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					</table>India’s average solar irradiance <a href="https://www.mercomindia.com/high-irradiance-low-installations-the-solar-gap-in-eastern-and-central-india" target="_blank" rel="noopener">ranges</a> from 3.5 kWh/m²/day to 5.5 kWh/m²/day. Differences between satellite-derived irradiance estimates and actual ground conditions can create uncertainty in generation forecasts and financial models, affecting project economics and investment decisions.
Irradiance data discrepancies can distort generation forecasts, affecting project performance and economics. Overestimated irradiance can inflate expected energy output, while lower actual generation can reduce revenues, cash flows, returns, and bankability. Further, inaccurate readings can also mask technical underperformance and hinder plant optimization.
Developers and technical consultants address these discrepancies by comparing multiple datasets and validating satellite estimates against ground measurements before finalizing resource assessments.
Ground Validation
Companies such as <a href="https://abenergia.co.in/" target="_blank" rel="noopener">AB Energia Solutions</a>, a solar engineering, procurement, and construction company, identify and quantify differences early, assess the suitability and representativeness of available data, and calibrate the resource assessment where required.
According to Abhijeet Sengupta, Chief Technology Officer AB Energia compares irradiance data from multiple independent sources, including ground-measured data wherever available, to identify site-specific variations. The company then uses the most representative and technically appropriate dataset for generation modeling.
Nishant Sood, MD at solar commercial and industrial solar solutions provider <a href="https://www.candi.solar/in/our-work" target="_blank" rel="noopener">Candi Solar India</a>, said the company uses both ground-based and satellite irradiance data because the two have different error profiles and require validation. For ground-based data, its technical teams verify that sensors and physical devices are calibrated and functioning properly and check the geometric relationship between global horizontal irradiance, direct normal irradiance, and the solar zenith angle.
For satellite data, Candi Solar uses aggregated sub-hourly data over longer intervals to reduce spatial and temporal noise. It continuously compares satellite and sensor measurements to determine whether deviations fall within expected ranges and can be explained by different error profiles. The third-party provider carries out satellite-data validation.
According to a source at public sector energy company <a href="https://sjvn.nic.in/" target="_blank" rel="noopener">SJVN</a>, at least 12 months of continuous ground measurements help capture seasonal variations in irradiance, cloud cover, and atmospheric conditions. For large or financially sensitive projects, 18 to 24 months of quality-controlled ground data can provide better validation. The objective is to validate long-term satellite resources against site-specific measurements rather than replace satellite data with ground measurements.
Data Discrepancies
The discrepancy between ground-measured irradiance and satellite communication (SATCOM)-based solar resource mapping in India is location-specific and depends on satellite data quality, atmospheric conditions, terrain, and the availability of reliable ground measurements.
SATCOM datasets provide modeled estimates instead of direct measurements at project sites. Differences can arise from cloud cover, aerosols, dust, humidity, local topography, and satellite data resolution.
The SJVN source said discrepancies can be smaller at sites with uniform terrain and stable atmospheric conditions and more pronounced in areas with complex terrain, high aerosol loading, or rapidly changing weather.
Regional Variations
Rapidly changing clouds may not always be captured accurately at a project’s precise location and time. Dust, smoke, aerosols, pollution, humidity, water vapor, ozone, terrain, and errors in satellite model inputs can also affect estimates. Ground sensors measure irradiance at specific locations, while SATCOM data usually represents irradiance over a grid or defined area.
According to the SJVN source, high aerosol and pollution levels in the Indo-Gangetic Plain can cause irradiance to be overestimated or underestimated if aerosol conditions are not represented accurately. Dust and desert aerosols are important factors in Rajasthan and Gujarat. Complex terrain can create local variations in the Himalayas and other hilly regions, while the Western Ghats and Northeast can present challenges due to orographic clouds and localized weather. Coastal and floating solar sites can also be affected by humidity, fog, mist, cloud cover, and changing atmospheric conditions.
Most of India <a href="https://www.mercomindia.com/indias-solar-irradiation-below-normal-in-2025-global-anomalies-reach-20" target="_blank" rel="noopener">recorded</a> below-normal solar irradiation in 2025, ranging from below 1% to less than 8% of the norm, with southwest coastal anomalies below 10%. An unusually long monsoon largely contributed to this development. Globally, irradiation anomalies ranged from below 14% to 20% above long-term averages.
Project Performance
If irradiance assumed during project planning is consistently higher than the resource available at the site, expected generation and capacity utilization factor (CUF) can be overestimated.
Sengupta said material discrepancies between irradiance datasets can influence estimated energy generation and a project’s CUF.
Sood said Candi Solar does not consider CUF a useful plant-level performance metric because it is too broad. Irradiance deviations can proportionally affect technical performance and, depending on contractual terms, commercial performance. Inaccurate readings can also hinder performance optimization by masking technical underperformance or indicating underperformance where none exists.
Financial Impact
Discrepancies in irradiance estimates can affect project financial modeling and bankability because expected generation determines projected electricity sales and revenue.
According to SJVN, sustained generation below projections can reduce revenues and cash flows, affecting expected financial returns and a project’s ability to meet financial obligations.
Sengupta said even relatively small variations in estimated generation can affect projected revenues, cash flows, and project returns.
Sood said irradiance is among the most sensitive inputs for generation estimates, which are key inputs for project financials. The irradiance assumption used in project financials depends on market pricing dynamics, financial risks, and scenario analysis.
Validation Standards
The SJVN executive said developers and lenders should avoid relying on a single meteorological or satellite-derived dataset. Two or more independent sources should preferably be compared, with reliable ground measurements used for validation wherever available. Where datasets differ significantly, a reasonable and appropriately conservative estimate should be used for energy yield assessment and financial modeling, with the associated uncertainty reflected in the assessment.
Stakeholders suggest that the Central Electricity Authority could introduce uniform irradiance-related guidelines as India pursues its target of achieving 500 GW of installed non-fossil fuel-based electricity capacity by 2030.
Technology and Data Investments
While advances in satellite sensors, spatial and temporal resolution, atmospheric modeling, cloud detection, aerosol estimation, artificial intelligence, and machine learning have improved the accuracy and consistency of satellite-based solar resource assessments, ground validation remains important because satellite models cannot always fully capture local terrain, microclimates, and other site-specific conditions.
Stakeholders call for expanding reliable ground-based measurement stations across climatic and geographical regions, regularly calibrating and maintaining equipment, and standardizing data collection, validation, quality assessment, and reporting. Investment is also required to develop accessible long-term solar resource databases, better integrate ground and satellite datasets, and strengthen institutional capacity and technical expertise.

