By AGILE Consulting Engineers, Solar PV and Battery Energy Storage Systems (BESS) specialists.
Industry data on construction rework consistently points to the same root cause: poor information reaching the field, not poor workmanship in the field. On solar PV and Battery Energy Storage System (BESS) projects, where electrical, structural and civil scopes all have to line up precisely, that gap between design intent and buildable documentation is where most cost and schedule blowouts actually originate.
Table of Contents
- Where Construction-Phase Risk Actually Comes From
- What RFIs and Variations Cost a Project
- Clearer Contractor Scope
- Coordination Between Disciplines
- Procurement and Equipment Accuracy
- Commissioning and Handover
- What Happens When Detailed Design Is Rushed
- What to Do Next
- FAQ
Where Construction-Phase Risk Actually Comes From
Every solar PV or BESS project carries construction risk, but not all of that risk originates on site. A meaningful share of it is built into the project at the design stage, well before a contractor mobilises. When drawings are issued at a preliminary or concept level and treated as construction-ready, the contractor is left to make decisions the designer should have made, decisions about exact cable routing, equipment clearances, structural connection details or protection settings, under time pressure and without the full context the original design team had.
Detailed design exists precisely to close that gap. It takes a project from a concept that demonstrates feasibility to a fully resolved set of documents, drawings, schedules and specifications, that a contractor can build from without having to guess at the designer’s intent.
What RFIs and Variations Cost a Project
A Request for Information (RFI) is the formal mechanism a contractor uses to ask the design team to clarify something the drawings do not answer. A steady stream of RFIs is a normal part of any construction project, but a high volume of them, particularly ones asking about information that should have been resolved in design, is a reliable signal that the documentation was not detailed enough for the field to build with confidence. Each RFI has a cost, not just the time to answer it, but the time the crew spends waiting, or working around the uncertainty, while it is being answered.
Variations follow a similar pattern. Where an RFI reveals a gap in the documentation, a variation is often the commercial consequence of resolving it, additional cost, additional time, or both, because the work now differs from what was originally scoped and priced. Reducing the number of RFIs a design generates is one of the more direct ways to reduce the number of variations a project ends up carrying.
Clearer Contractor Scope
A detailed design package gives a contractor a scope of work with defined boundaries, specific equipment, exact quantities in a Bill of Quantities (BOQ), and clear interface points between electrical, structural and civil packages. That clarity matters commercially as much as technically. A contractor pricing a well-defined scope can price it with less contingency, because there is less unknown to price against. A contractor pricing a loosely defined scope has to build in a buffer for the unknowns, and those buffers tend to surface later as claims or disputes if the actual conditions differ from what was assumed at tender.
Clear scope also reduces a subtler risk, disputes between trades over where one package’s responsibility ends and another’s begins. A detailed design that explicitly documents interface points, for example, where the electrical contractor’s cable containment scope meets the civil contractor’s trenching scope, removes a common source of on-site friction before it happens.
Coordination Between Disciplines
Solar and BESS projects sit at the intersection of electrical, structural and civil engineering, and a design error in one discipline often only becomes visible when it collides with another discipline on site. A foundation designed without final confirmation of equipment footprint and weight, a cable route that conflicts with a structural member, a switchroom sized before the final equipment list was locked in, these are the kinds of issues that detailed, coordinated design is meant to catch on paper rather than in the field.
Coordination during design, checking that the electrical, structural and civil documentation agree with each other before anything is issued for construction, is comparatively cheap. The same coordination problem discovered mid-construction usually means demolition, redesign and delay, all layered on top of the cost the clash coordination would have avoided in the first place.
Procurement and Equipment Accuracy
Detailed design also reduces risk indirectly, through procurement. A design that has finalised equipment selection, inverters, Power Conversion Systems (PCS), transformers, switchgear, produces an accurate Bill of Quantities (BOQ) that procurement can order against with confidence. A design still in flux tends to produce a BOQ that changes after ordering has started, which creates its own cascade of cost and schedule risk, cancelled orders, expedited freight, or equipment substitutions made under pressure rather than by design.
Getting equipment selection genuinely locked down during detailed design, rather than treating it as provisional right up until construction, removes a meaningful source of downstream disruption.
Commissioning and Handover
Risk reduction from detailed design extends past construction into commissioning. A system built from a well-resolved design, with an accurate Single Line Diagram (SLD), a clear protection schedule and coordinated documentation across disciplines, is generally easier to commission, because the commissioning team is verifying a design intent that was actually followed, rather than reverse-engineering what got built against what was drawn. That has flow-on effects for handover, operational readiness and the quality of as-built documentation the owner ultimately receives.
What Happens When Detailed Design Is Rushed
Compressing the detailed design phase to protect an overall project schedule is a common instinct, particularly under commercial pressure to reach financial close or start construction. It is also, in the industry’s own project data, one of the more reliably counterproductive moves available, because the time saved at the design stage is rarely proportional to the time lost later resolving avoidable RFIs, variations and rework. A detailed design phase that is allowed to run its proper course tends to produce a construction phase that runs more predictably, which is usually the outcome a tight overall schedule actually needs.
What to Do Next
If your project is approaching construction with drawings that still carry provisional equipment selections, unresolved interface points between disciplines, or protection settings yet to be finalised, that is the moment to close those gaps, not after the contractor has started asking questions about them. AGILE’s solar and BESS system design service delivers coordinated electrical, structural and civil detailed design intended to give contractors a scope they can build from with confidence.
FAQ
What is the difference between preliminary design and detailed design?
Preliminary design establishes feasibility and general system concept, while detailed design resolves every element to a construction-ready level, including finalised equipment selection, coordinated drawings and complete schedules a contractor can price and build from.
How do RFIs affect a construction schedule?
Each RFI represents a gap in the design documentation that needs to be resolved before work can proceed with certainty, and a high volume of RFIs typically slows the overall program even when individual responses are quick.
Does detailed design cost more upfront?
It generally requires more design investment before construction starts, but that investment is commonly offset, and often exceeded, by the reduction in variations, rework and delay it enables during construction.
Why does discipline coordination matter so much on solar and BESS projects?
These projects combine electrical, structural and civil scopes that physically intersect, such as foundations, cable routes and equipment enclosures, so an uncoordinated design is more likely to produce clashes that only surface once construction is underway.
Can rushing detailed design actually make a project slower overall?
Yes, compressing the design phase often shifts unresolved decisions into construction, where resolving them through RFIs and variations typically takes longer and costs more than resolving them on paper would have.
What role does the Bill of Quantities play in reducing risk?
An accurate BOQ, based on finalised equipment selection, allows procurement to order with confidence and gives contractors a firm basis for pricing, reducing the risk of late equipment changes disrupting the construction schedule.
