What is insolation?
Insolation (or solar insolation) is the total solar radiation energy received at a location over a given time, typically expressed in kilowatt-hours per square metre (kWh/m²). It is the location-specific solar resource that drives all solar generation. Monthly and annual insolation are the standard reference numbers; daily values appear in some analyses.
Insolation is closely related to irradiance but they are not the same. Irradiance is the instantaneous power per area (watts per square metre); insolation is the integrated energy per area over time. A daily insolation of 5.4 kWh/m² equals 5.4 'peak sun hours' (PSH = insolation ÷ 1 kW/m² STC reference).
Indian annual insolation varies sharply by region. Rajasthan and Gujarat receive 2,000 to 2,200+ kWh/m² annually. Most of central and southern India sits at 1,700 to 2,000 kWh/m². Eastern states and the Northeast are lower (1,200 to 1,750 kWh/m²). This variation explains why solar payback differs across India even with identical hardware.
Why insolation matters
For solar EPCs and developers, insolation is the foundation of yield modelling. Annual kWh per kWp generation is the product of insolation, Performance Ratio, and orientation/tilt factors. Accurate insolation data drives honest payback projections.
For customers, location-specific insolation explains why solar payback varies across India. The same hardware produces more electricity in Jaipur than in Kerala or Sikkim.
For utility-scale developers, insolation data is critical to project siting decisions. Land in higher-insolation regions delivers more kWh per acre and better project economics.
For policymakers, insolation maps inform deployment strategy. State-level promotion of solar accounts for resource availability.
How insolation is measured and used
- Ground measurement. Pyranometers at meteorological stations measure surface irradiance.
- Satellite-derived data. Satellite measurements modelled into ground-level irradiance estimates.
- Integration over time. Hourly irradiance summed to daily, monthly, and annual insolation.
- Resource maps. Annual and seasonal insolation published as maps and datasets.
- Plane-of-array adjustment. Horizontal GHI adjusted for module tilt and orientation.
- Yield modelling. Annual kWh per kWp = POA insolation × Performance Ratio.
- Project finance modelling. Insolation-based yield drives revenue projections.
Real example: insolation comparison for two Indian sites
Site A. Jodhpur, Rajasthan. Annual GHI: 2,150 kWh/m². POA insolation at 25° south tilt: ~2,250 kWh/m². At Performance Ratio 0.80: 1,800 kWh per kWp per year.
Site B. Guwahati, Assam. Annual GHI: 1,450 kWh/m². POA insolation at 25° south tilt: ~1,500 kWh/m². At Performance Ratio 0.78: 1,170 kWh per kWp per year.
Difference. Same hardware produces 53 percent more energy per kWp in Jodhpur. This explains why utility-scale solar concentrates in western India and why Indian solar tariffs vary regionally.
Benefits of using insolation data
- Foundation of yield forecasting. Annual kWh prediction.
- Location-specific economics. Honest project IRR.
- Site comparison. Pick the best sites for utility-scale projects.
- Public data availability. MNRE, NREL, NASA all publish free data.
- Monthly resolution. Captures seasonal variation.
- POA-adjusted modelling. Tilt and orientation incorporated.
- Standardised metric. kWh/m² used globally.
Limitations
Spatial resolution. Maps typically at km-level grid; microclimate can differ.
Satellite-vs-ground discrepancy. Different methodologies sometimes diverge.
Year-to-year variability. Annual averages mask significant variability.
Cloud-cover modelling. Highly variable in monsoon regions.
Dust and pollution effects. Local atmospheric conditions reduce surface insolation.
Long-term trend. Some evidence of long-term changes in surface insolation; not always captured.
Insolation across India
| Region | Typical annual GHI (kWh/m²) |
|---|---|
| Rajasthan | 2,050 to 2,250 |
| Gujarat | 1,950 to 2,150 |
| Madhya Pradesh | 1,850 to 2,050 |
| Karnataka | 1,800 to 2,000 |
| Tamil Nadu | 1,800 to 2,000 |
| Maharashtra | 1,800 to 2,000 |
| Andhra Pradesh / Telangana | 1,800 to 2,000 |
| Uttar Pradesh | 1,700 to 1,900 |
| Bihar / Jharkhand | 1,600 to 1,800 |
| West Bengal | 1,500 to 1,700 |
| Kerala | 1,600 to 1,800 |
| Northeast states | 1,200 to 1,600 |
Quick facts
| Term | Solar Insolation (Solar Radiation, GHI) |
|---|---|
| Unit | kWh per square metre (kWh/m²) |
| Relation to PSH | Daily PSH = Daily insolation ÷ 1 kW/m² |
| Indian range | 1,200 to 2,200+ kWh/m² annual |
| Horizontal vs POA | GHI is horizontal; POA accounts for tilt and orientation |
| Data sources | MNRE / NISE, NREL NSRDB, NASA POWER, Solargis, Vaisala |
| Use | Yield forecasting, site comparison, project economics |
Common mistakes about insolation
- Confusing insolation and irradiance. Insolation is energy per area over time; irradiance is instantaneous.
