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Glossary of battery storage, EMS, PPC, and solar monitoring terms
One sentence definitions of the terms a developer, asset owner, or utility meets in battery storage controls, grid interconnection, solar monitoring, tariffs, and the standards that govern them. Each term has its own page with context, related terms, a primary source, and a link to the guide that covers it in depth.
60 terms. Updated . Written by the engineers who build and operate these systems.
Control stack
- Battery energy storage system (BESS)
- A battery energy storage system (BESS) is a battery, its power conversion system, and its controls, installed to store electricity and deliver it back to a site or the grid on command.
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- Battery management system (BMS)
- A battery management system (BMS) is the controller inside a battery that measures cells, protects them, and publishes the charge and discharge limits the battery can safely accept.
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- Data acquisition system (DAS)
- A data acquisition system (DAS) is the hardware and software that polls every device on a solar or storage site and records the readings on a fixed interval with a trustworthy timestamp.
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- Data logger
- A data logger is the device on a solar or storage site that polls inverters, meters, and sensors over Modbus or other protocols and forwards the readings to a monitoring platform.
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- Dispatch algorithm
- A dispatch algorithm is the control logic inside an energy management system that decides what a battery does, such as charging in cheap hours or holding a facility under a demand ceiling.
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- EMS run mode
- An EMS run mode is the state an energy management system is started in, which decides whether its commands reach real equipment.
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- Energy management system (EMS)
- An energy management system (EMS) is the software that decides when a battery charges, when it discharges, and how much power it moves.
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- Grid forming inverter
- A grid forming inverter sets its own voltage and frequency and can hold up a microgrid alone, while a grid following inverter synchronizes to a voltage that already exists.
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- Historian
- A historian is the time series database that stores every measurement and event a SCADA or energy management system collects, so operators can chart, audit, and report on what the plant did.
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- Inverter based resource (IBR)
- An inverter based resource (IBR) is a generating or storage resource, such as solar PV, wind, or a battery, that connects to the grid through power electronics rather than a rotating machine.
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- Islanding
- Islanding is the condition in which a solar or storage system keeps energizing a section of the electrical system after it has been disconnected from the utility grid.
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- Microgrid
- A microgrid is a group of interconnected loads and energy resources within a defined electrical boundary that acts as a single controllable entity and can connect to or disconnect from the grid.
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- Power conversion system (PCS)
- A power conversion system (PCS) is the bidirectional inverter that converts a battery’s DC power to AC for the grid and back again to charge it.
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- Power plant controller (PPC)
- A power plant controller (PPC) is the real time controller that holds a solar or storage plant at its setpoint or utility command at the point of interconnection.
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- SCADA
- SCADA (supervisory control and data acquisition) is the system that polls a site’s devices, raises alarms, stores history, and relays telemetry to the utility, without running any optimization.
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- Setpoint
- A setpoint is the target value a controller sends to a device, such as the active power in kilowatts an energy management system commands a battery inverter to deliver.
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- Site controller
- A site controller is the industrial computer at a solar or storage site that runs the energy management system and power plant controller locally, so dispatch continues when the cloud connection drops.
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Grid and interconnection
- Ancillary services
- Ancillary services are the grid support functions, such as frequency regulation, operating reserves, voltage support, and black start, that a grid operator buys to keep the system stable.
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- Curtailment
- Curtailment is a reduction in a plant’s output below what it could produce, ordered by the utility or grid operator or required by an export limit at the point of interconnection.
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- Frequency droop
- Frequency droop is a control rule that changes a plant’s active power in proportion to the grid frequency’s deviation from nominal, raising output when frequency falls and lowering it when frequency rises.
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- Frequency regulation
- Frequency regulation is a grid service in which a battery follows a signal from the grid operator every few seconds, charging and discharging to hold system frequency at nominal.
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- Interconnection agreement (IA)
- An interconnection agreement is the contract between a plant owner and the utility or grid operator that sets the technical and operating conditions for connecting the plant to the grid.
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- Point of interconnection (POI)
- The point of interconnection (POI) is the electrical location where a solar or storage plant connects to the utility system and where its grid code compliance is measured.
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- Ramp rate control
- Ramp rate control limits how fast a plant’s active power may change, in megawatts or kilowatts per minute, so a cloud passing or a dispatch step does not shock the grid.
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- Volt-var control
- Volt-var control adjusts a plant’s reactive power as a function of measured voltage, absorbing reactive power when voltage is high and injecting it when voltage is low.
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Battery metrics
- C-rate
- C-rate is the rate at which a battery is charged or discharged relative to its capacity, where 1C empties the battery in one hour and 0.5C empties it in two.
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- Depth of discharge (DoD)
- Depth of discharge (DoD) is the share of a battery’s capacity that has been withdrawn, so a battery at 30 percent state of charge is at 70 percent depth of discharge.
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- Power rating and energy capacity
- Power rating, in kW or MW, is how much a battery can deliver at one moment; energy capacity, in kWh or MWh, is how much it can deliver in total before it is empty.
