BESS Basics for Beginners

General Terms

BESS (Battery Energy Storage System)
A system that stores electrical energy in batteries for later use. It includes batteries, inverters, controllers, and other components.

ESS (Energy Storage System)
A broader term for systems that store energy in various forms, including batteries, mechanical systems, or thermal systems.

Battery
An enclosure that contains cells, electrical hardware and firmware. A certified battery includes a BMS that safeguards the health of the cells and gatekeeps charging and discharging.

Management Systems

EMS (Energy Management System)
A software system used to monitor, control, and optimize energy generation, storage, and consumption within a facility.

BMS (Battery Management System)
A system designed to monitor and manage a battery’s performance, ensuring safety, longevity, and optimal operation.

BESS Variants

Modular
A BESS designed with modular components, allowing scalability and easier maintenance by adding or replacing individual modules.

Cabinet
A Cabinet is a compact, enclosure for an energy storage solution, typically used in commercial, industrial, or residential settings, it can contain battery modules, PCS/Inverters, power electronics, and thermal management housed in a cabinet.

Container
An energy storage system can be housed in a container-like structure. The container can either be the same dimensions as traditional shipping containers or have a more specific form. Some containers cannot be entered into, and some have doors that makes it possible to access the content for service.

Power System

DC (Direct Current)
An electric current that is uni-directional, so the flow of charge is always in the same direction. As opposed to alternating current (AC), the direction and amperage of direct currents do not change. It is used in many household electronics and in batteries.

AC (Alternating Current)
An electric current that periodically reverses direction and changes its magnitude continuously with time, in contrast to direct current (DC), which flows only in one direction. Is used in all electricity Grids.

Inverter
A device that converts DC power stored in batteries to AC power for use in standard electrical systems. It is also used to convert AC power in the grid to DC power so that the battery can be charged and discharged. There are also DC/DC inverters that can change voltage to higher or lower.

PCS – Power Conversion System
The power conversion system typically includes an inverter. Sometimes circuit breakers etc are included in this as well as they are needed when an inverter is added.

kW or kVA (kilo Watt or kilo Volt-Ampere)
Units used to indicate the power output/input from/to a BESS. This property is highly dependent on the inverter and together with the battery it can deliver a certain power.

Performance Metrics

C-rate (Charge/Discharge Rate)
A measure of how quickly a battery charges or discharges relative to its capacity. For example, a 1C rate means the battery fully charges or discharges in one hour.

RTE (Roundtrip Efficiency)
The percentage of energy that can be recovered from a BESS compared to the energy initially stored. Higher RTE means less energy loss.

SoC (State of Charge)
The current charge level of a battery, expressed as a percentage of its total capacity.

SoH (State of Health)
A measure of a battery’s overall condition and its ability to deliver the expected performance compared to its original specifications.

DoD (Depth of Discharge)
The DoD of a battery is a set available range of the SoC. Lithium batteries will have a longer lifespan when they are not 100% charged and discharged down to 0%, but rather in the range of 90% down to 10% SoC, which equals 80% DoD (90 minus 10).

kWh (kilo Watt hours)
The amount of energy stored in a BESS in the SoC 0-100% range. Usually, kWh is the unit used for storage up to 1000 kWh where you instead use MWh as the unit to describe the amount of energy stored in a BESS.

MWh (Mega Watt hours)
Unit used to describe the amount of energy that can be stored in a BESS.

Cycle
The total energy thruput corresponding to a battery going from full to empty and back to full charge. A full “cycle” is usually a combination of many smaller charges up and down, amounting to the rated energy of the battery.

Available Energy
Available energy is used to describe what metrics are available for the end user to use. For example, DoD is the pre-set available range of a battery (Usable energy) compared to the Rated Energy (100% SoC down to 0% SoC).

Applications and Use-cases

VPP (Virtual Power Plant)
A network of distributed energy resources, including BESS, aggregated and managed as a single entity to provide energy services to the grid.

Ancillary Services
Services provided by BESS to stabilize and support the electrical grid, such as frequency regulation and voltage support.

Load Shifting
The process of storing energy during low-demand periods and using it during high-demand periods to optimize energy costs and efficiency.

Peak Shaving
Reducing energy consumption during peak demand times to lower electricity costs and avoid overloading the grid.

Power Boost
A Polarium developed configuration in the BESS and EMS to enable more power (kW) available to the EV Chargers than the limit of the Grid connection size.

C&I BESS

C&I BESS (Commercial and Industrial)
Batteries specifically designed for use in commercial and industrial settings, often larger and more robust than residential systems.

Electrical functionality, offered in some BESS-products

Reserve Power

Reserve Power refers to stored energy within a BESS that is kept on standby for back-up or peak demand periods. It ensures stability, backup power, and grid balancing, helping industries, businesses, and utilities maintain reliability during unexpected shortages or failures.

UPS (Uninterruptible Power Supply)

A UPS (Uninterruptible Power Supply) is a battery-powered backup system that provides instant power during outages or voltage fluctuations. Unlike traditional backup generators, a BESS-based UPS offers seamless, reliable energy for critical loads, preventing downtime and damage from power disruptions.