Quick Facts
What DNI is
Direct Normal Irradiance (DNI) is the solar irradiance from the direct beam of sunlight, measured on a surface held perpendicular (normal) to the sun’s direction. DNI captures only the direct sun-disk component, excluding scattered (diffuse) light from the rest of the sky.
DNI is the relevant metric for:
Concentrating Solar Power (CSP): Mirrors or lenses focus only direct light. Diffuse light cannot be concentrated.
Concentrator Photovoltaics (CPV): Similar requirement; only direct light can be concentrated onto small high-efficiency cells.
Single-axis tracker analysis: Trackers improve energy capture more in high-DNI conditions.
Solar plant design optimisation: Understanding DNI separately from DHI helps tilt and tracker decisions.
The unit is watts per square metre (W per sq m), same as other irradiance measures.
DNI versus GHI versus DHI
The three irradiance components form a relationship:
GHI = DNI × cos(zenith angle) + DHIWhere:
GHI: Global Horizontal Irradiance (W per sq m).
DNI: Direct Normal Irradiance (W per sq m).
DHI: Diffuse Horizontal Irradiance (W per sq m).
Zenith angle: Angle between sun and vertical (0 when sun overhead, 90 at sunrise/sunset).
The cosine factor accounts for the geometric projection of DNI onto a horizontal surface.
At Indian solar noon (zenith angle about 25 to 30 degrees):
Typical DNI: 800 W per sq m.
Typical DHI: 100 W per sq m.
cos(30) = 0.866.
GHI = 800 × 0.866 + 100 = 793 W per sq m.
For solar plant design, DNI and DHI together give a more detailed picture than GHI alone. Software like PVsyst uses all three to model POA irradiance.
DNI in India
India has substantial DNI variation by region:
| Region | Annual DNI (kWh per sq m per year) | Solar Application |
|---|---|---|
| Western Rajasthan | 2,200 to 2,500 | Premium for CSP, trackers |
| Kutch, Gujarat | 2,100 to 2,400 | Excellent for trackers |
| Andhra Pradesh, Telangana | 1,800 to 2,200 | Good for trackers |
| Karnataka, Maharashtra inland | 1,700 to 2,000 | Good for trackers |
| Madhya Pradesh | 1,700 to 2,000 | Good for trackers |
| Tamil Nadu | 1,500 to 1,800 | Moderate; humidity reduces DNI |
| Coastal Maharashtra, Goa | 1,300 to 1,600 | Lower; humidity, monsoon |
| Punjab, Haryana | 1,400 to 1,700 | Moderate; pollution effects |
| Bihar, Jharkhand | 1,300 to 1,600 | Lower; humidity |
| West Bengal, Northeast | 1,000 to 1,400 | Lower; monsoon, humidity |
The highest DNI regions (Rajasthan, Gujarat) support both PV with trackers and CSP. Lower DNI regions favour fixed-tilt PV without trackers.
DNI measurement
DNI is measured by a pyrheliometer:
A pyrheliometer has a narrow field of view (about 5 degrees) that sees only the direct sun and immediate circumsolar region.
The pyrheliometer is mounted on a solar tracker that keeps the sensor pointed at the sun.
The sensor responds only to direct sunlight, ignoring diffuse light from the rest of the sky.
The measurement is in W per sq m of direct beam irradiance.
Pyrheliometers cost significantly more than pyranometers due to the precision tracker and narrow field of view design. Utility-scale CSP and CPV plants install pyrheliometers as standard. Most PV plants use only pyranometers (for GHI and POA), since DNI is less critical for fixed-tilt PV.
DNI and tracker design
Single-axis trackers improve energy capture by following the sun’s east-to-west motion. Trackers benefit most in high-DNI conditions:
In high-DNI (Rajasthan, Gujarat): Trackers add 15% to 25% to annual energy versus fixed-tilt.
In moderate-DNI (Karnataka, Andhra Pradesh): Trackers add 12% to 20%.
In low-DNI (Northeast, monsoon-heavy): Trackers add 8% to 15%.
The cost-benefit of trackers depends on local DNI. High-DNI regions justify trackers more readily.
The trade-off:
Tracker CAPEX: 10% to 20% higher than fixed-tilt.
Tracker O&M: Slightly higher due to motor maintenance.
Tracker energy gain: 15% to 25% in high-DNI sites.
Net: Trackers improve LCOE in high-DNI regions.
For Indian utility-scale projects in Rajasthan and Gujarat, single-axis trackers with bifacial modules are now the standard configuration.
DNI and CSP
Concentrating Solar Power (CSP) uses mirrors or lenses to focus sunlight onto a small receiver, generating high-temperature heat that drives a turbine.
CSP requirements:
High DNI: Annual DNI above 2,000 kWh per sq m is the threshold for viable CSP.
Direct beam only: CSP cannot use diffuse light.
Land availability: CSP plants are larger per MW than PV.
Water for cooling (some designs): Significant water consumption.
In India, CSP has had limited deployment compared to PV. Indian CSP plants total about 200 MW capacity, deployed in Andhra Pradesh and Rajasthan during 2010-2014. The technology has been largely displaced by PV with batteries for the same applications (dispatchable solar generation).
DNI versus diffuse light fraction
The DNI fraction of GHI varies with conditions:
Clear desert sky (high DNI conditions): DNI fraction 75% to 85% of GHI.
Cloudy/overcast: DNI fraction 0% to 20% of GHI (most light is diffuse).
Monsoon-affected sites: DNI fraction 40% to 60% of GHI annually.
Coastal humid sites: DNI fraction 50% to 65% of GHI annually.
For Indian conditions, DNI fraction is highest in dry western and northwestern regions and lowest in monsoon-heavy and humid coastal regions.
Common DNI mistakes
Confusing DNI with GHI. Different metrics measuring different things.
Using DNI for fixed-tilt PV design. Fixed-tilt PV uses both direct and diffuse light; POA irradiance is more relevant.
Ignoring DNI when designing trackers. Trackers benefit most in high-DNI conditions.
Assuming high GHI means high DNI. Two sites with the same GHI can have very different DNI/DHI splits.
Using outdated DNI data. Older datasets may not capture recent pollution-driven changes.
Best practices
For tracker plant design, use site-specific DNI data.
For CSP feasibility, verify annual DNI above 2,000 kWh per sq m as minimum threshold.
For utility-scale plants in high-DNI Indian regions, evaluate tracker plus bifacial configuration.
For lender-grade economics, use multi-decade DNI data from quality satellite-derived sources.
For monitoring, install pyrheliometer alongside pyranometer for comprehensive resource characterisation.
Standards and references
DNI measurement follows WMO Guide to Meteorological Instruments. Pyrheliometers comply with ISO 9059 and ISO 9060. Satellite-derived datasets follow industry-standard processing methodologies. NIWE publishes DNI data for Indian sites.
Related glossary terms
- Solar Irradiance
- Global Horizontal Irradiance
- Diffuse Horizontal Irradiance
- Peak Sun Hours
- Insolation
- Pyranometer
- Met Station
Key takeaways
Direct Normal Irradiance (DNI) is the solar irradiance from the direct beam of sunlight, measured perpendicular to the sun’s direction. It excludes scattered diffuse light. High-DNI regions like Western Rajasthan (annual DNI 2,200 to 2,500 kWh per sq m) support both tracker-mounted PV and Concentrating Solar Power applications. DNI is measured by pyrheliometers and is critical for tracker plant design and CSP feasibility. Indian annual DNI ranges from 1,000 (Northeast, monsoon-heavy) to 2,500 (western Rajasthan).