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- Round trip efficiency (RTE)
- Round trip efficiency (RTE) is the energy a battery returns divided by the energy put into it over a full charge and discharge cycle, measured at the AC terminals.
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- State of charge (SOC)
- State of charge (SOC) is the energy remaining in a battery expressed as a percentage of its usable capacity, with 100 percent full and 0 percent empty.
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- State of health (SOH)
- State of health (SOH) is a battery’s present capacity, and sometimes its internal resistance, expressed as a percentage of its value when new.
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- Storage duration
- Storage duration is a battery’s energy capacity divided by its power rating, in hours: a 2 MW, 8 MWh system is a four hour battery.
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Economics and tariffs
- Behind the meter (BTM)
- Behind the meter describes solar or storage installed on the customer’s side of the utility meter, serving the site’s own load and lowering its bill.
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- Demand charge
- A demand charge is the part of a commercial electricity bill priced in dollars per kilowatt on the highest average power the facility drew in any single interval, usually fifteen minutes, during the billing period.
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- Energy arbitrage
- Energy arbitrage is charging a battery when electricity is cheap and discharging it when electricity is expensive, earning the price difference less the round trip losses.
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- Front of the meter (FTM)
- Front of the meter describes solar or storage connected directly to the utility or transmission system, selling energy and services into markets rather than serving one customer’s load.
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- Load forecasting
- Load forecasting is predicting how much power a facility or grid will draw in coming intervals, from historical meter data, weather, schedules, and calendar effects.
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- Peak shaving
- Peak shaving is discharging a battery during the intervals when a facility draws the most power from the grid, so its measured peak demand and the demand charge billed on it are lower.
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- Revenue stacking
- Revenue stacking is earning from several storage applications with one battery, such as demand charge reduction, arbitrage, frequency regulation, and backup, by allocating its state of charge among them.
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- Solar self consumption
- Solar self consumption is storing surplus solar generation in a battery during the day and using it on site later, instead of exporting it to the grid at a low price.
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- Time of use tariff (TOU)
- A time of use tariff is an electricity rate that charges different prices for energy, and often for demand, depending on the hour, the day of the week, and the season.
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Monitoring and performance
- Availability
- Availability is the share of time a solar or storage plant, or a piece of its equipment, was able to operate, usually reported as a percentage over a month or a year.
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- Capacity factor
- Capacity factor is the energy a plant produced over a period divided by the energy it would have produced running at full rated power the whole time.
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- Performance ratio (PR)
- Performance ratio (PR) is a solar plant’s actual energy output divided by the output expected from the measured irradiance at nameplate rating, a dimensionless measure of how well the plant converts sunlight.
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Standards, protocols, and programs
- DNP3
- DNP3 is the SCADA protocol, standardized as IEEE 1815, that utilities use to exchange time stamped measurements, events, and controls between a control center and outstations such as a plant controller.
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- IEC 62443
- IEC 62443 is the international series of standards for cybersecurity of industrial automation and control systems, covering the asset owner, the system integrator, and the product supplier.
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- IEEE 1547
- IEEE 1547 is the standard for interconnecting distributed energy resources, such as rooftop and commercial solar and batteries, with distribution systems.
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- IEEE 2800
- IEEE 2800 is the standard that sets minimum interconnection and performance requirements for inverter based resources, such as solar, wind, and batteries, connecting to transmission and subtransmission systems.
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- Modbus
- Modbus is the open master and slave protocol that most inverters, meters, and battery systems use to expose their readings and accept commands as numbered registers.
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- MQTT
- MQTT is a lightweight publish and subscribe messaging protocol in which devices publish readings to topics on a broker and any authorized subscriber receives them.
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- NERC CIP
- NERC CIP is the set of mandatory Critical Infrastructure Protection reliability standards that govern the cybersecurity of the bulk electric system in North America.
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- Performance Data Provider (PDP)
- A Performance Data Provider (PDP) is a company approved by the Self Generation Incentive Program to collect metered data from an incentivized storage system and report it to the program administrator.
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- Self Generation Incentive Program (SGIP)
- The Self Generation Incentive Program (SGIP) is the California program that pays incentives for behind the meter energy storage and other distributed generation, with monitoring and performance rules attached.
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- SunSpec Modbus
- SunSpec Modbus is an open information model that standardizes the Modbus register maps of solar inverters, meters, and storage systems, so one driver can read many brands.
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Operations and security
- Audit log
- An audit log is the tamper evident record of who did what in a control system and when, covering logins, configuration changes, mode switches, and every setpoint written.
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- Commissioning
- Commissioning a storage site in an EMS is describing the plant to the software: its devices, how they are wired, which logic dispatches them, what the tariff is, and which systems may read the result.
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- Event log
- An event log is the time ordered record of what a control system noticed: alarms raised and cleared, devices lost and recovered, mode changes, and limits reached.
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- Role based access control (RBAC)
- Role based access control (RBAC) grants each user of a control system the permissions of a role, such as viewer, operator, or administrator, rather than individual rights.
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- Vendor agnostic
- A vendor agnostic energy management system controls batteries, inverters, and meters from any manufacturer through open protocols, instead of only the hardware its maker sells.
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