A 400 kW rooftop array is not a difficult thing to buy, and it is not a difficult thing to install. What makes these projects go wrong is sequence: modules go up before the roof has been properly read, or a string gets re-arranged on the day because a row did not fit the way the drawing said it would.
This is the field sequence we follow on export projects, written up against a 400 kW factory rooftop build in Butuan City, Philippines. Nothing in it is exotic. That is the point — the failures are boring, and they are preventable.
Why a factory roof is a good candidate in the first place
A three-shift factory consumes most of its electricity during daylight. That is the entire argument for rooftop PV on an industrial load: generation and consumption overlap, so the array displaces grid energy directly instead of needing storage to time-shift it.
The grid stays connected as the balancing source. Solar covers what it covers; the utility covers the rest.
That arrangement is worth stating explicitly early, because it decides what the owner should expect. This is a bill-reduction system, not a backup system. If the requirement includes riding through outages, that is a different design conversation with different hardware in it — and discovering that difference after the array is on the roof is expensive.
Stage 1: the survey is an engineering task, not a sales visit
Before any hardware is discussed, the roof has to answer a fixed set of questions. I keep them as a list, because skipping one of them is usually where the trouble starts later.
| Question | Why it is asked now |
|---|---|
| Usable area, after setbacks and obstructions | Sets the ceiling on array size before anyone quotes a number |
| Orientation and tilt of each roof plane | Sets the generation profile against the load profile |
| Shading from nearby structures, and at what time of day | A shaded string underperforms its neighbours and drags the MPPT input it shares |
| Condition of the metal sheets | Decides whether the roof needs work before it carries an array |
| Cable route from array to inverter room | Decides penetrations, trenching and how much of the DC run is exposed |
The one that gets skipped most often is the last one. Cable routing gets treated as an installation detail, and then it turns out the only practical path crosses a roof penetration nobody wants to make.
Plan the walkway while you are still on the roof with a tape measure
A 400 kW array covers a large area. If access routes are not designed in from the start, cleaning and inspection become physically difficult within the first year — and modules nobody can safely reach are modules that quietly stop producing.
It is a layout decision, not an afterthought, and it is far cheaper to take at survey stage than to retrofit. The walkway has to be in the drawing before the rails are, because rails and walkway sections share the same fixing pattern.
Stage 2: substructure before modules
Rails, clamps and walkway sections go down first, and the mounting system is specified per project — against the sheet profile of that particular roof and against the local wind load. Neither of those is a default.
A clamp engineered for one standing-seam profile does not reliably transfer load on another. And wind load is a function of the site, the building height and the applicable local code, not of the array size.
Field note: if a mounting quote arrives before anyone has seen the roof profile, it is a placeholder. Ask what sheet profile and what wind speed the clamp spacing was calculated against.
Getting the substructure right is what makes the module installation fast. Every row is straight, every clamp point stays accessible, and the array remains serviceable for the life of the system.
Stage 3: modules row by row — and why the string plan is not negotiable
With the mounting complete, crews work row by row: modules are lifted, seated, clamped and connected into strings. On this build the roof crew and the electrical crew worked in parallel, one finishing the array while the other prepared the inverter and protection panels.
What must not happen is improvising the strings on the roof. The electrical reason is worth spelling out:
- String length sets the voltage. The number of modules in series determines the string's open-circuit voltage, and the inverter's MPPT input has a window — a floor below which tracking is inefficient and a ceiling above which the inverter will not operate at all.
- Cold raises open-circuit voltage. Module voltage carries a negative temperature coefficient, so the highest voltage a string ever sees is not on a hot afternoon but on a cold, clear morning. String length has to be chosen so the worst-case cold voltage stays inside the inverter's limit.
- Strings sharing an MPPT input should match. If one string on an input is shaded or shorter than its neighbours, that input is pulled toward the weakest string rather than delivering the sum of the best ones.
All three are resolved on paper, before shipment. Re-arranging strings on the roof because a row did not fit means at least one of the three is now unchecked.
Stage 4: commissioning is a sequence, not a switch
The tests run in a specific order, and the order exists because each step protects the next one.
- Insulation resistance — before anything is energised, confirm the DC installation is not faulted to earth.
- Polarity — confirm each string is wired the way the drawing says, before it reaches the inverter.
- String open-circuit voltage and current — compare against the expected value from the string plan. A string reading low is a wiring or shading problem, and it is much cheaper to find here than after handover.
- Inverter start-up — only once the DC side is verified.
- Monitoring configuration — so the owner has a baseline from day one, which is what makes later performance questions answerable at all.
Field note: the string-by-string voltage record taken at commissioning is the most useful document in the handover file. A year later, when somebody asks whether the array is underperforming, that record is the only thing that turns the question into an answer.
What goes in the handover file
For this project: the as-built layout, the bill of materials, and the documentation for each major component. Those three are what the owner's own maintenance people — or a third-party O&M contractor — will need, and all three are easier to produce during the build than to reconstruct afterwards.
For export projects the same discipline applies before shipment. Configuration records, inspection photographs and packing lists travel with the order, so that what arrives matches what was engineered.
The part that is not in scope
Worth stating plainly, because it is the most common mismatch between expectation and contract. On export projects the scope covers the system: design, hardware, factory testing, documentation and packing. It does not include installation crews abroad or a local service organisation. That work is done by the buyer's own contractor.
Two things follow from that.
First, the engineering package has to be complete enough for a contractor who has never seen the equipment to install it correctly — which is exactly why the string plan and the as-built drawing matter more here than they would on a domestic job.
Second, anything that depends on local conditions has to be confirmed locally, and confirmed before the inverter is specified rather than after it has shipped. Utility interconnection rules are the obvious one. How exported energy is treated — and therefore what the array is actually worth on the bill — is settled with the distribution utility, not nationally. For the Philippines, that process is described here.
Full field notes
The build this is written against, with the photographs, is documented here: How a 400 kW Rooftop Solar System Comes Together.
If you are working on a comparable roof, the sequence above is the part worth protecting. Hardware choices can be revisited; a compressed sequence usually cannot be.
Jay Zhou is a solar engineer working on commercial and industrial PV and storage systems for export markets. Photographs are from Force Solar installation records and describe the referenced project configuration; every system is sized against its own load data.
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