Most solar projects don't fail. They just quietly become something other than what was sold — a system a little smaller, a yield a little lower, a timeline a little longer. Almost always, the cause traces back to a stage where nobody defined what "done" meant. This guide walks the five stages of a turnkey EPC project and names, at each one, exactly what you should be signing off.
On this page
What EPC actually means
Engineering, Procurement, Construction. In a turnkey EPC contract, one company designs the system, buys the components, builds it, commissions it, and stays accountable for whether it performs.
The alternative — a designer here, a supplier there, a local contractor to mount it — is often cheaper on paper and reliably more expensive in practice. When generation falls short, each party points at the others. The value of an EPC contract is not that it's cheaper. It's that there is one throat to choke.
The question that reveals everything
Ask a prospective contractor: "If this plant generates 15% less than your proposal says, what happens?" The quality of the answer — and whether it's in the contract — tells you more than any brochure.
Stage 1 — Assess
Everything downstream is determined here. A weak assessment cannot be rescued by good construction.
What happens
- Consumption analysis. Twelve months of TANGEDCO bills, totalled. For commercial sites, the daytime load curve and shift pattern.
- Sanctioned load check. Your system cannot exceed it without a separate enhancement application — the rule that most often derails a net-metering approval. See our net metering guide.
- Site survey. Usable shadow-free area, measured, not estimated. Roof type, age and condition.
- Shadow study. Across the day and across the seasons. A December parapet shadow is longer than a June one.
- Structural check. Can the roof carry the array and the wind load?
- Generation model. Site-specific, annual, monsoon-inclusive.
What you sign off
A proposal that states the system size, the expected annual generation in units, and the assumptions behind that number. Not a peak-day figure. Not "up to." A modelled annual yield, with its inputs visible.
Gate 1
Do not proceed past a proposal that cannot tell you: expected units per year, the self-consumption assumption, and what's excluded from the price. Everything after this stage is built on those three numbers.
Stage 2 — Engineer
Where a design becomes a buildable, defensible thing.
What happens
- Array layout and string design — how modules are grouped electrically, matched to the inverter's operating window and the roof's shading reality.
- Single-line diagram (SLD) — the electrical architecture, including protection, isolation and earthing.
- Structural design — mounting geometry, foundation or ballast detail, calculated for wind load, not judged by eye.
- Waterproofing detail at every roof penetration.
- Bill of materials (BOM) — every component named by make, model and rating.
- Approvals package — the technical documents the utility requires.
What you sign off
The SLD and the bill of materials. This is the moment "branded inverter" must become a model number. Once the BOM is fixed and signed, substitution requires your consent — which is the entire point of fixing it.
Gate 2
A signed BOM naming every component is the single most effective protection a customer has. Without it, the panels that arrive are whatever was cheapest that month.
Stage 3 — Procure
Unglamorous, and where quality is actually decided.
What happens
- Ordering against the approved BOM — modules, inverter, mounting structure, DC and AC cable, protection devices, monitoring.
- Inspection on receipt. Serial numbers recorded, glass checked for transit damage, datasheets matched to the BOM.
- Compliance check. Depending on the scheme and metering arrangement, module and cell sourcing may need to satisfy applicable domestic-content requirements. For net-metered and open-access projects this has been tightening — confirm what applies to your project before ordering, because it constrains which modules are eligible.
- Warranty documentation collected and retained — panel, inverter, structure.
What you sign off
Delivery documentation matching the BOM, with module serial numbers recorded. If the delivered make differs from the signed BOM in any respect, that is a variation requiring your written agreement — not a detail to discover during commissioning.
Stage 4 — Build
The visible part, and usually the fastest.
What happens
- Civil and structural — foundations or ballast, structure erection, roof waterproofing at penetrations.
- Mechanical — module mounting, clamping to specified torque, maintaining the air gap beneath.
- DC electrical — string wiring, cable management (properly tied and supported, never resting on the roof), junction boxes, DC isolation.
- AC electrical — inverter mounting with ventilation clearance, AC cabling, isolator, connection to your distribution board.
- Earthing and surge protection — the part that decides whether a lightning season kills your inverter.
- Pre-commissioning tests — string voltage and current, insulation resistance, earth resistance, polarity.
- Inspection and grid synchronisation — utility inspection, net meter installation, and the plant brought online under an approved metering arrangement.
What you sign off
Pre-commissioning test results, in writing. String currents that match the design. Insulation resistance within limits. Earth resistance measured, not assumed. These are the numbers that prove the plant was built as designed — and they're trivially easy to skip.
Gate 3
Do not accept "commissioned" without test results. A plant can be energised and generating while carrying an insulation fault that will fail in the first monsoon.
Stage 5 — Sustain
Twenty-five years, of which the first day is the least interesting.
What happens
- Handover pack — as-built drawings, SLD, BOM, all warranty certificates, test reports, O&M manual.
- Monitoring configured — and, critically, your access credentials, not only the vendor's.
- Training — what to look at weekly, what to never touch, who to call.
- O&M contract commences — cleaning schedule, preventive maintenance, response times, annual testing, performance guarantee.
What you sign off
The handover pack and monitoring access. A plant handed over without documentation is a plant nobody can maintain properly in year seven, when the original crew has moved on. Our maintenance checklist covers what should happen from here.
Realistic timelines
| Project | Typical duration | What sets the pace |
|---|---|---|
| Residential rooftop | 1–3 weeks from approval | Approvals, not installation |
| Commercial & industrial | 4–10 weeks | Capacity, structural work, grid-sync approvals |
Note the phrase from approval. The physical installation of a residential system is a matter of days. The application, technical feasibility check at the distribution transformer, net meter agreement and inspection scheduling are what consume the calendar.
This is why "who files the paperwork" is not an administrative footnote in a quotation. It is the schedule.
Where projects go wrong
The proposal never stated an annual yield
Without a committed generation figure, underperformance has no definition, and therefore no remedy.
The BOM was never fixed
"Tier-one components" is a category, not a commitment. Components get substituted downward between quotation and delivery, and nobody has grounds to object.
Structural design by eye
Wind load is a calculation. Madurai gets seasonal winds strong enough to punish a structure that was sized by habit.
Waterproofing treated as an afterthought
The leak appears in the first monsoon, in the room below, and by then the array sits on top of the problem.
No pre-commissioning test results
The plant generates on day one. The fault it was built with surfaces in year two, outside anyone's memory of who was responsible.
Monitoring access retained by the vendor
You cannot manage what you cannot see. Insist on your own credentials at handover.
O&M discussed but never written
Twenty-five years is too long to rely on goodwill and a phone number.
Frequently asked questions
What does EPC mean in solar?
Engineering, Procurement and Construction — one contractor designs, buys, builds, commissions and remains accountable for performance, rather than you coordinating separate parties.
How long does a solar project take?
Residential rooftop: typically one to three weeks from approval. Commercial and industrial: four to ten weeks, depending on capacity, structural work and grid-synchronisation approvals.
What should I sign off at each stage?
Four gates: the proposal with stated generation assumptions; the SLD and bill of materials naming every component; the pre-commissioning test results; and the handover pack with warranties, drawings, test certificates and your own monitoring access.
What is grid synchronisation?
Connecting the completed plant to the utility grid under an approved metering arrangement, after inspection. In Tamil Nadu that means TANGEDCO approval and a bidirectional meter.
What causes most delays?
Approvals and paperwork, not construction. An EPC contractor who files them on your behalf removes the most common source of delay.
One contract. One accountability.
We assess, engineer, procure, build and sustain — with generation assumptions stated up front and TANGEDCO approvals handled end to end. Free site assessment in Madurai.