Global Irradiance Data Calculator

Input Data

Surface azimuth uses south = 0°, east = -90°, west = 90°. Sunshine hours can override the basic clearness estimate.

Latitude (°)

Use positive north and negative south.

Day of Year

Example: 172 is near late June.

Base Clearness Index

Typical range is 0.30 to 0.75.

Measured Sunshine Hours

Leave 0 to rely on the base clearness index.

Possible Sunshine Hours

Optional. Leave blank to use astronomical day length.

Altitude (m)

Higher altitude slightly reduces atmospheric losses.

Cloud Factor (0 to 1)

0 means clear sky. 1 means dense cloudiness.

Surface Tilt (°)

Tilt from horizontal.

Surface Azimuth (°)

South-facing is 0° in this calculator.

Ground Reflectance

Typical values: 0.20 soil, 0.60 snow.

Array Area (m²)

Used for incident solar energy and output estimates.

Module Efficiency (%)

Modern modules often fall near 18% to 23%.

Performance Ratio (%)

Captures wiring, temperature, inverter, and mismatch losses.

Example Data Table

Scenario	Latitude (°)	Day	Clearness	Tilt (°)	GHI (MJ/m²/day)	POA (kWh/m²/day)
Coastal summer array	24.86	172	0.62	25	25.40	7.34
Desert shoulder season	26.20	95	0.68	20	22.75	6.41
Cloudy monsoon case	31.50	220	0.36	30	14.10	3.57
High-altitude clear site	35.10	300	0.70	35	18.90	5.62

Formula Used

1) Declination angle
δ = 23.45 × sin[360 × (284 + n) / 365]

2) Earth-sun distance correction
d_r = 1 + 0.033 × cos(360 × n / 365)

3) Sunset hour angle
ω_s = arccos[-tan(φ) × tan(δ)]

4) Day length
N = 24 × ω_s / π

5) Daily extraterrestrial horizontal radiation
H₀ = (24 × 60 / π) × G_sc × d_r × [cosφ × cosδ × sinω_s + ω_s × sinφ × sinδ]

6) Angstrom-Prescott sunshine relation
K_t ≈ a + b × (n / N), where a = 0.25 and b = 0.50

7) Global horizontal irradiation
GHI = K_t,adjusted × H₀

8) Diffuse fraction by Erbs correlation
The calculator applies the daily Erbs model to split global radiation into diffuse and beam components.

9) Tilted plane irradiation
POA = Beam × R_b + Diffuse × (1 + cosβ)/2 + GHI × ρ × (1 - cosβ)/2

This model is useful for planning and screening studies. It is not a replacement for high-resolution ground measurements or bankable energy simulation.

How to Use This Calculator

Enter the site latitude and day of year.
Provide either a base clearness index or measured sunshine hours.
Set altitude and cloud factor to reflect site atmosphere.
Enter panel tilt, azimuth, and ground reflectance.
Add array area, module efficiency, and performance ratio.
Press the calculate button to generate irradiance and output estimates.
Review the result cards, detailed table, and graph.
Use the CSV or PDF buttons to export the calculated summary.

FAQs

1. What does this calculator estimate?

It estimates daily extraterrestrial radiation, global horizontal irradiation, diffuse share, beam potential, tilted plane irradiation, and expected array energy from practical site inputs.

2. What is the clearness index?

The clearness index compares radiation reaching the ground with radiation available at the top of the atmosphere. Higher values usually mean clearer skies and stronger solar resource.

3. Why can sunshine hours override the clearness input?

Measured sunshine duration often gives a stronger site-specific estimate of sky clarity. When entered, the calculator converts that duration into an effective clearness value.

4. What is POA irradiation?

POA means plane-of-array irradiation. It represents solar energy striking the tilted surface, which is more relevant for panel performance than horizontal radiation alone.

5. Why does altitude affect the result?

Higher elevations typically have less atmospheric mass above the site. That can reduce scattering and absorption, so the calculator applies a modest positive adjustment.

6. Is the estimated DNI exact?

No. It is an engineering estimate based on daily energy splitting and noon geometry. For detailed concentrating or tracking studies, use measured or modeled hourly DNI datasets.

7. Can I use snow or bright roofs in reflectance?

Yes. Ground reflectance can be raised to capture snow, pale surfaces, or reflective rooftops. Higher reflectance increases the ground-reflected contribution on tilted arrays.

8. Is this suitable for bankable design?

It is best for feasibility checks, quick comparisons, and educational use. Final investment decisions should rely on validated weather files, loss modeling, and detailed simulation.