Satellite-Ground Irradiance Gaps Raise Risks for Solar Projects in India

India has witnessed up to 7% below average solar irradiation in the last four years, reflecting the concerns of local asset managers about a decline in irradiance levels, according to global solar data assessor and forecaster <a href="https://solargis.com/" target="_blank" rel="noopener">Solargis</a>.
According to the Solargis irradiance map, factors like aerosols and cloud cover had impacted resource availability, and this could result in solar farms underperforming, impacting investor confidence in one of the world’s fastest-growing solar markets.
Solargis highlights that the challenges in performance can be addressed with investment in robust resource data throughout the lifetime of every solar project. Such an investment will help developers monitor irradiance levels and deploy changes in the plant like hours of sunlight, satellite-based solar radiation data capture, and solar storage options.
Intelligent resource investment
The Solargis report highlights intelligent resource investment to study factors leading to a decrease in irradiance in India. For instance, the data retrieved from ground-mounted sensors were prone to errors due to shading, irregular or insufficient cleaning of panels, misalignment of instruments, and calibration issues.
A satellite-based approach to monitoring irradiance allows for quality control and error detection and helps calculate the performance ratio (PR). A satellite-based resource is beneficial to developers with a smaller portfolio.
Another smart resource investment in studying irradiance would be pyranometers — a device that gives information about solar irradiance in particular geography by measuring the flux density in solar radiation. The installation and maintenance of a pyranometer are relatively expensive and more practical when used in large-scale solar projects.
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Referring to the varying irradiance levels impacting the generation capacity of solar projects, the CEO of Solargis, Marcel Suri, observed, “Controlling the weather is outside of our capabilities, but what solar developers, owners, and operators can control is their knowledge. Vital, trusted intelligence on the solar resource, albedo, and specific geographical conditions at project sites can be the difference between an asset that retains its value throughout its lifecycle and one which fails to live up to its potential.”
The Solargis report also highlights that globally, long-term averages set by solar developers remain deviated due to the variations in irradiation levels. It states that India, North America, and Australia are three such global economies that face challenges posed by varying irradiance levels, ultimately slowing down power production in solar plants.
The last five years of maps in Australia show significant irradiance variations throughout the continent compared to long-term averages. The fluctuating weather conditions were extreme during 2019-2020, leading to the disastrous bushfire ‘Black Summer’ in Australia. Solargis is now analyzing the long-term data to understand climate drivers like El Nino and La Nina.
The above-average recordings in the northwest of North America reflect anomalies such as the recent heat dome, which witnessed the temperature peak at a record high of 49.6 degrees Celsius. The Solargis maps show below-par irradiation in the north and southeast America compared to the last decade’s averages versus long-term trends. According to Solargis, the long-term data-driven approach will help asset owners in North America ready their solar projects to withstand variable weather conditions. It will also be crucial as the Investment Tax Credit (ITC) phases out, impacting long-term revenue security.
To track the irradiance levels, a research team from the <a href="https://home.iitd.ac.in/index.php" target="_blank" rel="noopener">Indian Institute of Technology</a> (IIT), New Delhi, <a href="https://mercomindia.com/iit-delhi-researchers-develop-portable-solar-tracking-towers/" target="_blank" rel="noopener">developed</a> non-mechanical solar tracking towers with highly reflective mirrors, vertically mounted in a particular way that they fall in the line of the sun during different phases of a day. The team found that the deployment of mirrors enhanced the irradiance on the solar photovoltaic panels during the entire day. The solar irradiance increased by more than 50%, resulting in 20-25% more power generation than solar panels with conventional mounting techniques.

