By AGILE Consulting Engineers, Solar PV and Battery Energy Storage Systems (BESS) specialists.
“We’ve had it designed” is one of the more misleading sentences in solar and Battery Energy Storage System (BESS) development, because it can mean anything from a single-page concept sketch to a fully coordinated construction package. Preliminary design and detailed design are not two versions of the same document with more or less detail added. They are different engineering exercises, done for different purposes, with different levels of certainty attached to what they produce.
Table of Contents
- Why the Terms Get Used Loosely
- Purpose of Preliminary or Concept Design
- Typical Preliminary Design Deliverables
- Level of Certainty at Preliminary Stage
- Purpose of Detailed Design
- Typical Detailed Design Deliverables
- Level of Certainty at Detailed Design Stage
- Why the Distinction Matters for Risk and Financing
- What to Do Next
- FAQ
Why the Terms Get Used Loosely
Part of the confusion is that both stages produce drawings, both involve engineers, and both are genuinely called “design” in everyday conversation. Some project frameworks also insert an intermediate stage, often called Front End Engineering Design (FEED) or scheme design, which adds another layer of terminology. Stripped back to what actually matters for a solar or BESS project, the useful distinction is not the label used but the question each stage is trying to answer and how much engineering certainty sits behind its output.
Purpose of Preliminary or Concept Design
Preliminary design exists to evaluate whether a project concept is viable and to support the decisions that come before significant capital is committed: site selection, indicative budgeting, planning and approvals strategy, and the investment case put to financiers or the project board. It answers “is this worth pursuing, and roughly what will it look like and cost” rather than “exactly how do we build it.”
Because the purpose is evaluation rather than construction, preliminary design deliberately does not chase the level of precision that construction requires. Spending detailed design effort on a project that has not yet cleared its basic viability hurdles is, in most cases, effort spent in the wrong order.
Typical Preliminary Design Deliverables
A preliminary design package for a solar or BESS project typically includes an indicative site layout, a parametric energy yield estimate, a high-level single line diagram (SLD) showing the intended electrical configuration in concept form, a budget-level cost estimate built from benchmark ratios, and a first-pass view of grid connection feasibility. Equipment is usually described generically, for example “central inverters, approximately X MW,” rather than by specific make and model, and quantities are indicative rather than measured.
These deliverables are designed to support decisions such as land acquisition, initial grid connection enquiries, and funding approval to proceed to the next stage, not to support procurement or construction.
Level of Certainty at Preliminary Stage
Cost, weight, power and performance figures at preliminary stage are, in engineering terms, essentially rule of thumb estimates drawn from benchmarks and comparable projects rather than from calculated first principles. That is not a criticism of the work, it reflects what the stage is designed to deliver efficiently. The appropriate way to treat preliminary design figures is as a planning-grade range, with contingency built in to reflect the acknowledged uncertainty, rather than as numbers a contract should be signed against.
Purpose of Detailed Design
Detailed design exists to produce a construction-ready basis for building the project: drawings precise enough to construct from, specifications precise enough to procure against, and quantities precise enough to tender and contract on. It answers “exactly how do we build this, with what equipment, to what standard, and at what measured quantity,” and it is the stage where design decisions get locked in ahead of construction rather than left open.
Typical Detailed Design Deliverables
A detailed design package typically includes fully coordinated electrical drawings and SLDs, structural drawings and foundation schedules informed by site-specific geotechnical investigation, civil layouts covering earthworks, drainage and access, a specified equipment list by make and model, protection and control philosophy and settings, a coordinated cable and conduit schedule, and a Bill of Quantities (BOQ, the itemised schedule of materials and work quantities used to price and manage construction) measured from the finalised drawings rather than estimated from benchmarks. Where relevant, it also incorporates the outcomes of the formal grid connection process, since equipment and protection requirements can be shaped by connection study findings.
Level of Certainty at Detailed Design Stage
Detailed design estimates are based on parts counts, actual drawing dimensions and specified equipment weights and ratings, rather than benchmark ratios. This is what allows the design to be tendered with genuine price certainty and built without the contractor needing to make significant design decisions on site. It does not mean detailed design eliminates all uncertainty. Site conditions can still differ from what was assumed, and variations still happen. But the baseline the project is working from is measured and coordinated, not estimated.
Why the Distinction Matters for Risk and Financing
Confusing the two stages creates real project risk. Financing decisions made against preliminary-grade cost estimates that are later treated as fixed can produce budget shortfalls once detailed design reveals actual site conditions and specified equipment costs. Construction contracts signed against preliminary-grade drawings invite variation claims, because the contractor is effectively pricing uncertainty rather than a defined scope. Understanding which stage a set of drawings actually represents, and treating its outputs with the appropriate level of confidence, is a basic risk management discipline for any project owner, financier or developer working through a solar or BESS pipeline.
What to Do Next
If your project has an investment-grade case built on preliminary design and the next milestone is construction readiness, that transition is where engineering certainty needs to catch up with commercial ambition. AGILE’s solar and BESS system design service covers this full progression, from refining preliminary assumptions through to coordinated, construction-ready detailed design documentation.
FAQ
Is preliminary design the same as a feasibility study?
They overlap significantly and the terms are often used interchangeably in practice, both being concerned with evaluating viability and supporting early investment decisions rather than producing construction-ready documentation.
Can construction start from a preliminary design package?
It is not advisable. Preliminary design deliverables are indicative and benchmark-based, and building from them typically shifts design risk and cost uncertainty into the construction phase.
How much do costs typically change between preliminary and detailed design?
It varies by project, but movement in either direction is common once site-specific data, specified equipment and measured quantities replace the benchmark assumptions used at preliminary stage.
Does every project need a formal preliminary design stage?
Most do, since it is the mechanism by which viability, indicative budget and financing approval are established before the more significant investment of detailed engineering is committed.
What triggers the move from preliminary to detailed design?
Typically financial close, board or investment committee approval, or an equivalent decision gate confirming the project will proceed, since detailed design represents a substantially larger engineering investment.
Is a Front End Engineering Design (FEED) stage the same as detailed design?
Not quite. FEED generally sits between preliminary and detailed design, narrowing down technical options and firming up scope, but it typically does not reach the fully measured, construction-ready precision of detailed design.
