POA (Plane of Array) Irradiance

POA (Plane of Array) Irradiance is the total amount of solar radiation—direct, diffuse, and ground-reflected—that strikes the surface of a solar module at its actual tilt and azimuth. It represents the true irradiance conditions under which photovoltaic modules operate, making it one of the most critical inputs in solar designing, energy forecasting, solar shading analysis, and system performance modeling.

Unlike Global Horizontal Irradiance (GHI) or generic tilted irradiance values, POA irradiance aligns exactly with the mounting structure and geometry of the solar array. This makes it essential for accurate production estimates, stringing & electrical design, inverter loading ratio (ILR) evaluation, and long-term financial modeling.

POA irradiance is foundational across the entire solar workflow—from early feasibility studies and solar proposals to engineering validation, commissioning, and ongoing system monitoring.

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Key Takeaways

POA irradiance represents the actual sunlight striking solar modules.
It combines direct, diffuse, and reflected irradiance.
Shading, tilt, azimuth, and albedo significantly impact POA.
POA is essential for accurate design, forecasting, and financial modeling.
Effective POA is the most realistic metric for energy yield predictions.

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What It Is

POA irradiance measures how much usable sunlight actually reaches the plane of a solar module after accounting for real-world installation conditions, including:

Tilt angle
Azimuth orientation
Shading losses from trees, buildings, and obstructions
Diffuse Horizontal Irradiance (DHI)
Ground-reflected radiation (albedo)
Tracking system movement for single-axis and dual-axis trackers

In simple terms, POA irradiance tells solar designers how much sunlight the modules truly see, making it far more actionable than horizontal or normal-incidence irradiance values.

POA is heavily used in:

Solar layout optimization
Stringing & Electrical Design
Shadow Analysis
Performance simulations
AC/DC ratio planning
Energy yield predictions for residential, commercial, and utility-scale systems

How It Works

POA irradiance is calculated by transforming irradiance data from standard measurement planes into the actual plane of the solar array, using sun position, array geometry, and site conditions.

1. Start with Irradiance Components

Meteorological datasets such as TMY or EPW provide:

These inputs are essential for accurate modeling inside solar design software.

2. Convert Direct Irradiance to the Array Plane

Direct beam radiation is projected onto the module plane based on:

Tilt
Azimuth
Sun angle, often validated using the Sun Angle Calculator

3. Model Diffuse Sky Irradiance

Diffuse light is redistributed across the sky dome using established models:

Isotropic
Hay & Davies
Perez (most accurate and widely used)

4. Add Ground-Reflected Irradiance

Reflected irradiance depends on albedo, which varies with snow, sand, vegetation, or concrete—especially impactful in bifacial systems.

5. Apply Shading and Soiling Adjustments

Near- and far-shading losses are quantified using Shadow Analysis, including obstructions like chimneys, trees, parapets, and terrain.

6. Combine to Get POA

POA = POA_direct + POA_diffuse + POA_reflected − shading losses

This final POA value becomes the primary input for production engines, performance simulations, and yield calculations.

Types / Variants

1. POA (Unshaded)

Ideal plane-of-array irradiance assuming no shading.

Used in theoretical maximum and feasibility studies.

2. POA (Shaded / Effective POA)

Adjusted for shading, soiling, and mismatch losses.

Used for realistic energy yield and ROI modeling.

3. POA for Tracking Systems

Calculated dynamically as modules rotate throughout the day.

Critical for utility-scale trackers and backtracking logic.

4. Spectrally Adjusted POA

Used when evaluating technology-dependent responses such as HJT, thin-film, or bifacial modules.

How It’s Measured

POA can be measured directly on-site using professional instrumentation.

1. Pyranometers

Mounted at the same tilt and azimuth as the array
Measures global POA irradiance
Common in commissioning and performance validation

2. Reference Cells

Small PV cells mounted in the array plane
Closely mimic real module behavior
Sensitive to spectrum and temperature

Units

W/m² (instantaneous)
kWh/m²/day, month, or year (energy over time)

Typical peak POA values range from 900–1100 W/m² under clear skies with optimal tilt.

Practical Guidance

For Solar Designers

Always model POA using site-specific meteorological data.
Optimize tilt and azimuth for annual POA, not just noon output.
Use Shadow Analysis to quantify losses from obstructions.
Validate assumptions using pyranometer or reference cell data during commissioning.

For Installers

Ensure sensors are mounted in the same plane as modules.
Verify tilt and azimuth against engineering drawings.
Maintain sensor cleanliness and calibration.

For EPCs & Developers

POA is central to energy yield modeling, P50/P90 analysis, and bankability studies.
Tracking systems require dynamic POA calculations.
Seasonal effects such as snow, dust, and haze should be included as POA loss factors.

For Sales Teams

POA-based estimates are far more accurate than GHI-based claims.
Use POA-driven simulations inside Solar Proposals to communicate realistic energy outcomes and ROI.

Real-World Examples

Residential Rooftop System

A 6 kW rooftop system is evaluated using Shadow Analysis, revealing chimney shading that reduces morning POA by 12%.

The effective POA is then used for string sizing, annual yield estimates, and proposal creation.

Commercial Flat Roof

A 500 kW ballasted array balances tilt, spacing, and POA.

Increasing row spacing improves winter POA enough to justify higher racking costs.

Utility-Scale Solar Farm

A 50 MW single-axis tracker project uses dynamic POA calculations to optimize backtracking, reducing shading losses and improving annual yield.

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Releated Terms

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