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Greener Upgrades blog series #3

A Guide to Battery Energy Storage Systems (BESS)

What is a Battery Energy Storage System (BESS)?

Battery Energy Storage Systems (BESS) have gained significant attention due to them offering numerous benefits including reducing emissions, fuel usage and cost. However, this is a complex technology, and it is important to consider numerous factors before choosing the right BESS for your application. In this article, we discuss key terminologies and equipment needed for battery storage integration. By using batteries, we are helping our customers optimise their temporary power setups.


Battery Energy Storage Systems, or BESS, are innovative energy storage solutions that store electrical energy in batteries for later use. They play a crucial role in power stability on grid or off grid optimising energy usage, and reducing emissions, fuel usage and providing additional capacity.

Main components of BESS:

  • Battery cells: Battery cells store electrical energy by converting it into chemical energy and are commonly assembled in modules and packs to meet the desired capacity.
  • Inverters: These convert direct current (DC) energy to alternating current (AC). This allows electrical energy to be converted, stored, and reused.
  • Controller: Controls the battery cells’ performance for safe operation. It also maintains the balance of charge and allows the BESS to be monitored remotely. Controls are key to how the BESS will operate with other assets.

Common BESS terminology

Understanding key battery energy terminology is essential for effective utilisation. Here are some key terms:

  • State of Charge (SoC): represents the current energy level of a battery. It is often illustrated graphically to provide a visual representation of the battery's charge status.
  • Depth of Discharge (DoD): indicates the amount of a battery's capacity that has been used. 
  • Round-Trip Efficiency (RTE): is expressed as a percentage and represents the ratio of energy recovered from the BESS compared to the energy input, accounting for losses such as heat.
  • BESS cycle rate: how many times the BESS cycles each day (fully charges and discharges). 
  • Max charging current: the maximum charging current for a BESS is determined by battery capacity, chemistry, and the charger or inverter used in the system.
  • Hybridising: adding multiple power sources within a solution, this could be adding battery storage to a generator package solution or even to a solar package. A hybrid power package is made by combining two or more different forms of power.
  • Pass-through limit: defines the maximum amount of energy that can flow through a BESS without being stored. The pass-through limit is a vital consideration when pairing a generator with a BESS.
  • Spinning reserve: generating capacity that is active and prepared to provide immediate power during increases in power demand or unanticipated loss of other power sources.
  • Spinning reserve displacement: using stored energy in batteries to replace or reduce the need for typical sources of spinning reserve such as generators. 

How are Battery Energy Storage Systems rated?

Energy storage capacity

Measured in kilowatt-hours (kWh), this refers to the amount of energy that can be stored. If a battery energy storage system has a higher energy storage-to-power ratio, it is well suited for applications like spinning reserve displacement, storing excess renewable energy, and diesel and fuel displacement. 

Power assistance or power rating

Measured in kilowatt kW this refers to the highest power output the unit can provide at a specific time. If a BESS has a higher power-to-energy storage ratio This rating is often used for applications when power is required to be delivered quickly such as a tower crane start-up or as part of a generator load-on-demand package, providing power assistance to generators for motor start equipment. Also enhancing grid supplies with frequency support or power factor correction. 

BESS control systems

BESS control systems are incredibly important as they impact the efficiency of a power package. The level of controls required depends on the complexity and purpose of a project. Here is an overview of the controllers integrated into a project. 

  • Product/asset level controls: Each product is equipped with an asset controller, managing the PCS (Power Conversion System), BMS (Battery Management System), and other auxiliary systems within the battery storage product.
  • Microgrid controller: Aggreko is introducing a microgrid controller for medium and large-scale mobile battery assets. This controller optimises BESS performance, seamlessly integrates with our genset fleet, and enables the use of renewable sources like wind and solar. It serves as the central controller for an energy system and can connect with multiple assets. 

Integrating BESS into a hybrid power package

The equipment required to integrate BESS into a hybrid power package (generator & battery) varies significantly according to project requirements. But as a rule, the main components consist of communication cables, power cables, cables from the generator and a generator BESS charging cable.

At Aggreko, our experts would work with you to ensure that the right equipment is specified according to the individual project requirements.

Common applications for battery energy storage

The main applications where we use battery energy storage to support our customers are listed below:

  • Hybridising, using batteries to downsize or rightsize a generator
  • To overcome capacity constraints (due to a restricted power connection or limited generator power)
  • Transient loading (supporting high inrush currents like motor start machinery, tower cranes, etc)
  • Integration with renewables, to manage intermittent supply
  • Off-grid power resilience
  • Back-up power and power security
  • Providing virtual capacity for EV (Electric Vehicle) charging
  • Spinning reserve displacement (for load on demand or/ multi-asset setups)
  • To keep Stage V engines operating at optimal loads
  • To comply with legislation and regulations when working in low/no emission zones


Battery Energy Storage Systems (BESS) are transforming the energy landscape, offering flexibility, efficiency, and sustainability. At Aggreko, we offer a range of BESS solutions designed to meet various energy storage or power assistance needs. By understanding BESS chemistry, terminology, and integration requirements, we can help you make informed decisions to minimise emissions, reduce fuel consumption, and optimise your hybrid power setups while ensuring reliability and performance.

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