By AGILE Consulting Engineers, Solar PV and Battery Energy Storage Systems (BESS) specialists.
Design errors on solar and Battery Energy Storage System (BESS) projects rarely announce themselves at the design stage. They show up months later as an underperforming plant, a foundation that needs rework, or a protection system that trips when it should not. Industry commentary on plant performance consistently points to a handful of repeat offenders, and almost all of them are avoidable with the right rigour applied at the right stage.
Table of Contents
- Why the Timing of an Error Matters More Than the Error Itself
- Optimistic Yield Assumptions
- Inadequate Geotechnical Investigation
- Underspecified Protection Coordination
- DC to AC Ratio Misjudgement
- Multi-Discipline Coordination Gaps
- Avoiding These Errors Constructively
- What to Do Next
- FAQ
Why the Timing of an Error Matters More Than the Error Itself
Every design contains assumptions that turn out, in hindsight, to have been slightly wrong. That is normal engineering. What separates a manageable issue from a costly one is when it is caught. An error found during detailed design review costs a redraw. The same error found during construction costs rework, schedule delay and, often, a contractual dispute. Found after commissioning, it costs lost generation or storage revenue for the life of the asset. None of the error categories below are exotic. They are well documented in industry practice, and the pattern behind most of them is the same: a shortcut taken at an early stage that was never revisited before it became embedded in construction documentation.
Optimistic Yield Assumptions
Energy yield modelling carries inherent uncertainty, and industry analysis puts the typical uncertainty range for a PV plant’s final yield estimate at somewhere between roughly 4.5 and 15 percent depending on the quality of inputs used. That range is manageable when it is understood and priced into project assumptions. It becomes a problem when a generic simulation tool, outdated weather data, or borrowed assumptions about structural loads and wind or snow conditions are carried through into a bankable yield estimate without being tested against the specific site.
The practical fix is not complicated in concept, even if it is rigorous in execution: use site-specific irradiance and shading data, validate simulation software assumptions against the selected equipment’s actual performance characteristics, and treat the yield estimate as a living figure that gets refined as the design firms up, rather than a number locked in at feasibility stage and never revisited.
Inadequate Geotechnical Investigation
For ground-mount arrays, the foundation design is only as good as the geotechnical data behind it. Soil bearing capacity, groundwater conditions and, in some climates, frost action and pile uplift from adfreeze stresses can all materially change what foundation type and depth is appropriate. A desktop geotechnical assessment is a reasonable starting point at feasibility stage, but relying on it through detailed design and into construction is a recognised source of foundation rework, particularly where site conditions vary across a large ground-mount footprint.
Avoiding this comes down to sequencing: commissioning site-specific geotechnical investigation early enough in the detailed design programme that foundation design, and the structural loads it needs to accommodate, are based on measured data rather than assumption, with enough test points across the site to capture variability rather than a single average condition.
Underspecified Protection Coordination
Protection coordination, the design work that ensures fuses, relays and circuit breakers operate in the correct sequence to isolate faults without taking down more of the plant than necessary, is one of the areas most exposed to being underspecified when it is treated as a late-stage compliance exercise rather than an integral part of the electrical design. For a megawatt-scale plant, errors in protection coordination, transformer sizing or the underlying grid studies can lead to reduced output, more frequent forced outages, and can put the project at risk of not meeting its grid connection compliance obligations.
The constructive approach is to treat protection coordination as a design deliverable in its own right, developed alongside the single line diagram (SLD) rather than after it, and validated against the specific equipment selected and the actual grid connection requirements rather than generic settings carried over from another project.
DC to AC Ratio Misjudgement
The ratio between a plant’s DC nameplate capacity and its AC nameplate capacity, commonly called the inverter loading ratio, is a genuine design decision with real trade-offs, not a fixed industry number. Individual system ratios commonly sit between roughly 1.13 and 1.3, though ratios up to around 1.5 are seen in some modern designs, and the right figure for a given site depends on climate, module temperature behaviour, inverter clipping tolerance and the value placed on shoulder-season and winter generation versus midday clipping losses.
The error is not picking a particular ratio, it is picking one generically, without testing it against the site’s temperature profile, selected inverter characteristics and dispatch requirements. A ratio pulled from a rule of thumb or a previous project in a different climate can leave meaningful energy on the table, or conversely expose the plant to more clipping than the economics justify. Getting this right requires modelling the ratio against the specific site and equipment, not applying a default.
Multi-Discipline Coordination Gaps
A design can be technically correct discipline by discipline and still fail on site if the disciplines were not properly coordinated against each other. Reviewers checking a mature detailed design package are typically looking at coordinate and elevation control, cut and fill balance, drainage paths, road geometry and loads, potential trench conflicts, foundation reactions, constructability and, critically, consistency between the electrical, structural and civil packages. Gaps here are less about any single discipline being wrong and more about the interfaces between disciplines not being checked as a set.
This is why coordination reviews, run as a deliberate step rather than assumed to happen informally, are one of the more effective quality controls available on a detailed design programme.
Avoiding These Errors Constructively
None of the categories above are solved by any single silver bullet. What tends to work in practice is a combination of disciplined, staged validation: independent design review at key milestones rather than only at final issue, site-specific data collection sequenced early enough to inform rather than retrofit the design, protection and electrical coordination developed as first-class deliverables, and a formal cross-discipline coordination check before documentation goes to tender. None of this replaces good individual engineering, it is what catches the errors that individual engineering, working in isolation, tends to miss.
What to Do Next
If a project is heading into tender or construction and you want a second set of eyes on yield assumptions, geotechnical scope, protection coordination or DC to AC ratio decisions before they are locked in, that review is far cheaper now than it will be once steel is in the ground. AGILE’s solar and BESS system design service is structured around exactly this kind of coordinated, multi-discipline detailed design and review.
FAQ
What is the most common category of design error on solar and BESS projects?
Industry commentary points most consistently to yield modelling based on generic assumptions and inadequate geotechnical investigation for ground-mount foundations, since both tend to be set early and are not always revisited as the design matures.
Can these errors be fully eliminated?
Not entirely, since some uncertainty is inherent to engineering estimation, but disciplined staged review and site-specific data collection significantly reduce both the likelihood and the consequence of these errors.
Is a higher DC to AC ratio always better for energy yield?
No. A higher ratio increases shoulder-season and winter output but also increases clipping losses on clear days, so the right ratio depends on site climate and equipment characteristics rather than a single universally optimal figure.
Why does protection coordination matter beyond regulatory compliance?
Poorly coordinated protection can cause unnecessary outages across more of the plant than a fault actually affects, reducing availability and energy output even when the plant is otherwise performing as designed.
At what stage should geotechnical investigation happen?
Early enough in detailed design that foundation design is based on measured, site-specific data rather than a desktop assumption, since foundation rework during construction is one of the more expensive corrections to make.
How do multi-discipline coordination gaps typically get caught?
Through a deliberate coordination review that checks the electrical, structural and civil packages against each other before tender, rather than assuming that individually correct disciplines will automatically align on site.
