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Irradiance is the direct, diffuse, and reflected solar radiation that strikes a surface. Note that Irradiance is not the same thing as insolation although their units are both a power density. Solar irradiance is a measure of instantaneous power density (units kW/m2), so it varies throughout the day. Solar insolation is the total amount of solar energy received at a location in a specified time(units usually kWh/(m2 day)). The units are effectively the same because insolation has time in both the numerator and denominator so it cancels out. Insolation is often used for simple PV system design and irradiance in more complex systems.

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