AC ohmic loss

What this step does

The row measures the resistive loss of the AC cables between each inverter and its transformer. It acts on the inverter output and grows with the square of that output. The design loss fraction at the reference power sets the coefficient.

The math

1. Running total
2. Loss at a timestep

$$E_{after} = E_{before} - E_{row},\qquad E_{row} = \frac{\Delta t}{1000\,P_{DC}}\sum_i L_i \tag{1}$$ $$L_i = N_{inv}\,\frac{f_{AC}}{P_{ref}\,\text{PF}^2}\,P_{inv,i}^2 \tag{2}$$

where

Symbol	Meaning	Unit
$E_{before}$	the running total of the loss diagram before this row	kWh/kWp
$E_{after}$	the running total after this row	kWh/kWp
$E_{row}$	the row taken as a loss	kWh/kWp
$\Delta t$	the length of one timestep	hours
$P_{DC}$	the DC nameplate of the system	kWp
$L_i$	the power lost at timestep $i$ over the whole system	W
$N_{inv}$	the number of inverters	—
$f_{AC}$	the AC wiring loss at the reference power and unit power factor, as a fraction of the reference power	—
$P_{ref}$	the AC power reference of one inverter: its AC nameplate, or its share of the array's STC reference power, the DC nameplate reduced by the module and string mismatch derates (the Model tab's AC power reference step sets which)	W
$\text{PF}$	the power factor	—
$P_{inv,i}$	the output of one inverter at timestep $i$, taken as 0 when it is negative at night	W

Reading the number

A loss, negative. About two thirds of the design fraction over a year, so 0.3 to 0.7 % of the inverter output for the usual 0.5 to 1 %, on utility and rooftop systems alike. Long cables at low voltage make it large.

API

loss.ac_wiring

Symbols follow the symbol registry.