Solar Insolation Calculator

The Solar Insolation Calculator computes daily extraterrestrial solar insolation using established astronomical formulas from FAO Irrigation and Drainage Paper 56 and Duffie & Beckman’s Solar Engineering of Thermal Processes. This value (Ra) represents the solar energy available above the atmosphere and serves as the foundation for many solar radiation models. Solar Insolation is key in agriculture for estimating potential energy input to crops.

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Scientific Basis

Extraterrestrial insolation depends only on Earth-Sun geometry: distance variation, declination, latitude, and sunset hour angle. The calculation uses precise equations for inverse distance factor dr = 1 + 0.033 cos(2πJ/365), declination δ = 0.409 sin(2πJ/365 - 1.39), sunset hour angle ωs = acos[-tan(φ) tan(δ)], and Ra integration over daylight. This method is peer-reviewed and used globally in ET0, crop models (DSSAT, APSIM), and solar PV sizing.

Importance of Extraterrestrial Solar Insolation

Ra is the theoretical maximum solar input before atmospheric losses. It is essential for calculating clear-sky radiation, estimating actual surface insolation via transmissivity models (e.g., Hargreaves), computing reference evapotranspiration (Penman-Monteith limited-data version), assessing solar PV yield potential, and modeling crop growth in data-scarce regions like rural Bangladesh. Accurate Ra ensures reliable downstream calculations in irrigation, renewable energy, and climate adaptation.

User Guidelines

• Latitude: positive for Northern Hemisphere (e.g., Barishal ~22.7°N), negative for Southern.
• Day of Year: Jan 1 = 1; use online converters if needed (leap years up to 366).
• Results are theoretical (top-of-atmosphere); real surface insolation is 30–70% lower due to clouds/atmosphere.
• For monthly averages, compute for the 15th of each month or average daily values.

When and Why to Use This Tool

Use when designing solar-powered irrigation, estimating ET0 without radiation data, assessing photovoltaic feasibility on farms, teaching solar geometry, or inputting baseline values into models. Critical in tropical agriculture where seasonal variation affects planting and water needs.

Purpose of the Solar Insolation Calculator

To provide fast, accurate, science-based extraterrestrial insolation values supporting sustainable farming, renewable energy, and environmental research — accessible to farmers, students, and professionals without complex software.