Wiring (ohmic) losses:
Remember that the ohmic loss goes with the square of the current, therefore of the Power !
The basic loss parameter is the resistance of the wiring: Pwirloss = Rw * I² [W or kW]
However in PVsyst the loss parameter may also be expressed as a loss percentage when running at STC.
Therefore: as an example, if we admit a system of 10 kW with a loss of 2% at STC (i.e. under 1000 W/m²):
- Under 1000 W/m2, the loss will be R * Istc² = 20 W (2 % of 10'000 W)
- Under 500 W/m2, the current will be half, the loss will be R * (Istc / 2)² = R * Istc/4 = 5 W, i.e. 1% of 5000 W
In other words, with 2% loss at STC, when running at half the power (under 500 W/m²), the relative loss will be 1% and under 250 W/m2 it will be 0.5%.
The loss has to be evaluated at each simulation time step according to the actual power, and the cumulated loss over the year will be of the order of 60% of the specified value in % of STC (depending on climate).
Transfo iron loss:
The Iron loss is a permanent loss (as soon as the transformer is connected to the grid). It is a 24/24H loss (or eventually about half of this time if you switch OFF the line connexion by night). The iron loss only depends on the grid voltage, therefore it is constant.
Only the Ohmic part of the transfo loss is related to the yield, and obeys the rule described above.
During the sizing time, the overload estimation results of a very quick and coarse calculation, using the histogram representation of the output of the array along the year.
This histogram involves global parameters like an average array temperatures for each power class, and doesn't take into account the inverter's Pnom dependency on the temperature, as well as all array losses. Moreover, it is based on the monthly irradiation values of the project's site, which may not be the same as the Meteo file's values.
Therefore, the loss estimation of this sizing tool is not quite accurate, and is often overestimated.
The referennce ("exact") value can only be obtained with the detailed hourly simulation. This gives usually lower overload losses, as all the the losses of the array are correctly taken into account.
The parameters define an unavailability duration. The unavailabîlity periods (up to 5) may be specified explicitly, or you can ask for a random distribution.
Now a failure in winter or in summer, or by clear/covered day, of by night/day, will not have the same consequences on the production of course. Therefore the energy loss is not equal to specified duration.
In the present time, it is not possible in PVsyst to specify an unavailability loss with a pre-defined annual value.