What Is Balance of Plant (BoP) in BESS?

Balance of Plant (BoP) is a term that originates from power engineering, where it refers to everything that is not the main generating unit — the supporting infrastructure required to make a power plant operational.

In BESS, the main equipment is the DC block, the power conversion system (PCS), the medium-voltage transformer and RMU, the high-voltage substation, the power plant controller/SCADA, and — where applicable — the RTM controller. Additional items required to design, build, and ensure the complete BESS plant is operational are typically categorized as Balance of Plant.

Main BESS equipment vs Balance of Plant scope categories.

Why BoP Is Difficult to Define

The general definition of BoP may be simple, but the difficulty starts when you try to define who is responsible for what. The line between designing and executing BoP work is not always clear, and the split between engineering and execution can sit with different parties depending on the project.

BoP is a scope of work. It describes the different categories of work that every BESS project needs to have executed. Which party performs each category, and whether they are responsible for the design, the execution, or both, can vary significantly depending on the overall contractual terms for the project.

This is why if you ask three different stakeholders in the industry, you may receive three different descriptions of what BoP means. Not because they are wrong, but because they are describing BoP from their experience — a scope of work that was defined and divided in a particular way on their projects. The categories that make up BoP may be consistent, but how they are contracted and executed can differ from project to project.

The Scope

Civil Works (CBoP)

Site preparation and levelling. Access roads. Drainage. Foundations — concrete pads or screw-pile installations depending on the site. Cable trenching. Fencing and perimeter security. CCTV. PFAS collection where environmental regulations require it.

Electrical Works (EBoP)

Cable installation, pulling, routing, termination, glanding, and labelling. Cable tray and trenching. Earthing and lightning protection. Protection and communication systems — relay installation, fibre optic and communication cabling.

Equipment Installation

Physically installing the main equipment on site — lifting DC blocks onto foundations, positioning PCS units, mounting auxiliary equipment like power quality monitors, transient fault recorders, and communication cabinets. Equipment manufacturers typically provide supervision and oversight while the BoP workforce does the hands-on work.

Auxiliary Power

Auxiliary power transformers and equipment — the electrical supply needed to support the plant and the workforce during construction and operation.

Grid Connection Infrastructure

The physical infrastructure connecting the plant to the grid — installation of power quality monitors, energy meters, transient fault recorders, CTs and VTs, protection relays, and associated cabling between the plant and the point of connection.

Commissioning

Equipment and plant-level commissioning activities — energisation, subsystem testing, PPC/SCADA integration, safety system checks and safety system verification.

Site Acceptance Tests (SAT)

Grid code compliance testing, frequency regulation pre-qualification, and overall plant performance verification.

How BoP Gets Contracted

The scope categories of Balance of Plant exist on every BESS project. However, how they get assigned and executed depends on who is leading the project and what contract structure is in place.

EPC-Led

Under an EPC contract, the EPC contractor is responsible for the complete BESS plant delivery — engineering, procurement, construction, and having the system integration responsibility.

In practice, the EPC may subcontract most of the execution work for the BoP while preparing or executing the engineering work in-house. Typically, a local civil works company is subcontracted for site clearance, groundworks, and foundations. An electrical contractor is appointed to handle cabling, termination, other installation work, and to assist with installation, commissioning, and site acceptance. The work is often split between different contractors depending on the EPC’s in-house capabilities, but the EPC manages all internal engineering, project management, and subcontractor coordination of the interfacing work, and takes accountability for the complete delivery. The subcontractors report to the EPC, not to the buyer.

Developer-Led

The buyer procures the main BESS equipment directly from the manufacturers and splits the overall execution of the project between different independent contractors. The developer might appoint a single BoP contractor to manage all scope categories, or — more commonly — split the scope across multiple contracts: one for civil works, one for electrical works, a grid compliance consultant, and so on.

Under a split model, the developer takes on the system integration role — managing the same interfaces an EPC would manage, and ensuring that the equipment from different manufacturers works together and the plant meets its intended performance requirements. Developers with strong in-house engineering and execution teams tend to outsource less, while developers who are primarily commercial organizations need to bring in all technical capabilities through subcontractors and consultants.

Why Defining BoP Matters

Balance of Plant is a scope of work. System integration is a function. These are fundamentally different concepts, and a BoP contractor should not be expected to act as a system integrator — regardless of how the contract is structured.

System integration requires plant-level knowledge, equipment interface management, and grid code expertise that sits outside the BoP scope. Clearly defining BoP — what’s included, who is responsible for each category, and where the boundary with system integration lies — is essential for every stakeholder involved in a BESS project. When that definition is clear, contractors know their scope and interfaces are managed. When it’s not, the gaps become the source of disputes, delays, and cost overruns.

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learnBESS covers the full BESS project lifecycle — from procurement and Balance of Plant through commissioning and operations — in our course modules.