Solar Rooftop Factory Permits in Thailand 2026 — RG.4, ERC, Building Permit & Grid Interconnection

Many factories want solar to cut their power bill 30–60%, but stall on a single question: "what permits do I need, and how painful is it?" The good news: in late 2024 the government removed the RG.4 factory license for rooftop solar — the process is now much shorter. This guide lays out exactly what remains to be done in 2026, end to end, so you can plan before you start.

Want to know how many kWp makes sense for your factory and the payback period? Enter your monthly bill in the Solar ROI Calculator before you file paperwork — instant numbers, free.

Overview: after the unlock, 3–4 steps remain

For a rooftop, self-consumption system outside an industrial estate:

Step	Authority	Key threshold	Status in 2026
Factory license (RG.4)	DIW	Rooftop, any size	✅ Exempt (Ministerial Reg. 2024)
Power-generation license	ERC	≤ 1,000 kVA = exemption filing	⚠️ Must notify/file
Building modification (Or.1)	Local authority	≤ 160 sq.m + engineer cert = exempt	⚠️ Size-dependent
Grid interconnection	PEA / MEA	Any grid-tied system + zero export	🔴 Always required

Step 1 — Factory license (RG.4): now unlocked ✅

Power generation used to fall under "factory category 88," requiring an RG.4 license. But Ministerial Regulation No. 3 B.E. 2567, effective 28 December 2024, declares that rooftop solar power generation outside industrial estates, at any installed capacity, is NOT classified as a factory → no RG.4 and no DIW factory operating license required.

⚠️ Watch the exceptions:

• Ground-mounted solar farms may still fall under factory category 88 — a different case from rooftop.
• Factories inside an industrial estate (IEAT) follow a separate IEAT process — check with the estate.

Step 2 — ERC power-generation license (1,000 kVA threshold)

Even when it's not a factory, you still pass through the Energy Regulatory Commission (ERC) based on capacity:

• Installed ≤ 1,000 kVA → no full license, but you must file for exemption (you receive an exemption acknowledgement for energy-business operation) — submittable online via ERC's system.
• Installed > 1,000 kVA → a full power-generation license is required (more documents and time).

kVA ≠ kWp: ERC measures the threshold at the inverter/generator kVA (AC power), not the panel kWp (DC). Most systems are designed so total inverter capacity stays under 1,000 kVA to remain within the exemption.

Step 3 — Building modification permit (Or.1): 160 sq.m threshold

Under the Building Control Act, adding panel weight to a roof can count as a "building modification." There is an exemption if all conditions are met:

• Total panel area ≤ 160 square metres, and
• Total weight ≤ 20 kg/sq.m, and
• A licensed civil engineer certifies the roof structure's strength.

Meet all three = no Or.1 filing required. If the panel area exceeds 160 sq.m (most factories do), you must file a building-modification permit with the local authority, with drawings and calculations signed by an engineer.

Whether or not you qualify for the exemption, an engineer's roof-structure check should be done in every case — panels plus mounts carry wind load (see factory rooftop solar structural load assessment).

Step 4 — PEA / MEA grid interconnection (don't forget this)

The step many overlook, but it is always mandatory for any grid-tied (on-grid) system:

1. File a grid-connection (parallel-operation) application with MEA (Bangkok/Nonthaburi/Samut Prakan) or PEA (other provinces).
2. Attach a single-line diagram, inverter spec (must be on the utility's approved inverter list), and protection settings.
3. Commercial/industrial (C&I) users in Thailand cannot export power to the grid (zero export) → you must install a zero-export controller / anti-islanding device to prevent reverse flow.
4. The utility reviews, then schedules a commissioning test before authorizing live parallel operation.

The decision flow

flowchart TD
  A["Rooftop, self-consumption
outside an estate"] --> B["RG.4:
already exempt ✅
(2024 regulation)"]
  B --> C{"Inverter capacity
≤ 1,000 kVA?"}
  C -->|"Yes"| D["ERC:
file for exemption"]
  C -->|"No (>1,000 kVA)"| E["ERC:
full license"]
  D --> F{"Panel area
≤ 160 sq.m?"}
  E --> F
  F -->|"Yes + engineer cert"| G["Or.1:
exempt"]
  F -->|"No (over 160)"| H["Or.1:
file building permit"]
  G --> I["Grid interconnection
PEA/MEA + zero export 🔴
(always)"]
  H --> I
  I --> J["Commissioning test
→ parallel operation → live"]

Documents to prepare (checklist)

• Company affidavit + land title / building lease
• Single-line diagram + installation layout
• Inverter and panel specs + certification (inverter on the utility list)
• Structural calculations + licensed civil engineer's certification
• ERC exemption form / PEA-MEA interconnection application
• (If over 160 sq.m) building modification permit application

A typical end-to-end timeline is ~1–3 months, depending on size and document completeness — the usual bottlenecks are the structural drawings and the utility's inspection queue.

Cautions

• The rules changed most recently in late 2024 — re-verify current notices with ERC / the utility / the local authority before starting a real project.
• If you pursue BOI investment promotion or sit inside an IEAT estate, separate additional conditions apply.
• Do not begin installation before interconnection approval — the utility can order disconnection and impose penalties.

Let Sahawatthanakit handle it end to end

We deliver factory rooftop solar as turnkey EPC — roof survey, design, structural calculations with engineer certification, ERC filing + PEA/MEA interconnection, installation, and commissioning. You don't chase paperwork yourself.

• Phone: 02-096-2118 / 061-541-6939 / 096-109-4244 (Khun Mam)
• LINE: @406rrgvm
• Email: info@sahawatthanakit1988.com
• Free quote / consultation: click here

Frequently Asked Questions

Does a factory rooftop solar system need an RG.4 license? No longer — since Ministerial Regulation No. 3 B.E. 2567 took effect on 28 Dec 2024, rooftop solar outside industrial estates is not classified as a factory at any size, so no RG.4 is required (ground-mounted or in-estate systems may differ).

If I install ≤ 1,000 kVA, what do I do with the ERC? No full license, but you must file an exemption notification for energy-business operation with the ERC to receive an acknowledgement. Above 1,000 kVA requires a full power-generation license.

Can I sell surplus power back to the utility? For commercial/industrial (C&I) users, Thailand currently applies a zero-export policy — reverse flow into the grid is not allowed. You must design for self-consumption and install a reverse-flow prevention device. The economics come from offsetting your bill, not from selling power.

How long does interconnection take? The whole process is roughly 1–3 months depending on system size and document completeness; the bottlenecks are usually the engineer-certified structural drawings and the utility's inspection queue.

Do I need an engineer's certification? You should have one in every case, and it is required when the panel area exceeds 160 sq.m or when filing a building modification — a licensed civil engineer must certify that the roof can carry the panel weight and wind load.