India Witnessed Up to 7% Below Average Solar Irradiance in the Last Four Years: Report

Commercial and industrial solar investment firm CrossBoundary Energy’s (<a href="https://www.crossboundary.com/" target="_blank" rel="noopener">CBE</a>) latest report sheds light on how solar energy generation is forecasted, the specific challenges in measuring solar irradiation in Africa, and the negative consequences CrossBoundary Energy has seen as a result of inaccurate estimates.
CrossBoundary Energy finds that variations within the datasets commonly leveraged to estimate solar energy generation in Africa often cause projections to overstate solar irradiation. It argues that if these variations are not addressed during project development, inaccurate irradiance estimates can result in up to a 20% reduction in realized solar savings for commercial and industrial (C&amp;I) solar consumers and a 1-2% reduction in the internal rate of return (IRR) on investment for solar developers and investors.
CrossBoundary Energy aims to set the ball rolling for a broader conversation about raising the C&amp;I industry standard for calculating long-term solar energy generation to deliver better outcomes for clients through the report.
When CrossBoundary Energy measured the irradiation with its ground-based measurement systems, the deviation between actual measured and estimated irradiation differed by 3-5% compared with satellite data. The situation is much worse on close as sites outside of major cities experience even higher deviations. Estimates of sites nestled in highlands, valleys, or next to lakes could vary by 20%. This translates to 20% lower energy savings for solar customers and reduced IRR for businesses outside major metropolitan areas.
Ground-based measurements displayed higher accuracy as they could find even slight variations in the concentration of particles in the atmosphere and captures both direct irradiance and di­ffuse irradiance. The catch is that the distribution of ground-based validation sites is mostly clustered in Europe, South Africa, and overall in temperate regions, with few and sparsely distributed stations serving other parts of Africa, particularly in the tropics.
CrossBoundary Energy states that a study by DNV GL in 2020 confirmed that a 3-5% overestimation of a site’s solar production is likely to have an equivalent impact on revenues over the life of a project.
Solar irradiation inaccuracy is a challenge plaguing the whole industry. However, in regions like the U.S. and Europe, there are a large number of solar developers and customers driving a competitive market for solar irradiation data providers. This, in turn, has resulted in improvements to irradiation measurement accuracy.
The nascent African solar market has a limited number of data providers and ground-based monitoring stations. The majority of measurement stations that do exist are concentrated in urban areas.
CrossBoundary Energy argues that pragmatic action is required to secure continued African solar market growth and achieve the critical mass required to improve data diversity. Solar developers, buyers, and data providers have to interrogate irradiation data better and collaborate on long-term solutions.
It also suggests that solar developers should assess multiple solar irradiation data sources to evaluate a realistic range of solar irradiation. This will help to ensure that projections are not overly optimistic and avoid misrepresentation of project profitability. Businesses evaluating solar for their facilities should add evaluation of irradiation estimates and expected solar production to their bid evaluation processes.
According to a report by The Africa Solar Industry Association (<a href="http://afsiasolar.com/" target="_blank" rel="noopener">AFSIA</a>),  37 countries in Africa have installed more than 1 GW capacity.
Earlier, the African Union Commission and the International Renewable Energy Agency had <a href="https://mercomindia.com/africa-renewables-response-covid-pandemic/" target="_blank" rel="noopener">agreed</a> to promote renewable energy across the African continent and bolster Africa’s response to the COVID-19 outbreak.

Measuring Solar Irradiation a Challenge for Developers and Offtakers in Africa

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