GHI vs GTI (POA) – A Practical Field Guide for Solar Plant Engineers ☀️⚡

In utility-scale solar plants, SCADA often shows:

POA (GTI) > GHI

Is this correct?
✅ Yes — in most cases, this indicates healthy irradiance measurement and proper sensor behavior.

Understanding why this happens is critical for accurate PR analysis, energy yield validation, and revenue assurance.

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☀️ Irradiance Basics (Field-Oriented View)

🔹 GHI – Global Horizontal Irradiance

➡️ Measured on a 0° horizontal surface
➡️ Includes:

• Direct Normal Irradiance (projected on horizontal plane)

• Diffuse sky radiation

• Ground-reflected component

🔹 GTI / POA – Plane of Array Irradiance

➡️ Measured at module tilt angle (e.g., 8–15° in many Indian plants)
➡️ Represents the actual irradiance received by the modules

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📊 Why POA is Typically Higher than GHI

When modules are tilted toward the sun:

✔ Better geometric alignment with direct solar rays
✔ Higher beam component capture
✔ Additional contribution from:

• Diffuse radiation 🌤️

• Ground reflection (Albedo) 🌍

This effect is more pronounced during:

• 🌅 Morning hours

• 🌇 Evening hours

• ☀️ Clear sky days

At solar noon, the difference narrows but POA generally remains slightly higher in fixed-tilt systems.

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📈 Practical Field Benchmark (O&M Reference)

$$\textbf{POA / GHI Ratio = 1.02 to 1.08}$$

✅ This range indicates:

• Proper sensor orientation

• Correct leveling

• Healthy calibration

• No abnormal shading

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⚠️ When Should Engineers Investigate?

🚨 POA / GHI > 1.12
Possible causes:

• GHI sensor soiling

• GHI leveling error

• Sensor drift

• Obstruction/shading

🚨 POA / GHI < 0.90
Possible causes:

• POA sensor shading

• Incorrect tilt angle

• POA sensor misalignment

• Heavy soiling

👉 Sudden ratio deviation = Early warning sign of measurement error.

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🔧 Pyranometer Best Practices (Utility-Scale Plants)

✔ Use same make & model for GHI & POA
✔ Follow same calibration standard (ISO / IEC compliant)
✔ Maintain regular cleaning schedule
✔ Annual recalibration (as per manufacturer guidelines)
✔ Ensure proper bubble leveling

📌 Installation Guidelines

• GHI → Mounted horizontal (0°)

• POA → Mounted at exact module tilt angle

Even a 2–3° tilt deviation can distort readings.

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📉 Why GHI Appears Lower in SCADA?

Most common reasons:

✔ Natural geometric tilt advantage
✔ Dust accumulation on GHI sensor 🧽
✔ Shadow from nearby structures ⚠️
✔ Sensor drift over time
✔ Poor leveling

Always check physical condition before concluding performance issues.

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⏱️ 2-Min Daily O&M Quick Check

At solar noon (clear sky):

• GHI ≈ 900–1000 W/m²

• POA ≈ 950–1050 W/m²

👉 Large deviation?
Immediately verify:

• Sensor cleanliness

• Leveling

• Cable termination

• Data logger inputs

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🧠 Advanced O&M Engineering Insights

✔ Always use POA for PR calculation (never GHI)
✔ Validate irradiation trend with daily generation curve
✔ Compare cumulative daily irradiation vs plant output
✔ Sudden POA/GHI ratio shift = predictive maintenance trigger
✔ Cross-check with satellite irradiation if doubt persists

Well-maintained irradiation data = Accurate performance analytics.

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🚀 Final Technical Takeaway

✔ Slightly higher POA than GHI = Normal & expected
✔ Stable POA/GHI ratio = Reliable instrumentation
✔ Accurate irradiance data = Correct PR & revenue confidence
✔ Smart monitoring = High-performing solar asset

Strong fundamentals + disciplined O&M = Sustainable generation excellence ⚡☀️

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