PVWatts v5 vs v1 nominal inverter efficiency

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PVWatts v5 vs v1 nominal inverter efficiency

Hi Paul,

I took a look at the "11 System Losses" section of the "PVWatts Version 5 Manual.pdf" and there stands that:

- nominal inverter efficiency: 0.96
- reference inverter efficiency: 0.9637

PVWatts v1 uses the following two (from "11 Inverter Model" section "PVWatts Version 1 Technical Reference.pdf"):

- nominal inverter efficiency: 0.92
- reference inverter efficiency: 0.91

I understand that the latter is old and outdated, and that the former values from PVWatts v5 represent the current inverter characteristics.

Still I am confused how could these two (inverter efficiency) be mixed, when it comes to conversion of System Loss to PVWatts DC-to-AC derate factor?

PVWatts_v1_derate_factor = (1 - total_loss/100) * PVWatts_v5_nominal_inverter_efficiency = (1 - total_loss/100) * 0.96

Why do we use the PVWatts_v5 nominal inverter efficiency? Shouldn't we use the PVWatts_v1 nominal inverter efficiency?

PVWatts_v1_derate_factor = (1 - total_loss/100) * PVWatts_v5_nominal_inverter_efficiency = (1 - total_loss/100) * 0.92

Thank you for the reply.

Paul Gilman


I think the purpose of those equations is to show how to convert the loss percentages to an equivalent derate factor so that compare the V5 losses (converted to a derate factor) to the V1 derate factor. What you call "PVWatts_v1_derate_factor" might be more accurately called "V5 default losses expressed as a derate factor to compare with V1 derate factor."

Best regards,


Thank you for the reply Paul.

So basically the nominal inverter efficiency of 0.96 used to get the "V5 default losses expressed as a derate factor" is independent of the nominal inverter efficiency value used in PVWatts v1? Meaning nominal inverter efficiency in PVWatts can take any other value, while upper 0.96 can stay the same?

Paul Gilman

One of the differences between PVWatts V5 and V1 described in the Technical Reference for PVWatts V5 (PDF 714 KB) is that the nominal inverter efficiency is an input in V5 and was a built-in assumption in V1.

If you are trying to calculate a V1 derate factor that is equivalent to a set of V5 losses and V5 inverter with 0.96 nominal efficiency, then you should use the V5 nominal inverter efficiency in the conversion equation.

Best regards,


Once again thank you Paul.
You should require a pay raise for all the support work you do in here.

Kind regards,

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