Quick Facts
What insolation is
Insolation is the total solar energy received per unit area over a defined period, typically measured in kWh per sq m. The term comes from “incoming solar radiation” and represents the cumulative energy delivered by the sun to a surface.
Unlike irradiance (which measures instantaneous power per unit area in W per sq m), insolation measures the integrated total energy over time. Daily insolation, monthly insolation, and annual insolation are the standard reporting periods for solar applications.
Numerically, daily insolation in kWh per sq m per day is equal to the daily Peak Sun Hours (PSH). A site with 5 PSH receives 5 kWh per sq m per day of insolation.
For solar plant design and economics, insolation is the foundational input. The plant’s annual energy output equals the product of installed kWp, annual POA insolation, and Performance Ratio.
How insolation differs from irradiance
Two related metrics:
Irradiance: Instantaneous solar power per unit area. Measured in W per sq m. Varies through the day from 0 (night) to peak (around solar noon).
Insolation: Cumulative solar energy per unit area over time. Measured in Wh per sq m or kWh per sq m. Integration of irradiance over the defined period.
For a typical Indian site:
Peak irradiance: 800 to 1100 W per sq m at solar noon.
Daily insolation: 3.8 to 6.2 kWh per sq m per day.
The daily insolation is the area under the irradiance curve from sunrise to sunset.
For solar plant design, both metrics matter:
Irradiance for instantaneous output calculations (peak inverter sizing, hot operating conditions).
Insolation for energy production projections (annual revenue, payback period).
Insolation across India
| City | Daily Insolation (kWh per sq m per day) | Annual Insolation (kWh per sq m per year) |
|---|---|---|
| Jaisalmer (Rajasthan) | 6.0 to 6.2 | 2,190 to 2,260 |
| Bikaner (Rajasthan) | 5.9 to 6.0 | 2,150 to 2,190 |
| Ahmedabad (Gujarat) | 5.5 to 5.7 | 2,010 to 2,080 |
| Jaipur (Rajasthan) | 5.5 to 5.7 | 2,010 to 2,080 |
| Bhopal (Madhya Pradesh) | 5.3 to 5.5 | 1,935 to 2,010 |
| Bengaluru (Karnataka) | 5.0 to 5.2 | 1,825 to 1,900 |
| Hyderabad (Telangana) | 5.2 to 5.4 | 1,900 to 1,970 |
| Chennai (Tamil Nadu) | 5.0 to 5.2 | 1,825 to 1,900 |
| Mumbai (Maharashtra) | 4.8 to 5.0 | 1,750 to 1,825 |
| Delhi | 4.7 to 4.9 | 1,715 to 1,790 |
| Kolkata (West Bengal) | 4.4 to 4.6 | 1,605 to 1,680 |
| Patna (Bihar) | 4.4 to 4.6 | 1,605 to 1,680 |
| Guwahati (Assam) | 4.2 to 4.4 | 1,535 to 1,605 |
These values are annual averages. Year-to-year variation is 3% to 6%, with monsoon strength being the main driver.
Seasonal patterns in Indian insolation
Indian insolation varies through the year:
Pre-monsoon (March to May): Highest insolation. Clear sky, high sun, long days. Daily insolation can reach 6.5 to 7.0 kWh per sq m per day in clear conditions.
Monsoon (June to September): Reduced insolation. Cloud cover blocks direct sunlight. South Indian sites see the largest reduction; central and northwestern sites see less.
Post-monsoon (October to November): Recovery toward annual average.
Winter (December to February): Lower insolation due to shorter days and lower sun angle. Northern sites drop more than southern.
For comprehensive solar projections, monthly insolation data is more useful than annual averages.
Insolation, POA, and panel tilt
Standard insolation data refers to Global Horizontal Insolation (GHI): total solar energy on a horizontal surface. For solar plants, panels are tilted, receiving a different amount of insolation.
Plane of Array (POA) insolation: Total solar energy on the tilted module surface.
For Indian latitudes (8 to 30 degrees), optimal-tilt panels receive 5% to 10% more annual POA insolation than horizontal:
8 degrees latitude (Thiruvananthapuram): POA insolation about 3% to 5% above GHI.
20 degrees latitude (Mumbai): POA insolation about 6% to 8% above GHI.
28 degrees latitude (Delhi): POA insolation about 8% to 12% above GHI.
For solar plant design, POA insolation is the more relevant metric. Site-specific calculations using actual tilt, azimuth, and shading are needed for accurate projections.
Insolation in solar plant economics
Insolation directly drives solar plant energy production:
Annual Energy = Installed kWp x Annual POA Insolation (kWh per sq m) x Performance RatioFor a 100 kWp plant in Ahmedabad with POA insolation of 2,050 kWh per sq m and PR of 0.82:
Annual Energy = 100 x 2,050 / 1000 x 0.82 = 168,100 kWh.
Note the conversion: insolation in kWh per sq m divided by 1 kW per sq m gives equivalent hours of full-sun operation.
This figure drives the plant’s revenue and economic viability.
Insolation data sources
Free sources:
NIWE Solar Atlas: India-specific, free. Monthly and annual data.
NASA SSE: Global satellite-derived data, free. Useful for basic estimates.
NREL System Advisor Model (SAM): Free software with built-in data.
PVGIS: European Commission’s database with India coverage.
Paid sources:
Solargis: Premium quality, multi-decade satellite-derived data. Used in project finance.
Meteonorm: Premium data with location-specific time series.
Site-specific monitoring:
For utility-scale plants, on-site pyranometers (in a met station) provide actual measured insolation.
Comparing actual measurements to satellite-derived data validates the resource assumption used in financial models.
Common insolation mistakes
Confusing insolation with irradiance. Different units and different physical quantities.
Using GHI (horizontal) when POA (tilted) is needed. POA is what panels actually receive.
Treating annual insolation as fixed across years. Real variation is 3% to 6%.
Ignoring seasonal patterns. Monsoon-heavy regions need monthly analysis.
Using single-year data for long-term projections. Multi-year averages from quality datasets are needed.
Best practices
For project economics, use multi-decade insolation data from quality satellite-derived sources.
For utility-scale projects, install on-site pyranometers for measured insolation data.
For Indian designs, use POA insolation rather than GHI for tilted modules.
For lender-grade economics, use P90 (90% probability) insolation, not P50 (median).
For long-term projections, account for year-to-year variability through sensitivity analysis.
Standards and references
Insolation measurement follows WMO Guide to Meteorological Instruments. Pyranometers comply with ISO 9060. Satellite-derived datasets follow industry-standard processing methodologies.
Related glossary terms
- Solar Irradiance
- Peak Sun Hours
- Global Horizontal Irradiance
- Direct Normal Irradiance
- Diffuse Horizontal Irradiance
- Pyranometer
- Met Station
- Tilt Angle
Key takeaways
Insolation is the total solar energy received per unit area over a defined period, measured in kWh per sq m. It is the cumulative integration of irradiance over time. Indian daily insolation ranges from 3.8 to 6.2 kWh per sq m per day; annual insolation from 1,400 to 2,250 kWh per sq m per year. The metric directly drives solar plant energy production: annual energy equals kWp times annual POA insolation times Performance Ratio. For project economics, multi-year insolation data from quality satellite datasets is essential.