By AGILE Consulting Engineers, Solar PV and Battery Energy Storage Systems (BESS) specialists.
A Bill of Quantities looks like paperwork until the day a tender comes back 20 percent over budget and nobody can say why. At that point it stops being paperwork and becomes the document everyone wishes had been tighter three months earlier. For solar and Battery Energy Storage System (BESS) projects, the BOQ is one of the few artefacts that connects the engineering design directly to what a contractor will actually charge, and getting it right is a detailed design discipline in its own right, not an afterthought bolted on at the end.
Table of Contents
- What a BOQ Actually Is
- Where the BOQ Sits in the Design Process
- What a Solar and BESS BOQ Typically Itemises
- BOQ Versus a Bill of Materials
- Its Role in Procurement and Tendering
- Its Role in Cost Control and Variations
- What Goes Wrong When a BOQ Is Rushed
- What to Do Next
- FAQ
What a BOQ Actually Is
A Bill of Quantities (BOQ) is a structured schedule that itemises every material, component and, in most formats, every labour activity needed to build a project, along with the measured quantity of each. For a solar or BESS asset, it is the document that takes a design that exists as drawings and specifications and turns it into a list that a contractor can price line by line. It is not a cost estimate in itself. It is the quantity backbone that pricing, procurement and progress claims all get built on top of.
The distinction matters because a BOQ is only as good as the design it is derived from. If the array layout, cable routes or foundation schedule are still moving, the quantities attached to them will move too, which is why a credible BOQ is normally issued once the detailed design has reached a stable, construction-ready state rather than at concept stage.
Where the BOQ Sits in the Design Process
Quantities exist in some form at every stage of a project, but their reliability changes markedly as the design matures. At feasibility stage, quantities are typically parametric, derived from benchmark ratios such as steel tonnes per megawatt or cable metres per string, and are useful for indicative costing only. By the time detailed design is complete, quantities should be measured directly off construction drawings, single line diagrams (SLDs, the electrical drawings showing how equipment is connected in a simplified single-conductor format) and civil layouts, which is what gives a BOQ its procurement-grade accuracy.
This progression is worth understanding because a BOQ issued too early, before layouts and cable schedules are locked, invites either padded contingency from bidders pricing uncertainty, or disputes later when as-built quantities diverge from what was tendered.
What a Solar and BESS BOQ Typically Itemises
For a ground-mount or rooftop solar array, a BOQ generally works through the project discipline by discipline. On the equipment side that means modules by wattage and count, inverters by rating and count, and transformers and switchgear associated with the power evacuation path. On the structural and civil side it covers mounting structure steel by tonnage or by table count, foundation piles or ballast quantities, access roads, and drainage works. Electrical items include DC cable by size and length, AC cable by core configuration and length, earthing material such as earth pits and bonding conductors, and lightning protection components.
A BESS adds its own set of line items on top of this: battery enclosures or containers, power conversion system units, thermal management and fire suppression equipment, and the additional protection and metering equipment associated with integrating storage into the same connection point as the generation asset. Where a project is a hybrid solar and BESS asset, the BOQ needs to clearly separate shared infrastructure, such as the substation and grid connection assets, from generation-specific and storage-specific equipment, because that split usually maps directly onto how the project is procured and financed.
A properly structured BOQ also typically nominates labour components and installation activities alongside the materials themselves, which is what separates it from a simple materials list and is part of why it can be used as the basis for lump sum or schedule of rates contracts.
BOQ Versus a Bill of Materials
It is worth being precise about a distinction that gets blurred in practice. A Bill of Materials (BOM) is a procurement list of physical components, generally without labour or installation activities attached. A BOQ builds on a BOM by adding measured quantities tied to construction activities and, usually, unit rates or pricing structure. In practice this means a BOM might tell a procurement team what to buy, while a BOQ tells a contractor what to build and how much of it, which is why the BOQ is the document that underpins tender pricing rather than just equipment ordering.
