String vs Central vs Micro Inverter + Power Optimizer — Choosing Solar Inverter Architecture

When designing a solar system for a factory or commercial building, choosing the inverter architecture matters as much as choosing the panels — it determines how much you generate under shade, how granular your monitoring is, how safe the system is, and the cost per watt.

There are four main options — string, central, microinverter, power optimizer. This article makes clear which fits which job.

1. What an Inverter Does, and What "Architecture" Means

An inverter converts DC from the panels → AC for factory use / grid export. At its heart is MPPT (Maximum Power Point Tracking) — circuitry that forces each point to operate at maximum power.

"Architecture" is the question of at what level MPPT happens — whole string? whole array? per panel? The finer the level, the better the shading/mismatch tolerance, but the more expensive.

flowchart TD
    subgraph STR["String"]
        P1[N panels in series] --> I1["String Inverter
string-level MPPT"]
    end
    subgraph CEN["Central"]
        P2[many strings→combiner] --> I2["Large Central Inverter
aggregate MPPT"]
    end
    subgraph MIC["Microinverter"]
        P3[each panel] --> I3["Micro at panel
panel-level MPPT→AC"]
    end
    subgraph OPT["Power Optimizer"]
        P4[each panel+optimizer] --> I4["panel-level DC-DC
→ central String Inverter"]
    end

2. Comparison Table

3. Each Option in Depth

String Inverter — the C&I (commercial & industrial) standard

• Panels in series form strings, feeding an inverter with 2-4 MPPTs
• ✅ Low cost per watt, high efficiency, easy service/spares in Thailand
• ❌ A shaded/dirty panel in one string drags the whole string; an inverter failure takes down a big block; high-voltage DC on the roof

Central Inverter — for MW scale

• One large inverter takes many strings via combiner boxes
• ✅ Lowest cost per watt at large scale, centralized O&M
• ❌ Single point of failure; coarse MPPT; long DC runs — suited to solar farms, not typical factory roofs

Microinverter — per-panel MPPT + AC at the panel

• ✅ Maximum harvest under shade/mismatch; per-panel monitoring; no high-voltage DC on the roof (safe/built-in rapid shutdown); modular
• ❌ Highest cost per watt; many units on the roof (harder to access, but one failure = only one panel lost)

Power Optimizer (DC Optimizer) — the middle path (MLPE)

• A DC-DC box at each panel does per-panel MPPT + monitoring + rapid shutdown, but still sends DC to a central string inverter
• ✅ Nearly the per-panel benefits of micro, but the central inverter is easier to swap/service
• ❌ Often tied to a brand-specific ecosystem; adds cost + rooftop hardware

MLPE (Module-Level Power Electronics) = micro + optimizer — the group delivering panel-level MPPT and safety.

4. Safety + Grid Interconnection Standards

flowchart LR
    A["Inverter passes
IEC 62109 (safety)"] --> B["Anti-islanding
IEC 62116"]
    B --> C["array design
IEC 62548"]
    C --> D["interconnection approval
MEA / PEA"]
    D --> E["(option) Rapid shutdown
NEC 690.12"]

• IEC 62109 — safety of PV power converters (baseline requirement)
• IEC 62116 anti-islanding — the inverter must stop grid export instantly when the grid goes down, for utility-worker safety — a condition of MEA/PEA interconnection
• IEC 62548 — array design standard
• Rapid shutdown (NEC 690.12) — de-energizes high-voltage rooftop DC in an emergency; micro/optimizer have it built in, string needs add-on devices

5. How to Match the Architecture to the Job

flowchart TD
    Q1{MW-scale
solar farm?} -->|Yes| CEN[Central Inverter]
    Q1 -->|No, C&I roof| Q2{Shading/multi-orientation
/complex roof?}
    Q2 -->|No, single open plane| STR[String Inverter]
    Q2 -->|Yes| Q3{Need zero DC
on roof + higher budget?}
    Q3 -->|Yes| MIC[Microinverter]
    Q3 -->|Mid budget + easy inverter service| OPT[Power Optimizer]

• String — large single-plane, single-orientation, unshaded factory roof + value focus → the standard, most cost-effective choice in Thailand
• Central — only for MW-scale solar farms
• Microinverter — complex/shaded/multi-orientation roof + need for maximum safety (zero rooftop DC) + can absorb higher cost
• Power Optimizer — want per-panel benefits + rapid shutdown but prefer an easy-to-service central inverter, at a mid budget

Conclusion

There is no "best" architecture, only "fit for the job" — for most Thai factory roofs (large, single-orientation, unshaded), a string inverter is the most cost-effective. Micro/optimizer (MLPE) pay off with shading/multiple orientations, per-panel monitoring needs, or rapid-shutdown safety, while central is reserved for MW-scale solar farms.

Whichever you choose, the inverter must pass IEC 62109 + anti-islanding IEC 62116 and obtain MEA/PEA interconnection approval.

Sahawatthanakit (1988) handles survey, design and installation of Solar systems for factories and warehouses — our engineering team selects the inverter architecture to match your actual roof, shading and load profile, with ROI assessment.