Solar Irradiance Calculator
Use solar time instead of clock time for best consistency.
Example Data Table
| Latitude | Day | Solar Time | Tilt | Azimuth | DNI | GHI | POA Total |
|---|---|---|---|---|---|---|---|
| 30° | 172 | 12.50 | 25° | 0° | 1,146.49 W/m² | 1,303.99 W/m² | 1,255.50 W/m² |
| 35° | 80 | 11.75 | 30° | 10° | 875.20 W/m² | 782.64 W/m² | 904.15 W/m² |
| 18° | 220 | 13.10 | 15° | -20° | 1,021.34 W/m² | 1,011.77 W/m² | 1,046.21 W/m² |
These rows are illustrative reference cases for comparison.
Formula Used
δ = 23.45 × sin[360 × (284 + n) / 365]
ω = 15 × (solar time − 12)
cos θz = sin φ sin δ + cos φ cos δ cos ω
I0n = Gsc × [1 + 0.033 × cos(360n / 365)]
AM = 1 / [cos θz + 0.50572 × (96.07995 − θz)^−1.6364]
τb = exp(−k × AMalt)
DNI = I0n × τb
DHI = I0h × diffuse fraction × (1 − 0.3 × τb)
GHI = DNI × cos θz + DHI
POA total = POA beam + POA sky diffuse + POA ground reflected
POA sky diffuse = DHI × (1 + cos β) / 2
POA ground = GHI × albedo × (1 − cos β) / 2
This tool estimates site irradiance from geometry, atmospheric attenuation, diffuse share, altitude, and surface orientation. It is useful for scoping, sensitivity checks, and educational analysis.
How to Use This Calculator
- Enter site latitude and the day of year.
- Provide solar time for the exact analysis moment.
- Set surface tilt and azimuth for the panel plane.
- Enter local altitude, diffuse fraction, and ground albedo.
- Adjust atmospheric coefficient to match clarity or haze.
- Press calculate to view DNI, GHI, and plane-of-array irradiance.
- Use CSV or PDF export to save the calculated table.
Frequently Asked Questions
1. What does this solar irradiance tool calculate?
It estimates extraterrestrial irradiance, air mass, direct normal irradiance, diffuse horizontal irradiance, global horizontal irradiance, and total plane-of-array irradiance for a tilted surface.
2. Why does the tool ask for solar time?
Solar time aligns the sun position with the site’s actual solar geometry. It improves irradiance calculations when compared with ordinary civil clock time.
3. What is the atmospheric coefficient?
It is a simplified attenuation factor for atmospheric losses. Higher values represent heavier haze, dust, humidity, or pollution, which reduce beam irradiance.
4. What is diffuse fraction?
Diffuse fraction represents the share of sky-scattered light used in the estimate. Larger values raise diffuse irradiance and reduce the dominance of direct beam energy.
5. How should I set surface azimuth?
Use 0 degrees for a south-facing plane. Enter negative values for east-facing surfaces and positive values for west-facing surfaces.
6. Why are some results zero?
When the sun is below the horizon, the model sets beam and plane-of-array energy to zero. This typically happens at night or during very low sun angles.
7. Can I use this for system design decisions?
Yes, for early-stage studies and sensitivity checks. For final engineering, use local weather files, measured irradiance, shading analysis, and bankable simulation software.
8. Does altitude affect irradiance results?
Yes. Higher altitudes reduce air mass and often increase beam transmittance, which can raise direct irradiance compared with similar low-elevation sites.