Average annual solar radiation arriving at the top of the Earth’s atmosphere is roughly 1361 W/m2. The arriving radiation is distributed across the electromagnetic spectrum, from Ultra Violet, visible light to Infrared light. About half of it is infrared light.
Our thick atmosphere consist of 78% nitrogen and 21% oxygen, with trace amounts of water vapor and carbon dioxide. Ozone layer at the stratosphere (10-50km above Earth surface) absorbs most of the Sun’s Ultraviolet (UV) radiation. Other molecule such as water and carbon dioxide also absorb some of the spectrum of the solar radiation, creating characteristic absorption bands.
The Sun’s rays are attenuated as they pass through the atmosphere, leaving maximum normal surface irradiance at approximately 1000 W /m2 at sea level on a clear day. Ignoring clouds, the daily average insolation for the Earth is approximately 6 kWh/m2 .
Solar insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kW•h/(m2•day)).
Most of the Earth surface with higher solar insolation is located at cloudless desert. Clouds consisted of water droplets, frozen crystals and particles suspended in the atmosphere. It had great potential to reduce most of the sun radiation to reach the surface.
Obviously, to have a more efficient solar PV system, you need to place your system in a location that had allot of solar insolation.
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