What is irradiation
Irradiation is the total solar energy received per unit area over a specified time period, expressed in kWh/m² or MJ/m². It is the time integral of irradiance (instantaneous power per area). Daily irradiation is typical for solar PV design; annual irradiation is the standard for resource benchmarking; hourly irradiation supports detailed yield modelling.
For Indian solar, irradiation is the foundational measurement for resource assessment, yield prediction and plant siting. Daily irradiation in India averages 4.5-6.5 kWh/m²/day (varying by location and season). Annual irradiation ranges from 1,200 kWh/m²/year in the north-east to over 2,200 in western Rajasthan and Ladakh. See Solar Irradiance Data India, State-Wise Peak Sun Hours for the full state-by-state GHI, DHI and DNI table.
Why irradiation matters
Yield prediction. Direct proportional relationship to PV plant generation. See Solar Yield in India, kWh-per-kW Benchmarks by State for a worked verification method that sanity-checks generation estimates against irradiation.
Plant siting. High-irradiation sites favour utility-scale plants.
Tariff economics. Higher irradiation supports lower PPA tariffs.
Bankability. Multi-year irradiation data underpins project finance.
Performance benchmarking. PR is normalised against irradiation.
Irradiation types
| Type | Description |
|---|---|
| GHI (Global Horizontal Irradiation) | On horizontal surface |
| DNI (Direct Normal Irradiation) | On surface perpendicular to sun |
| DHI (Diffuse Horizontal Irradiation) | Sky-scattered on horizontal |
| POA (Plane of Array Irradiation) | On tilted module surface |
| GTI (Global Tilted Irradiation) | Same as POA |
Technical deep dive
Irradiation by region (annual, kWh/m²/year, on horizontal GHI)
| Region | Annual GHI |
|---|---|
| Western Rajasthan, Ladakh | 2,000-2,400 |
| Andhra Pradesh, Telangana | 1,800-1,950 |
| Karnataka, Tamil Nadu | 1,750-1,900 |
| Maharashtra, MP | 1,700-1,850 |
| Gujarat (non-Kutch) | 1,800-1,950 |
| Delhi, Haryana, Punjab | 1,650-1,800 |
| UP, Bihar | 1,500-1,750 |
| West Bengal, Odisha | 1,500-1,700 |
| Kerala, Goa | 1,500-1,750 |
| North-east | 1,200-1,500 |
Unit conversions
| From | To | Factor |
|---|---|---|
| kWh/m² | MJ/m² | × 3.6 |
| MJ/m² | kWh/m² | / 3.6 |
| kWh/m²/day | W/m² (average) | / 24 × 1000 |
| Wh/m² | kWh/m² | / 1000 |
POA vs GHI
POA irradiation typically exceeds GHI for south-facing tilt angle-optimised modules:
- POA gain at optimum tilt: +5 to +15 percent over GHI
- POA reduction at horizontal: 0% (POA = GHI)
- POA reduction at flat south-facing wall: -50% to -70%
Seasonal variation (Delhi example, daily GHI)
| Month | Daily kWh/m² |
|---|---|
| January | 4.0 |
| April | 6.5 |
| July | 4.5 (monsoon) |
| December | 3.8 |
Measurement instruments
| Type | Use |
|---|---|
| Thermopile pyranometer (Class A) | Reference quality |
| Silicon-cell pyranometer | Commercial monitoring |
| Pyrheliometer | DNI measurement |
| Albedometer | GHI and reflected |
Real-world applications
Solar plant siting, the same groundwork SurgePV’s shading analysis tools use to translate raw irradiation data into a site-specific generation estimate.
Bankable yield modelling.
PR calculation.
PPA tariff inputs.
Tariff orders.
Performance benchmarking.
Best practices
Use multi-year averaged irradiation (10+ years).
Cross-validate satellite data against ground stations.
Document irradiation source in yield reports.
Apply P50 and P90 distributions for project finance.
Account for inter-annual variability (typically ±5 percent).
Indian market context
NIWE Solar Atlas provides Indian ground station data.
Solargis and NSRDB provide bankable satellite-derived irradiation.
Meteonorm and PVGIS used for residential and small commercial.
SECI tender bids use validated multi-year irradiation.
PM Surya Ghar residential uses location-based estimates, and homeowners comparing quotes can cross-check the assumed irradiation against Heaven Green Energy’s solar calculator before signing.
Standards and certifications
| Standard | Scope |
|---|---|
| ISO 9060 | Pyranometer classification |
| IEC 61724 | Performance monitoring |
| WMO CIMO Guide | Meteorological measurement |
| NREL NSRDB methodology | Satellite-derived |
Key takeaways
Irradiation is solar energy per unit area over time, measured in kWh/m² or MJ/m². Indian annual irradiation ranges from 1,200 (north-east) to over 2,200 (Rajasthan, Ladakh). Key types: GHI, DNI, DHI, POA. Multi-year ground-validated satellite data is the bankable standard. Irradiation underpins yield prediction, plant siting and PPA tariff economics. Inter-annual variability of ±5 percent affects yield projections.
Further reading
For how Irradiation plays out in real projects, these guides go deeper:
- How Much Electricity Does Solar Generate in India?
- Solar Yield in India, kWh-per-kW Benchmarks by State
Sources
- ISO 9060, Pyranometer classification.
- IEC 61724, Performance monitoring.
- NIWE Solar Resource Atlas.
- Solargis Solar Resource Database.
- NSRDB, NREL.
- Meteonorm Global Solar Radiation Database.
- PVGIS Photovoltaic Geographic Information System.
- WMO Guide to Meteorological Instruments.
- IEA PVPS Task 16 publications.
Frequently Asked Questions
What is irradiation in simple words?
Irradiation vs irradiance?
What is the unit?
Indian daily irradiation?
Indian annual irradiation?
Is irradiation same as GHI?
How is irradiation measured?
Is irradiation same as insolation?
How does irradiation relate to yield?
What is the highest irradiation in the world?
Is Indian irradiation high?
How does monsoon affect irradiation?
What is plane-of-array irradiation?
How accurate is satellite-derived irradiation?
Where is irradiation data found?
From the Blog
CFO overseeing solar financing, procurement, and channel partner enablement at Qbits.