# ohmic losses - loss fraction at STC

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Dear André,

I have a doubt regarding the meaning of the parameters expected to define the ohmic losses in PVsyst. The percentage we have to introduce as "loss fraction at STC" is the percentage of the voltage drop or it is the percentage of the power losses of our design.

Best regards.

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Ohmic loss causes the convertion of electrical energy into heat energy. Loss fraction at STC (1000 W/m², 25°C) is the percentage of power loss.

Wiring ohmic loss Pw = Rw * Isc²

Where:

Rw = global wiring resistance of the full system.

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. Edited by dtarin
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Voltage drop and power losses never could be the same. Really they are different concepts. The formulas for calculation are 2*L*R*I for voltage drop in DC (measured in volts) and 2*L*R*I^2 for power losses in DC (measured in Wats). Since they have different reference values for percentage calculation, by chance it could have the same percentage, but I have not seen such coincidence in any situation. Normally the percentage of power losses are somehow lower than the percentage of voltage drop.

Similar situation happens with ac cables between inverter and transformer or between transformer and injection point.

As far as I have understand after reading another posts and tutorials of PVsyst, I believe that the software estimates an equivalent resistance to later calculate the power losses taking into account the estimated resistance and actual current. The situation is that you could estimate such resistance wit voltage drop percentage or with the power losses percentage. But I am not sure which value is expecting the software. I hope André could clarify this issue.

Best regards,

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• 3 weeks later...

The wiring losses are basically calculated using the resistance Rwiring, which is the basic parameter used in the simulation (and stored as a parameters).

Now PVsyst defines an equivalent value expressed as the percentage of the loss under STC conditions (i.e. PNom).

This value is more convenient for a first approach of your system development, as the default value is well understandable and doesn't depend on the system nominal power. However in the last steps of your PVsyst system evaluation, you should calculate this Rwiring explicitly according to your real wiring conditions.

NB: the specified percentage as function of the STC power will not be your final energy loss. See our FAQ Why the losses in the results are different than those specified ?