How to Vet a Factory Solar EPC Contractor in Thailand: 12 Questions to Ask Before You Sign
A solar system is an asset that must perform for 25+ years — signing with the wrong EPC is expensive to fix later. This page does not re-explain what an EPC is (there's a separate guide for that). It gives you the buyer's own question set to interview every bidder before you decide — the questions a strong EPC answers instantly and a weak one stumbles on.
The best way to choose a factory solar EPC is to interview every bidder with the same question set and compare answers. The three questions that expose weakness fastest: (1) Which legal entity backs the panel and workmanship warranty, and will it still exist in 25 years? (2) Who handles O&M after year one, and at what price? (3) Can you visit a live reference site? Vague answers are a warning.
The 3 questions that expose a weak EPC fastest
| Ask this | A strong EPC answers | Red-flag answer |
|---|---|---|
| Who backs the warranty — will they last 25 years? | Names the entity behind panel/inverter/workmanship warranty + company age + portfolio | "Fully guaranteed" but won't say by whom, or a brand-new company |
| Who does O&M after year one — at what price? | In-country O&M team + a priced O&M contract + a guaranteed response time | "Just call us anytime" — no written contract, price, or SLA |
| Can I visit a live reference site? | Offers a similar-size site to visit + lets you talk to the actual owner | Dodges, cites confidentiality, or only has renders / stock photos |
Use these three to screen first, then go deep with the full 12-question set below on whoever passes.
Why choosing the wrong EPC costs more than you think
Here's the asymmetry: a factory solar installer can disappear in 2-3 years, but the panel warranty is written for 25. If the entity backing it is gone, the warranty is nearly worthless. The common failures from a bad pick are an underperforming system, sub-standard wiring/installation that creates fire risk, and no one accountable when the inverter fails. This page helps you screen for the contractor who stays with you for the system's life — not just until the final invoice clears. Before signing, cross-check the 20-point solar procurement checklist and the list of Thai factory solar vendors. Also know the factory solar installation timeline (sign → COD) so you can hold the EPC to a real schedule, and read how to vet a solar installer and avoid scams to spot the warning signs before you pay a deposit.
The printable 10-point EPC vetting checklist
Print this checklist and walk every bidder through it, marking ✓ / ✗ on each line. Anything short of 10 ✓ means keep asking until it's clear — don't sign yet. In 2026, add one more check: that the EPC actually knows the current incentive rules (BOI / UGT2 / Direct PPA), because they are changing fast.
- 1) Verifiable track record — states cumulative MWp installed and has live sites you can visit
- 2) Names the warranty-backing entity, financially sound enough to last 25 years
- 3) Written, priced O&M contract with a response-time SLA
- 4) Performance / yield guarantee in writing
- 5) A licensed (กว.) electrical engineer signs off the design
- 6) Quality/safety standards (ISO 9001 / 45001) + utility registration
- 7) Names a bankable Tier-1 panel + inverter brand (not "or equivalent"), with an in-Thailand claims agent
- 8) Conducts an on-site roof/structure/electrical survey before quoting
- 9) The EPC handles ERC/PEA/MEA permitting + รง.4 / อ.6 end-to-end
- 10) Milestone-based payment (no abnormal upfront) + as-built handover + trains your team
Tip: press Ctrl/Cmd+P to print this page, then take the checklist into every interview and score each bidder on the same criteria.
Green flags vs red flags — the good and the dangerous signals side by side
As you talk to each EPC, watch which side of this table their answers and behavior fall on — the left is who to trust, the right is when to walk away.
| Green flag — good sign | Red flag — warning sign |
|---|---|
| Names the entity behind the panel/inverter/workmanship warranty | "Fully guaranteed" but won't say by whom, or dodges O&M questions |
| A priced O&M contract with a written response-time SLA | "Just call us anytime" — no contract, price, or SLA |
| Offers a live reference site and lets you talk to the actual owner | Won't allow a visit, cites confidentiality, or shows only renders/stock photos |
| A market-reasonable price with the cost build-up explained | A price far below market (usually lower-grade kit or scope cut out) |
| Gives you time to decide, no pressure, lets you read the full contract first | Pressure to sign today / deadline discount / asks to pay a personal account |
| A licensed engineer signs the design + has standards documentation (ISO) | No licensed engineer signs the design / no standards documents at all |
| Bills by milestone + delivers as-built docs + trains your team | Unusually high deposit + no as-built docs + hands over just the keys |
The 12 questions to ask every EPC before you sign
Use this as a standardized interview. Ask every bidder the same questions and record the answers in a table to compare directly — a vague answer tells you as much as a clear one.
1. Track record — ask for cumulative MWp installed and a project list
Ask for actual MWp installed, number of projects, and the names/locations of live sites. A strong contractor discloses verifiable numbers.
