Solar Insolation Data Calculator

Calculator Inputs

Site name

Use any project, site, or client reference.

Date

This sets the day number and solar geometry.

Latitude (deg)

North positive, south negative.

Longitude (deg)

Stored for reporting and exported summaries.

Panel tilt (deg)

Tilted plane assumes a south-facing surface.

Ground albedo

Typical values: soil 0.17 to 0.25, snow higher.

GHI (kWh/m²/day)

Leave zero to estimate from sunshine duration or components.

DNI (kWh/m²/day)

Optional direct normal energy input.

DHI (kWh/m²/day)

Optional diffuse horizontal energy input.

Sunshine hours

Used only when GHI is missing or zero.

Angstrom a

Default commonly starts near 0.25.

Angstrom b

Default commonly starts near 0.50.

Performance ratio (%)

Used to estimate 1 kWp daily energy output.

Example Data Table

Site	Date	Latitude	Tilt	GHI	DHI	DNI	POA Insolation
Karachi Roof A	2026-03-13	24.8607	25°	5.80	1.70	6.30	6.11
Lahore Plant B	2026-06-15	31.5204	20°	6.35	1.95	6.90	6.47
Quetta Farm C	2026-10-10	30.1798	30°	5.25	1.30	6.10	5.78

Values above are illustrative sample entries for layout and interpretation.

Formula Used

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

Sunset hour angle: ωs = cos^-1(−tan φ × tan δ)

Day length: N = 24 × ωs / π

Extraterrestrial daily radiation: H0 = (24×60/π) × Gsc × dr × [cosφ cosδ sinωs + ωs sinφ sinδ]

Angstrom-Prescott estimate: H / H0 = a + b × (n / N)

Clearness index: Kt = H / H0

Diffuse fraction: Estimated with the Erbs correlation from Kt

Tilted beam ratio: Rb = H_b,t / H_b using a daily geometric ratio for south-facing surfaces

Tilted plane insolation: Ht = Hb×Rb + DHI×(1 + cosβ)/2 + H×ρ×(1 − cosβ)/2

1 kWp energy estimate: Daily energy = Ht × Performance Ratio

In this page, H is daily global horizontal energy, H0 is extraterrestrial daily energy, Hb is beam horizontal energy, β is tilt angle, ρ is albedo, φ is latitude, and n is day number.

How to Use This Calculator

Enter a site name and choose the calculation date.
Provide latitude and longitude for the location.
Set the module tilt and ground albedo.
Enter measured GHI when available for the best daily result.
Add DNI and DHI if you have component data from instruments or reports.
If GHI is unavailable, leave it at zero and enter sunshine hours.
Adjust Angstrom constants when you have local calibration values.
Set the performance ratio to estimate daily 1 kWp delivered energy.
Press the calculate button to show the result above the form.
Use the CSV and PDF buttons to export the summary and monthly profile.

FAQs

1) What does this calculator measure?

It estimates daily solar resource metrics such as extraterrestrial radiation, clearness index, diffuse share, tilted plane insolation, peak sun hours, and approximate 1 kWp daily output.

2) What is the difference between GHI, DNI, and DHI?

GHI is total energy on a horizontal surface. DNI is direct beam energy normal to the sun. DHI is the scattered diffuse energy received on a horizontal plane.

3) When should I use sunshine hours instead of GHI?

Use sunshine hours when measured GHI is unavailable. The calculator applies the Angstrom-Prescott relation to estimate horizontal solar energy from sunshine duration and theoretical day length.

4) Why does tilt increase or reduce insolation?

Tilt changes how directly the panel faces the sun and sky dome. A suitable tilt can improve beam capture, while poor tilt may lower total plane-of-array energy.

5) What does the clearness index tell me?

The clearness index compares actual surface radiation with extraterrestrial radiation. Lower values suggest cloudy or hazy conditions, while higher values indicate clearer skies and stronger direct beam content.

6) Is the graph monthly or daily?

The summary table shows the selected day. The Plotly graph shows a monthly profile using representative days and the same calculated clearness behavior or sunshine-based estimate.

7) Does the tool account for panel azimuth?

This version assumes a south-facing surface for the tilted plane calculation. That is a common first-pass design assumption for northern hemisphere locations.

8) Can I use the 1 kWp result as final system production?

It is a planning estimate only. Final production depends on temperature, shading, wiring losses, inverter behavior, soiling, mismatch, downtime, and exact orientation.