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PV performance ratio and plane of array irradiance

  • soleire
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14 Jul 2021 12:59 #9826 by soleire
PV performance ratio and plane of array irradiance was created by soleire
I see that the performance ratio is not provided in the PVWatts model results using SAM 2020.11.29. I wanted to calculate this but the "annual POA total radiation (nominal) (kWh)" is also not listed? So to calculate the PR you can instead use the "plane of array irradiance" as listed in the monthly data tables?

For example, using the default set up (phoenix_az weather data, 4 kW system) I get a total POA from monthly values of 2364.678 kWh/m2. The AC energy is 6996.72 kWh and the DC power is 4 kW so I get:
PR = 6996.72 / (2364.678 x 4) = 0.7397 

And just to confirm, the sum of monthly "plane of array irradiance" (POA) values should equal the sum of "Weather file global horizontal irradiance" when the tilt is set to zero? I ask as I was seeing differences for a weather file that I uploaded from PVGIS (see attached).  

On a final note, the system losses is only 14.08% by default so the additional losses taking you down to overall PR of 73.97% are coming from things like temperature and spectral effects? Where can these details be viewed or is such a breakdown only possible for the detailed PV calculation?

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  • pgilman
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15 Jul 2021 22:04 #9831 by pgilman
Replied by pgilman on topic PV performance ratio and plane of array irradiance
Hi Scott,

Yes, you could use the monthly POA irradiance to calculate an annual value for the performance ratio equation.

The weather file GHI may not be the same as the POA values for a horizontal surface, depending on the source of the DNI, DHI, and GHI data in the weather file. PVWatts calculates the POA total irradiance using the DNI and DHI data from the weather file (along with sun angles and array surface angles).

PVWatts does not report values of losses due to temperature and spectral effects.

Best regards,
Paul.

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