Its Role in Procurement and Tendering
In a competitive tender process, the BOQ is the common baseline every bidder prices against. Because every contractor is pricing the same measured quantities, the BOQ is what makes bids genuinely comparable rather than each contractor scoping the work slightly differently and returning numbers that cannot be sensibly lined up against each other. This is one of the more underappreciated functions of a well-prepared BOQ: it removes a significant source of scope ambiguity before a contract is even signed, which reduces the likelihood of early variation claims.
For engineering procurement and construction (EPC) contracts specifically, the BOQ generally sits alongside the technical specification and drawing package as one of the core contract documents, and discrepancies between the BOQ and the drawings are a common source of early-stage contractual friction if the detailed design was not fully coordinated before the BOQ was issued.
Its Role in Cost Control and Variations
Once construction starts, the BOQ becomes a live cost control tool rather than a static tender document. Progress claims are typically assessed against the percentage of each BOQ line item completed, which gives both the contractor and the project owner a shared, auditable basis for payment. Where site conditions differ from what was assumed, for example a geotechnical condition requiring different foundation quantities, the BOQ is the reference point against which a variation is measured and priced.
This is also where a well-structured BOQ pays for itself over the life of a project. A BOQ broken down with enough granularity to isolate the specific items affected by a change allows a variation to be priced against genuine quantity movement, rather than opening the door to broader renegotiation of rates across the whole contract.
What Goes Wrong When a BOQ Is Rushed
The recurring pattern we see is a BOQ generated from an early or partially coordinated design, then carried forward into tender without being revisited once the design settles. Cable lengths in particular are sensitive to this, because route length depends on final equipment placement, trenching constraints and separation requirements from other services, all of which can shift meaningfully between preliminary and detailed design. Structural quantities are similarly exposed if foundation design has not yet incorporated site-specific geotechnical data.
The practical consequence is either a round of costly variations once construction reveals the gap, or a contingency-laden tender where every bidder has priced in a margin for the uncertainty they can see in the documentation. Neither outcome serves the project well, and both are avoidable by treating BOQ preparation as an output of a completed, coordinated detailed design rather than a parallel exercise run against a design still in motion.
What to Do Next
A BOQ is only as reliable as the design behind it. If your project is approaching tender and the quantities are still being derived from early-stage assumptions rather than a coordinated, construction-ready design, that is worth resolving before pricing goes out to the market. Teams looking to move from a stable design into a procurement-grade quantities package, with the electrical, structural and civil disciplines properly coordinated, often find it useful to bring that work together under a single detailed design process. AGILE’s solar and BESS system design service covers this ground, producing the drawings, specifications and supporting documentation that a defensible BOQ is built from.
FAQ
Is a BOQ the same as a cost estimate?
No. A BOQ measures and lists quantities of materials and work items. Pricing is applied to it separately by whoever is estimating or tendering, which is why the same BOQ can attract different prices from different bidders.
When in the project timeline should a BOQ be prepared?
Ideally once the detailed design is coordinated and stable, because quantities derived from drawings that are still changing will need to be revised, which undermines the BOQ’s use as a tender baseline.
Does a BESS project need a separate BOQ from the solar array?
Not necessarily separate, but the line items should be clearly segregated within one document, particularly where shared infrastructure such as the substation is being cost allocated between the generation and storage components.
Who typically prepares the BOQ for a solar or BESS project?
It is usually produced by the design engineer as an output of the detailed design, since accurate quantities depend on measured drawings, cable schedules and foundation designs rather than being estimated independently.
What happens if actual construction quantities differ from the BOQ?
This is managed through the project’s variation process, with the BOQ line items providing the reference point for pricing the difference, which is one reason a granular, well-structured BOQ reduces disputes.
Can a BOQ be reused across multiple similar projects?
The structure and item categories can be reused as a template, but the quantities themselves are site and design specific and need to be remeasured for each project, since layout, cable runs and foundation conditions rarely repeat exactly.
