What is GHI
Global Horizontal Irradiance (GHI) is the total solar radiation received on a horizontal surface at the Earth’s surface. It is the sum of direct beam radiation (Direct Normal Irradiance projected onto horizontal), diffuse radiation from the sky (Diffuse Horizontal Irradiance) and ground-reflected radiation (typically negligible for horizontal-facing).
GHI is measured in W/m² as instantaneous irradiance, or in kWh/m² as integrated irradiation over time. Daily GHI in India averages 4.5 to 6.5 kWh/m²; annual GHI ranges from 1,200 to 2,200 kWh/m² depending on region.
For Indian solar resource assessment, GHI is the foundational measurement, tracked in detail in QBits’ state-wise solar irradiance data. Plane-of-array irradiance (the irradiance actually hitting the tilted module) is derived from GHI, DNI and DHI through geometric transformations.
Why GHI matters
Solar resource baseline. GHI defines how much solar energy is available at a site, a figure worth cross-checking with QBits’ guide to maximising solar output in Indian climate conditions.
Yield modelling. P50 and P90 yield models use GHI as primary input.
Plant siting. High-GHI regions favour utility-scale solar plants.
Performance benchmarking. Performance Ratio is normalised against plane-of-array irradiance derived from GHI.
PPA tariff modelling. Higher-GHI sites can support lower PPA tariffs.
CUF prediction. CUF correlates strongly with GHI, as shown across the state benchmarks in QBits’ solar yield in India report.
GHI equation
Where:
- DNI = Direct Normal Irradiance
- θz = solar zenith angle
- DHI = Diffuse Horizontal Irradiance
Technical deep dive
Indian GHI by region (annual, kWh/m²)
| Region | Annual GHI |
|---|---|
| Western Rajasthan, Kutch | 2,000 to 2,200 |
| Ladakh, Spiti | 2,000 to 2,400 (altitude effect) |
| Andhra Pradesh, Telangana | 1,800 to 1,950 |
| Karnataka, Tamil Nadu | 1,750 to 1,900 |
| Maharashtra, MP, Chhattisgarh | 1,700 to 1,850 |
| Gujarat (non-Kutch) | 1,800 to 1,950 |
| Delhi, Haryana, Punjab | 1,650 to 1,800 |
| Uttar Pradesh, Bihar | 1,500 to 1,750 |
| West Bengal, Odisha | 1,500 to 1,700 |
| Kerala, Goa | 1,500 to 1,750 |
| North-east states | 1,200 to 1,500 |
GHI vs PSH
Peak Sun Hours (PSH) = GHI / 1 kW/m²
Seasonal variation
| Month | Approximate Delhi GHI (kWh/m²/day) |
|---|---|
| January | 4.0 |
| February | 4.8 |
| March | 5.8 |
| April | 6.5 |
| May | 6.8 |
| June | 5.5 (monsoon onset) |
| July | 4.5 (monsoon peak) |
| August | 4.0 (monsoon) |
| September | 4.8 |
| October | 5.2 |
| November | 4.3 |
| December | 3.8 |
GHI measurement
| Instrument | Description |
|---|---|
| Thermopile pyranometer (Class A) | High-accuracy reference |
| Silicon-cell pyranometer (Class B/C) | Lower cost, spectrum-limited |
| Satellite-derived | Spatial coverage |
| Reanalysis data (ERA5) | Historical archive |
Data sources
| Source | Coverage |
|---|---|
| NIWE Solar Resource Atlas | India ground stations |
| Solargis | Global satellite-derived |
| NSRDB | Global, including India |
| Meteonorm | Global synthesis |
| ISRO Bhuvan Solar | Indian satellite-derived |
Real-world applications
Solar plant siting and pre-feasibility.
Bankable yield modelling for project finance.
PR calculation for plant performance.
State-wise solar policy benchmarking.
Tariff order LCOE inputs.
Residential rooftop sizing, often estimated with a solar cost and savings calculator before a formal proposal is issued.
GHI quality classifications
| Class | Standard | Use |
|---|---|---|
| Class A (Secondary Standard) | ISO 9060 | Reference, scientific |
| Class B (First Class) | ISO 9060 | Commercial plants |
| Class C (Second Class) | ISO 9060 | Indicative |
| Reference cell | IEC 60904 | Calibration |
Common GHI problems
Soiled pyranometer reading low.
Shading on pyranometer (intermittent).
Calibration drift over years.
Satellite-derived GHI errors in cloudy regions.
Conversion errors between GHI and POA.
Best practices
Install calibrated pyranometers in array plane.
Clean sensors regularly.
Use ground-validated satellite data for long-term averages.
Apply P50 and P90 GHI for yield modelling.
Document GHI source and version in yield reports.
Match temporal resolution to project needs (hourly minimum for plant modelling).
Indian market context
NIWE operates a network of ground-based solar resource stations across India.
Solargis and NSRDB provide commercial satellite-derived GHI data for bankable yield modelling.
ISRO Bhuvan Solar provides free Indian satellite GHI data.
Major Indian utility solar projects use Solargis or Meteonorm P50/P90 data.
SECI tender bid documents reference GHI data for capacity allocation by state, and are typically supported by formal engineering resource center documentation during due diligence.
Standards and certifications
| Standard | Scope |
|---|---|
| ISO 9060 | Pyranometer classification |
| IEC 61724-1 | Performance monitoring including irradiance |
| WMO CIMO Guide | Meteorological measurement |
| NREL NSRDB methodology | Satellite-derived GHI |
Common mistakes
Using one year of GHI data instead of multi-year averages.
Confusing GHI with POA irradiance.
Skipping P90 sensitivity for project finance.
Treating satellite-derived data as fully ground-validated.
Ignoring soiling and shading on pyranometer.
Key takeaways
GHI is the foundational solar resource measurement, the total solar radiation hitting a horizontal surface. Indian GHI ranges from 1,200 kWh/m²/year in the north-east to over 2,200 in western Rajasthan and Ladakh. GHI is the primary input to yield modelling, plant siting and PPA tariff calculation. Calibrated measurements, multi-year averages and proper sensor maintenance determine bankable resource assessment.
Further reading
For how GHI plays out in real projects, these guides go deeper:
- Solar Irradiance Data India, Peak Sun Hours
- Solar Yield in India, kWh-per-kW Benchmarks by State
- India Rooftop Solar Capacity by State (2026)
- Solar Performance in Indian Winters, What to Expect
Sources
- NIWE Solar Resource Atlas.
- ISO 9060, Pyranometer classification.
- IEC 61724-1, Performance monitoring.
- Solargis Solar Resource Database.
- NSRDB National Solar Radiation Database, NREL.
- Meteonorm Global Solar Radiation Database.
- ISRO Bhuvan Solar.
- WMO Guide to Meteorological Instruments.
- IEA PVPS Task 16 Report on Solar Resource for High-Penetration Applications.
Frequently Asked Questions
What is GHI in simple words?
What is the unit of GHI?
What is the difference between GHI, DNI and DHI?
What is the average Indian GHI?
Which Indian regions have highest GHI?
How is GHI measured?
What is satellite-derived GHI?
Is GHI same as irradiation?
What is plane-of-array irradiance?
Why does Indian GHI vary by region?
How does GHI vary seasonally?
What is the typical Indian daily GHI?
How does GHI affect solar yield?
Where can I find Indian GHI data?
What is the relationship between GHI and PSH?
From the Blog
Co-founder & CEO driving Qbits Energy's product strategy and growth across India.