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Why PV does not perform at its rated power on a sunny day due to derating factors


Q:

Hi Dr.sean,
I used to be in project that works with PV but the drop is reaching 20% of the rated power. The type of PV (polycrystalline)
At which condition the PV will be more efficient regard the production of power.
suppose we have two PV panels at the same intensity of sun rays for example 1000 w/m2 but each PV have different ambient temp for example PV1 (25c) and PV2 (50c) now which one of these will produce more power?

A:

If polycrystalline PV has lost 20% of its rated power under Standard Test Conditions, then there would have to be a manufacturing defect.

If PV produced 20% of its rated power in the field, that would be good performance, because Standard Test Conditions are very unusual and because there are other losses before the electricity is converted to alternating current.

Also, Standard Test Conditions is for the solar cell temperature and usually the cell temperature is about 30C hotter than ambient temperature on a sunny day.

If the cell temperature is 25C different, then to calculate the difference in power, we would use the temperature coefficient of power for the calculation, which is usually about -0.45%/C

25C x -0.45%/C = 11.25% decrease in power

Other factors that would decrease power would be irradiance less than 1000W per square meter at the angle the light hits the solar modules and other derating, such as inverter efficiency, soiling, voltage drop, etc.

It is common on a sunny day for a PV array to be putting out 50% of the solar array power due to these factors.

Thanks,
Sean White


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