Global Horizontal Irradiance (GHI)

What GHI is

Global Horizontal Irradiance (GHI) is the total solar irradiance (direct plus diffuse) on a horizontal surface, measured in watts per square metre (W per sq m). Integrated over time, GHI gives total solar energy in Wh per sq m, kWh per sq m per day, month, or year.

GHI is the universal reference for solar resource assessment globally. It is the standard metric used in solar atlases, project finance models, and academic research.

The “global” in GHI means total (sum of direct and diffuse components). The “horizontal” means measured on a flat horizontal surface, regardless of solar panel orientation.

For solar plant design, GHI is the foundational resource input. Site-specific GHI is translated to plane-of-array (POA) irradiance using tilt and azimuth calculations.

Components of GHI

GHI has two physical components:

Direct Normal Irradiance (DNI): The solar radiation from the direct beam of sunlight (the disk of the sun). DNI is what concentrating solar systems track and absorb. DNI varies from 0 (cloudy days, low sun) to peaks of 950 to 1000 W per sq m (clear days with high sun).

Diffuse Horizontal Irradiance (DHI): Light scattered by the atmosphere reaching the horizontal surface from the sky dome (not from the direct sun disk). DHI is what makes the sky bright. DHI varies from very low (clear sky) to 100 to 300 W per sq m (overcast).

The relationship:

GHI = DNI × cos(zenith angle) + DHI

Where the zenith angle is the angle between the sun and vertical (zero when sun is directly overhead, 90 degrees at sunrise/sunset).

For typical Indian conditions at solar noon: DNI of 800 W per sq m, DHI of 100 W per sq m, sun zenith angle of 30 degrees:

GHI = 800 × cos(30) + 100 = 800 × 0.866 + 100 = 793 W per sq m.

How GHI is measured

By a pyranometer mounted on a horizontal surface:

The pyranometer’s thermopile (or silicon-cell) sensor absorbs solar radiation from the full sky hemisphere.

The sensor produces a voltage proportional to incident irradiance.

The voltage is calibrated and reported as W per sq m.

Data acquisition systems integrate the instantaneous values to produce hourly, daily, monthly, and annual totals.

Pyranometer accuracy:

Secondary Standard ISO 9060: 2% uncertainty annual.

First Class ISO 9060: 5% uncertainty annual.

Second Class ISO 9060: 10% uncertainty annual.

For utility-scale solar projects, Secondary Standard or First Class pyranometers are used.

GHI across India

Indian GHI varies substantially by region:

The variation reflects:

Latitude (lower latitudes have more sunlight).

Monsoon impact (heavier monsoon reduces annual GHI).

Pollution (urban areas have lower GHI due to particulates).

Elevation (higher elevation has thinner atmosphere, higher peak GHI).

GHI versus POA

For solar panel calculations, the relevant metric is Plane of Array (POA) irradiance:

GHI: On a horizontal surface.

POA: On the tilted module surface (matching the panel’s actual orientation).

For Indian latitudes, tilted panels at optimal angle receive 5% to 12% more annual irradiance than horizontal. The difference matters for accurate energy projections.

POA is calculated from GHI, DNI, DHI, panel tilt, and panel azimuth using radiation transfer models in software like PVsyst, SAM, or PVGIS.

For preliminary estimates, designers use:

POA = GHI × (1 + tilt correction).

For 25 degrees N latitude with south-facing 25 degree tilt: POA approximately 1.07 × GHI.

For detailed projections, software calculates POA from full irradiance data.

GHI in solar resource assessment

For a new solar project, GHI assessment involves:

Long-term historical GHI: Multi-decade averages reduce year-to-year variability. Critical for project finance.

Year-to-year variability: 3% to 6% typical, captured in P50, P90 statistics.

Monthly distribution: Important for understanding seasonal generation pattern.

Spatial variability: Different parts of large projects (above 100 MW) can have meaningfully different GHI.

Long-term trend: Stable over decades, but climate change may affect future GHI slightly.

For lender-grade projects, GHI assessment uses:

P50 GHI: Median value across multiple years.

P75 GHI: Value exceeded 75% of years.

P90 GHI: Value exceeded 90% of years.

Lenders typically use P90 to size debt service requirements conservatively.

GHI data sources

Free sources:

NIWE Solar Atlas: India-specific, free.

NASA SSE: Satellite-derived, global, free.

NREL SAM: Software with built-in data.

PVGIS: European Commission database with India coverage.

Paid sources:

Solargis: Premium quality, multi-decade satellite-derived. Used in project finance.

Meteonorm: Premium with location-specific time series.

For lender-grade projects, paid sources are typically used for higher accuracy and recent data.

Common GHI mistakes

Confusing GHI with POA. POA is what panels see; GHI is the horizontal reference.

Using single-year GHI for projections. Multi-decade averages are needed for accurate projections.

Ignoring seasonal variation. Monsoon-affected sites need monthly analysis.

Using outdated GHI data. Solar atlas accuracy has improved significantly; modern datasets give better results.

Treating satellite GHI as fully equivalent to ground measurement. Some calibration to local conditions may be needed.

Best practices

For project economics, use multi-decade GHI data from quality sources.

For utility-scale projects, install on-site pyranometers to validate satellite-derived data.

For Indian designs, use POA irradiance, not GHI, for tilted modules.

For lender-grade economics, use P90 GHI to bound the lower end of expected generation.

For long-term projections, account for year-to-year variability through statistical analysis.

Standards and references

GHI measurement follows WMO Guide to Meteorological Instruments. Pyranometers comply with ISO 9060. Satellite-derived datasets follow industry-standard processing methodologies. NIWE Solar Atlas is the authoritative Indian source.

Related glossary terms

• Solar Irradiance
• Direct Normal Irradiance
• Diffuse Horizontal Irradiance
• Peak Sun Hours
• Insolation
• Pyranometer
• Met Station
• Tilt Angle

Key takeaways

Global Horizontal Irradiance (GHI) is the total solar irradiance (direct plus diffuse) on a horizontal surface, measured in W per sq m or kWh per sq m. GHI is the universal reference for solar resource assessment globally. Indian annual average GHI is about 5.0 kWh per sq m per day, ranging from 3.8 (Northeast) to 6.2 (Jaisalmer). For solar plant design, GHI is translated to plane-of-array (POA) irradiance using tilt and azimuth calculations. For project economics, multi-decade GHI data from quality sources is essential.