameters at the connection point of the power plant. Its role must be distinguished from the control of an individual battery and from the planning of energy use.
For an integrated facility, it is specified where the task comes from, which device performs it, and how the result is monitored. A separate function name does not always mean a separate physical cabinet: part of the capabilities can be combined. In the terms of reference, it is important to record exactly the responsibility for regulation and data exchange, and not merely list abbreviations.
Battery status and performance indicators
Charge level
SOC, or State of Charge, shows the current charge level relative to the accepted base capacity. It is usually displayed as a percentage. For a correct reading, you need to know how the system estimates the charge and what capacity it uses as a basis.
The mark of 80% does not automatically mean that all of that charge is available to the load. Part of the range may remain unavailable due to operational constraints or a backup reserve. Also, the SOC does not show the permissible discharge power. It is checked separately, taking into account the state of the battery and the installation mode.
The same 50% on two batteries of different capacities also does not mean the same energy reserve. Work planning requires available kWh and allowable power, not just a charge indicator.
Battery Health
SOH, or State of Health, describes the change in battery performance compared to an initial or other established reference state. The assessment can be based on residual capacity, change in internal resistance, or other indicators. The method and conditions of evaluation need to be clarified.
SOH is not equal to the current charge and is not a ready prediction of the number of years of operation. The battery may be fully charged, but have less available capacity than at the beginning of operation. Compare benchmark results, not just percentages across screens.
Depth of discharge
DoD, or Depth of Discharge, characterizes the used portion of the base capacity during discharge. For a specific cycle, it is important to specify the initial and final charge levels. The working range is established in accordance with the documentation and system task.
Illustrative example: reducing SOC from 80% to 30% means using 50 percentage points for an unchanged evaluation base. This is not the same as a discharge from 100% to 30%. Therefore, the formula "100% minus the current SOC" cannot be used without clarification for the depth of any partial cycle.
Round-trip efficiency
RTE, or Round-Trip Efficiency, shows the ratio of the energy returned by the system after storage to the energy spent on charging in the agreed cycle. The indicator depends on the measurement limits and operating conditions.
Illustrative example: If the system received 100 kWh and returned 90 kWh under comparable conditions and the same initial and final charge levels, the RTE is 90%. This is an illustration, not a specification for a particular model. To compare offers, check whether the converter, transformer, cooling and other auxiliary consumption are included. The RTE of the battery unit should not be automatically transferred to the entire installation.
Connection and identification
Energy identification code
EIC, or Energy Identification Code, is a standardized identifier in the energy data exchange system. Its purpose depends on the type: the code can identify a participant, an area, or another relevant object. This is not a technical characteristic of the storage system.
In the documents, specify exactly which code is needed and what it identifies. The participant's code should not be replaced by the accounting point's code. An assigned ID alone does not mean that all contracts, permissions or accounting settings are ready. The procedure is defined for a specific role and configuration.
Connection point and commercial metering
The connection point describes the place of electrical connection of the installation to the corresponding network. Commercial metering provides electricity data for settlement and participants’ interactions. These concepts are related, but not interchangeable.
The diagram should show connections, measurements and limits of responsibility. The sum of the equipment's capacities does not automatically determine the permitted withdrawal or discharge through the connection point. Organizational differences are explained by a comparison of models of ESS use.
Which concepts are most often confused
| Pair | Distinction |
|---|---|
| kW and kWh | Power and amount of energy. |
| BMS and EMS | Battery control and coordination of energy modes. |
| EMS and PCS | Mode control and physical power conversion. |
| SOC and SOH | Current charge and evaluation of changes in battery characteristics. |
| RTE and Degradation | Cycle efficiency and change in characteristics during operation. |
How to use the glossary to review a proposal
- Mark unfamiliar abbreviations and ask for full names.
- Beside each number, note the unit, limit, and measurement conditions.
- Distinguish the battery, converter and software control functions.
- Check which parameters are documented and which require calculation for the facility.
For example, when opening a Sungrow PowerStack 255CS-2H card, use this order to read the specifications. The link helps to navigate to the equipment, but does not constitute an opinion on its suitability. A table of disputed terms should be added to the technical task: what is written, how the customer understands it, and what confirmation is required from the supplier.
For an operating solar power plant, it is also important to agree on who will review event logs and respond to deviations. On the page solar power plant maintenance, you can specify the scope of work for the solar part. Battery diagnostics, access to its data, and management program support must be separately included in the agreed scope of service.