2. Warranty-backing entity — who backs it, and for how long
Separate who warrants the panels (manufacturer), the inverters, and the installation/workmanship (the EPC itself). Ask about company age and financial standing — a workmanship warranty is only worth anything if the EPC survives.
3. O&M after year one — who, what contract, what price
Get a written O&M contract: price per kWp/year, scope (cleaning, inspection, parts), and a response-time SLA. Beware "free support" with no documentation behind it.
4. Performance / production guarantee
Ask whether they guarantee a minimum annual yield (e.g. kWh/kWp) and how they compensate if output falls short. Get it in writing.
5. Crew — in-house install or subcontracted
Ask whether the install crew is in-house or subcontracted, who supervises on-site, and whether a licensed (กว./Council of Engineers) electrical engineer signs off the design. Subcontracting isn't always bad — but you must know who owns quality.
6. Company certifications and standards
Ask for the business license, standards held (e.g. ISO 9001/45001), utility registration, and installation safety record. See the "certifications to verify" section below for detail.
7. Equipment brands — which panel and inverter, what tier
Ask for the actual model and brand (not "or equivalent"). Verify the manufacturer is a bankable Tier-1 with a warranty agent in Thailand. Compare options in our solar panel brand comparison, and see what Tier 1 solar panels really mean (BNEF list vs มอก./IEC).
8. Roof assessment and design — do they survey on-site?
A good contractor surveys the roof, load-bearing structure, and electrical system before quoting — not just from satellite imagery. See the 9-point roof assessment checklist.
9. Permitting and grid interconnection — who handles it
Ask whether the EPC handles all ERC/PEA/MEA permitting and the รง.4 / อ.6 paperwork end-to-end, or pushes it onto you. See the solar permits & approvals guide, and read the overview of rooftop solar regulations in Thailand so you know the full scope the EPC must cover.
10. Payment structure and contract
Beware an unusually high upfront demand. Standard contractors stage payments by milestone (deposit–delivery–COD) and include retention/defects-liability terms. Read the whole contract before signing.
11. Insurance and liability
Ask about third-party liability insurance during installation and whether the delivered system is covered by property/fire insurance. See solar insurance & warranty for factories.
12. Handover and training your team
Ask whether handover includes complete as-built documentation, training so your team can read the monitoring system, and a manual/emergency contact list. A good handover means you understand how the system works — not just getting the keys.
Red flags — when to walk away
Not every flag means fraud — but several at once is enough risk to look elsewhere.
- A price far below market — usually means lower-grade kit or scope cut out
- Pressure to sign today / a deadline-driven discount
- Won't let you visit a real site, or shows only renders
- Dodges questions on the warranty-backing entity and O&M
- Unusually high deposit, or asks to pay a personal account
- No licensed engineer signs off the design / no standards docs
Certifications and credentials to verify
Ask the EPC to show these documents — and verify them against official sources, not just their word.
Company registration + financial standing
Verify it's properly registered, how many years it has operated, and whether it can financially back a long-term warranty.
Licensed engineer (Council of Engineers)
Factory-scale electrical work must have a licensed electrical engineer sign off the design and supervise.
Utility registration + permitting track record
An active EPC knows the PEA/MEA/ERC process and has cleared approvals on many prior projects.
Quality/safety standards (ISO 9001 / 45001)
Not mandatory, but signals a managed process and a focus on on-site safety.
Manufacturer authorization / warranty agency for panels
Verify the panel brand has a claims agent in Thailand — otherwise cross-border warranty claims get painful.
Warranty and O&M terms to lock down in the contract
"25-year warranty" is just a headline. What matters is the three layers beneath it and the O&M terms.
| Warranty layer | Ask / lock down |
|---|---|
| Panel (product + performance) | How many years product cover, what the degradation (performance) terms are, and is there an in-Thailand claims agent |
| Inverter | How many years, can it extend, who replaces/repairs, and are parts readily available |
| Installation (workmanship) | How many years the EPC warrants the install, whether it covers leaks/structure, and whether the backing entity survives |
| O&M (after the workmanship period) | Price per kWp/year, scope, cleaning frequency, response-time SLA, and whether monitoring sends alerts |
Get all three warranty layers in writing before signing, and keep them with the as-built documents. More detail in solar insurance & warranty for factories and solar monitoring & O&M.
How CapSolar answers all 12 of these questions
We wrote this page because we want you to hold us to the same standard as everyone else. CapSolar designs and installs factory rooftop solar, backed by 80+ MWp / 150+ projects in Thailand, 100+ clients, and 85,000+ tons of CO₂ avoided. We run an in-country O&M team, a transparently priced O&M contract, and we'll take you to a live reference site. Ask us all 12 questions — then compare us against anyone else.
About this guide
Compiled by the CapSolar team from real factory-solar installation experience in Thailand and the questions B2B buyers actually ask during procurement. This is general buyer guidance, not individualized legal advice — please verify each provider's documents and certifications against official sources.
Last reviewed Jun 2026.
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