Here irradiance is used as energy (Watt-hours), rather than as power itself. However, solar irradiance is often used as power per area.

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Irradiance is a measurement of solar power and is defined as the rate at which solar energy falls onto a surface. The unit of power is the Watt (abbreviated W). In the case of solar irradiance we usually measure the power per unit area, so irradiance is typically quoted as W/m² - that is Watts per square meter. The irradiance falling on a surface can and does vary from moment to moment which is why it is important to remember that irradiance is a measure of power - the rate that energy is falling, not the total amount of energy.

https://www.3tier.com/en/support/solar-online-tools/what-are...

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Plane of Array (POA) Irradiance

In this step the irradiance data is transposed to the plane of the array. The sub-models included in this step include various array tracking algorithms, estimates for the reflectivity of the ground (albedo), and models to calculate the diffuse irradiance on the array from the sky.


https://pvpmc.sandia.gov/modeling-steps/1-weather-design-inp...
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SOA płaszczyzna ekspozycji modułów
http://winntbg.bg.agh.edu.pl/rozprawy2/10870/full10870.pdf

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Natężenie promieniowania słonecznego to chwilowa wartość gęstości mocy promieniowania słonecznego padającego w ciągu jednej sekundy na powierzchnię jednego m², prostopadłą do kierunku promieniowania. Wartość ta podawana jest zazwyczaj w [W/m²] lub [kW/m²].
http://solaris18.blogspot.com/2011/09/nasonecznienie-usonecz...