Solar Irradiance

What is solar irradiance

Solar irradiance is the power of solar radiation reaching a given surface per unit area, expressed in watts per square metre (W per sq m). When integrated over time, it becomes solar irradiation, measured in Wh per sq m or kWh per sq m per day, the figure that drives solar PV plant design.

The earth’s atmosphere receives roughly 1,361 W per sq m at the top of the atmosphere (the solar constant). After scattering, absorption, and reflection, the ground-level irradiance on a clear noon at most Indian sites peaks at around 800 to 1,100 W per sq m. Daily energy depends on how high this peak is, how long it lasts, and how clear the sky stays.

For solar PV designers, irradiance is the primary input. For a given panel and inverter, change the irradiance dataset and the predicted annual generation changes proportionally.

Three components: GHI, DNI, DHI

Solar radiation that reaches a horizontal surface has two parts. Direct sunlight comes from the disk of the sun. Diffuse sunlight comes from the rest of the sky after scattering through the atmosphere.

Global Horizontal Irradiance (GHI) is the total irradiance on a horizontal surface, including both components.

Direct Normal Irradiance (DNI) is the irradiance on a surface held perpendicular to the sun’s rays, including only the direct component.

Diffuse Horizontal Irradiance (DHI) is the irradiance on a horizontal surface from the diffuse part only.

The mathematical relationship is:

GHI = DNI x cos(z) + DHI

where z is the solar zenith angle (the angle between the sun and vertical). For a flat panel on the ground, GHI is the figure that matters. For a tracker or a tilted panel, designers compute plane-of-array irradiance using DNI, DHI, GHI, and the panel geometry.

Units and conversions

Irradiance: watts per square metre (W per sq m). At any single moment.

Irradiation: Wh per sq m or kWh per sq m, integrated over time (hourly, daily, annual).

Peak Sun Hours (PSH): the number of equivalent hours at 1,000 W per sq m that would deliver the same daily energy. A site with 5 kWh per sq m per day has 5 PSH. PSH is the most intuitive way to think about how much sun a location gets per day.

A solar panel rated 400 Wp on a site with 5 PSH per day produces approximately 400 multiplied by 5, or 2,000 Wh per day at the DC level before losses. Real AC output after PR is around 80% of this number.

India’s solar resource

India is a high solar resource country. The National Institute of Wind Energy (NIWE) Solar Atlas shows annual GHI between 4.0 and 6.2 kWh per sq m per day across most of the country, with the highest values in Rajasthan and northwest Gujarat.

These numbers describe the long-term average. Year-to-year variation is around plus or minus 4% to 6%.

How irradiance translates to plant output

A solar plant designer uses plane-of-array (POA) irradiance, the energy reaching the tilted module surface. For a south-facing tilted module in India, POA is usually 5% to 8% higher than GHI because the tilt captures more of the off-noon sun.

The module’s nameplate kWp is then multiplied by POA in kWh per sq m, divided by 1 kW per sq m (the STC reference), and reduced by the Performance Ratio:

Annual AC energy (kWh) = Plant kWp x Annual POA (kWh per sq m) x PR

For a 100 kWp plant in Ahmedabad with annual POA of 2,000 kWh per sq m and PR of 0.82, annual generation is 100 x 2,000 x 0.82, or 1,64,000 kWh.

Measuring irradiance

Pyranometers are the workhorse instrument for GHI and DHI. A first-class pyranometer (ISO 9060) typically achieves 5% measurement uncertainty over a year. Pyrheliometers, mounted on solar trackers, measure DNI with similar accuracy.

For large solar plants, on-site meteorological stations record GHI, POA, ambient temperature, module temperature, wind speed, and humidity in real time. The data feeds both the plant’s SCADA system and the long-term performance analysis used for IRR validation.

For desk studies and early-stage design, modelled satellite datasets such as NREL NSRDB, Meteonorm, and Solargis are widely used. They give long-term averages with a typical accuracy of 5% to 8% in well-mapped regions.

Why two adjacent sites can have different irradiance

Microclimates matter. A coastal site sees more morning haze. A site near industrial pollution sees lower DNI. A high-altitude site has thinner atmosphere and higher peak irradiance. A site downwind of a dust source sees more soiling losses.

Cloud patterns differ across distances of a few tens of kilometres. Anyone designing a solar plant in a new region should pull at least ten years of data from a regional satellite dataset, not extrapolate from a single nearby ground station.

Common mistakes when working with irradiance

Reading GHI from a generic table and ignoring tilt corrections. POA is what panels see, not GHI.

Confusing irradiance (instantaneous power) with irradiation (cumulative energy). A pyranometer logs both, but they are different physical quantities.

Using a one-year dataset without checking for anomaly years. Cyclones, dust storms, or El Nino events can skew a single year by 5% to 10%.

Forgetting to include diffuse irradiance for systems with steep tilt or in cloudy regions. Diffuse can account for 25% to 40% of annual irradiation in monsoon-affected zones.

Assuming peak irradiance exceeds the STC reference rarely. It does briefly at high altitudes and on clear cold days, and this can drive temporary inverter clipping.

Best practices

Use a long-term satellite dataset for the design phase. Validate against any nearby ground station if available.

Specify the dataset source and version in the design report. Different datasets give different answers, and project finance reviewers will ask which one was used.

For utility-scale projects, install a met station before financial closure to establish a site-specific baseline. Two years of measured data tightens the long-term forecast.

Translate annual irradiation into expected daily generation for the system size, then sanity check against the inverter’s expected output curve.

Related glossary terms

• Peak Sun Hours
• Global Horizontal Irradiance
• Direct Normal Irradiance
• Diffuse Horizontal Irradiance
• Pyranometer
• Tilt Angle
• Performance Ratio
• What is kWp

Key takeaways

Solar irradiance is the instantaneous power of sunlight on a surface, in W per sq m. Solar irradiation is the cumulative daily or annual energy, in kWh per sq m. India has one of the highest national solar resources globally, with annual GHI between 4.5 and 5.5 kWh per sq m per day on average. Designers use site-specific GHI, DNI, and DHI to model plane-of-array irradiation, then multiply by panel capacity and Performance Ratio to forecast plant output.