- Using national average for state-specific quotes. Wide variation.
- Forgetting POA adjustment. Tilted modules receive more than horizontal.
- Ignoring seasonal variation. Annual average masks monsoon dip.
- Skipping local microclimate. Dust, shading, pollution affect site-specific resource.
- Mixing data sources without verification. NASA, MNRE, Solargis can differ slightly.
- Treating insolation as constant year-over-year. Year-to-year variability exists.
Key takeaways
- Insolation is total solar energy received per area, measured in kWh/m².
- Foundation of all solar yield forecasting.
- Indian annual GHI ranges 1,200 to 2,200+ kWh/m²; Rajasthan highest, Northeast lowest.
- POA insolation adjusts horizontal GHI for tilt and orientation.
- Public data available from MNRE, NREL, NASA; commercial datasets from Solargis, Vaisala.
- Annual kWh per kWp ≈ POA insolation × Performance Ratio.
- Significant regional variation explains why Indian solar payback differs by location.
Frequently Asked Questions
What is solar insolation?
Solar insolation is the total amount of solar radiation energy received at a location over a given time, typically measured in kWh per square metre. It is the location-specific solar resource that drives all solar generation. Indian annual insolation ranges from about 1,200 kWh/m² (Northeast) to 2,200+ kWh/m² (Rajasthan).
Is insolation the same as irradiance?
Related but different. Irradiance is the instantaneous power per area (W/m²). Insolation is the integrated energy per area over time (kWh/m²). Irradiance is a snapshot; insolation is the cumulative resource.
How is insolation measured?
Through pyranometers at meteorological stations, or modelled via satellite-derived irradiance datasets. India has solar resource maps from MNRE, NISE, India Meteorological Department, and global datasets like NREL NSRDB and NASA POWER.
What is the difference between insolation and PSH?
Closely related. Peak Sun Hours (PSH) is daily insolation divided by 1 kW/m² (the STC reference). A daily insolation of 5.4 kWh/m² equals 5.4 PSH. Insolation expresses the energy directly; PSH expresses it as equivalent hours of full sunshine.
What is Global Horizontal Irradiance (GHI)?
GHI is the total solar irradiance received on a horizontal surface, including both direct (beam) and diffuse (scattered) components. Annual GHI is the most common way to express location-specific solar resource.
What is plane-of-array (POA) insolation?
POA insolation is the solar energy received on the actual tilted module surface, accounting for the tilt and orientation. POA insolation is higher than horizontal insolation for properly tilted modules in the northern hemisphere.
How does insolation vary across India?
Rajasthan and Gujarat: 2,000 to 2,200+ kWh/m² annual GHI. Karnataka, Maharashtra, Tamil Nadu: 1,800 to 2,000 kWh/m². Eastern states: 1,500 to 1,750 kWh/m². Northeast: 1,200 to 1,500 kWh/m². Hill states vary.
What affects insolation at a location?
Latitude, cloud cover, atmospheric conditions, dust and pollution, elevation, and seasonal variation. Lower latitudes and clearer skies typically have higher annual insolation.
Does insolation affect solar payback?
Directly. Higher insolation means more annual kWh per kWp, more savings, faster payback. A 3 kWp residential system in Rajasthan (high insolation) generates more annually than the same system in Kerala or the Northeast.
How is insolation used in solar design?
As the basis for annual yield forecasting. Annual kWh per kWp = annual insolation × Performance Ratio × orientation/tilt factor. PVsyst, NREL SAM, and similar tools take site insolation data as primary input.
Are Indian insolation maps public?
Yes. MNRE, NISE, and the National Renewable Energy Laboratory of India publish solar resource maps. Global datasets (NREL NSRDB, NASA POWER) are also free. Commercial datasets (Solargis, Vaisala) provide higher-resolution data.
Does seasonal insolation matter for design?
Yes for accurate forecasting. Monthly insolation varies significantly across the year, especially in monsoon-affected regions. Monthly modelling reveals seasonal generation patterns important for net-metering economics.
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- MNRE / NISE solar resource maps. India-specific irradiance data. mnre.gov.in
- NREL National Solar Radiation Database. Reference irradiance methodology. nrel.gov
- NASA POWER project. Global satellite-derived irradiance.
- Solargis and Vaisala 3TIER. Commercial solar resource datasets.
- India Meteorological Department. Surface measurement networks.
- National Institute of Solar Energy (NISE). Indian site characterisation.
- IEC 61724. PV system performance monitoring including irradiance reference.
Written by QuickEstimate Editorial, QuickEstimate Editorial (Surat).
Last updated: 4 June 2026.