# AC Loss at STC or Pnom

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Hello PVsyst,

I am simulating a 2.25MWp system in which I am defining AC loss (LV / MV / HV) 1.50 /  0.70 / 0.20%. But when I simulate it according to AC loss at STC level I get a 1948kWh/kWp/year specific generation. Similarly, with the same losses when I simulate it according to AC loss at the Pnom level, I get a 1944kWh/kWp/year specific generation.

So my question is why at the Pnom level generation gets reduced and how

Regards,

Shashank

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AC loss with Pnom is accurate as it is considered Inverter power for percentage loss calculation.

Example case: Inverter Power is 200kW, and DC power connected to the inverter is 250kWp.

AC loss at STC level = 1.5% of 250kWp which is wrong and loss values will be lesser.

AC loss at Pnorm (Inverter) = 1.5% of 200kW which is correct and loss values will be slightly higher than the previous case.

Regards,

Ajay.

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As you said that at STC level the loss is lesser than than at Pnom level.

But if I do 1.5% of 250kWp which comes 5kWp. Similarly, if I do 1.5% of 200kW which comes 4kW. So the loss at STC comes higher.

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The wiring loss basic parameter is the resistance of the wires. I.e. the Lenght and section, as well as the  metal.

Now the percentage definition is only another way of defining loss (i.e. the resistance).  It is not absolute: the percentage loss is relative to the operational power, and we can calculate that it is proportional to this power  (see the Help).

Therefore when defining a  loss in terms of percentage, you should specify at which reference power.  For this reference, PVsyst proposes 2 options: either the PNom of the inverter, or the PNom of the PV array.  So that for a same loss (same resistance), the percentage parameter will be lower for the PNom of the inverters than for the PNom of the PV array.

This is fully explained in the Help   "Project design > Array and system losses > Array ohmic wiring loss"

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