By AGILE Consulting Engineers, Solar PV and Battery Energy Storage Systems (BESS) specialists.
Every solar or Battery Energy Storage System (BESS) project has a moment where the numbers that got it financed stop being good enough to build from. A feasibility study is meant to answer whether a project is worth pursuing, not to tell a contractor exactly where to dig a trench. The gap between those two jobs is where detailed design earns its keep, and it is a bigger gap than most non-engineers assume.
Table of Contents
- Why Feasibility-Stage Assumptions Cannot Be Built From
- What Feasibility Studies Get Right, and Where They Are Deliberately Approximate
- The Translation From Concept to Construction-Ready
- What Typically Gets Refined
- What Typically Gets Corrected
- The Documentation That Emerges
- Coordination Across Disciplines During the Bridge
- What to Do Next
- FAQ
Why Feasibility-Stage Assumptions Cannot Be Built From
A feasibility study exists to answer a narrow, important question: is this project likely to be viable, and is it worth spending significantly more money to find out for certain. To answer that question efficiently, feasibility studies lean on parametric assumptions, industry benchmark ratios, indicative equipment selections and desktop-level site data. That is entirely appropriate for its purpose. It is also, by design, not precise enough to hand to a construction crew.
The commercial logic behind this is straightforward. Detailed engineering for a commercial solar project typically costs several times more than the feasibility study that preceded it, so it makes no sense to commit that level of engineering effort before a project has cleared its basic viability hurdles. The consequence is that everything produced at feasibility stage, from yield estimates to layout sketches to indicative costings, needs to be revisited, tested against real site data and, in many cases, materially revised once the project moves into detailed design.
What Feasibility Studies Get Right, and Where They Are Deliberately Approximate
A good feasibility study is honest about its own limits. It typically assesses the resource potential of the site, indicative costs, location and topography, likely shading, high-level geotechnical risk, proximity to existing network infrastructure and a first-pass site configuration and equipment sizing. It is a technical and financial screening exercise, not a design.
Where feasibility studies are deliberately approximate is in exactly the areas that matter most for construction: exact array layout and row spacing, final cable routing, foundation type and depth, protection settings, and the specific make and model of major equipment. These are left open at feasibility stage because refining them prematurely would cost money on a project that might not proceed, and because some of these decisions genuinely cannot be finalised without site-specific data that only gets collected once a project is committed to moving forward.
The Translation From Concept to Construction-Ready
Moving from feasibility to detailed design is not simply adding detail to the same drawings. It is a distinct engineering process that takes each feasibility-stage assumption and tests it against ground truth. A resource assessment based on satellite irradiance data gets checked against ground-level shading analysis and final array geometry. An indicative layout based on a rough site boundary gets redrawn against a topographic survey, easements, setbacks and any constraints identified during environmental and planning approvals. A budget-level equipment selection gets replaced with a specified, procurable equipment list once technical requirements and grid connection constraints are locked in.
This is the stage where a project genuinely becomes buildable. The output is no longer a set of assumptions that support an investment case, it is a coordinated set of drawings, specifications and schedules that a contractor can price, procure against and construct from without having to make design decisions on site.
What Typically Gets Refined
Some elements of the feasibility design survive largely intact but get sharpened. Energy yield modelling is refined using site-specific shading, final tilt and row spacing, and actual selected equipment performance data rather than generic assumptions. Array layout is optimised in more depth, working through row spacing, orientation and access road placement against the actual surveyed site rather than an indicative boundary. Cable routing and sizing get finalised against real trench paths and separation requirements rather than straight-line estimates. None of these represent a failure of the feasibility work, they represent exactly what detailed design is meant to do with the extra rigour and site information it has access to.
What Typically Gets Corrected
Other elements genuinely change, sometimes materially, once detailed design tests them properly. Geotechnical conditions discovered through site-specific investigation can alter foundation type or capacity in ways a desktop assessment could not have anticipated. Grid connection requirements identified through the formal connection process with the Distribution Network Service Provider (DNSP, the entity that owns and operates the local electricity distribution network) or the Australian Energy Market Operator (AEMO, the body that operates the National Electricity Market and sets technical connection requirements) can require equipment or protection changes not contemplated at feasibility stage. DC to AC ratio assumptions, made generically at feasibility stage, often get revisited once inverter selection, site temperature profile and clipping tolerance are properly modelled.
This is a normal and expected part of project delivery, not a sign that the feasibility study was poor. It is precisely why detailed design exists as a distinct, funded stage rather than being skipped in favour of building straight off early assumptions.
The Documentation That Emerges
The tangible output of this bridge is a construction-ready documentation package. That typically includes single line diagrams (SLDs) showing the electrical configuration, structural drawings and foundation schedules informed by geotechnical data, civil layouts covering access, drainage and earthworks, a coordinated cable and conduit schedule, protection and control philosophy, and a Bill of Quantities (BOQ, the itemised schedule of materials and work quantities used for tendering and cost control) that reflects the settled design rather than early parametric estimates. Together, this is what allows a project to go to tender with genuine price certainty rather than contingency-laden guesswork.
Coordination Across Disciplines During the Bridge
The reason this translation is genuinely engineering work, rather than administrative refinement, is that electrical, structural and civil decisions constrain each other. Inverter and transformer siting affects cable routing and voltage drop. Foundation design affects array layout tolerances. Access road placement affects both civil earthworks and construction sequencing. A detailed design process that runs these disciplines in isolation tends to surface conflicts late, generally during construction, which is the most expensive place to resolve them. Coordinating these disciplines against a single, shared model of the site is what separates a construction-ready design from a set of drawings that merely look complete.
What to Do Next
If your project has cleared feasibility and the next question is how to turn that investment case into something a contractor can build and price with confidence, that transition deserves a properly resourced detailed design process rather than an incremental add-on to the feasibility work. AGILE’s solar and BESS system design service is built around exactly this translation, taking a project from concept-stage assumptions through to coordinated, construction-ready documentation.
FAQ
How long does the move from feasibility to detailed design typically take?
It varies with project size and complexity, and is materially affected by how long site investigations, surveys and the formal grid connection process take, since detailed design cannot finalise around inputs that are still pending.
Does detailed design always increase project cost compared to feasibility estimates?
Not always, but it is common for costings to move in either direction once real site data, specified equipment and finalised quantities replace parametric assumptions, which is exactly why detailed design exists before financial close.
Can a project go straight to construction from a feasibility study?
It is not advisable. Feasibility-stage drawings and quantities are not precise enough for construction pricing or site works, and skipping detailed design tends to shift design risk onto the construction phase where it is more expensive to resolve.
What role does geotechnical investigation play in this transition?
It is one of the most consequential inputs, since foundation type and capacity for ground-mount structures depend on actual soil conditions, and desktop assumptions made at feasibility stage often need to be revised once site-specific data is available.
Who is responsible for reconciling feasibility assumptions with detailed design outcomes?
This typically sits with the detailed design engineer, who works through each feasibility-stage assumption against site-specific data and updates the design, documentation and quantities accordingly.
Does the DC to AC ratio decided at feasibility stage usually survive into detailed design unchanged?
Not always. It is commonly revisited once inverter selection, temperature profile and site-specific yield modelling are finalised, since the feasibility-stage figure is generally a benchmark assumption rather than a site-optimised value.
