Ohmic wiring loss

What this step does

The row measures the resistive loss of the DC cables, taken from the array energy. The Model tab's DC ohmic wiring step sets the method; equations 2 and 3 are those of its default method, I²R on the MPP current.

The math

1. Running total
2. Loss at a timestep (default method, I²R on the MPP current)
3. Wiring resistance from the design fraction (default method)

$$E_{after} = E_{before} - E_{row},\qquad E_{row} = \frac{\Delta t}{1000\,P_{DC}}\sum_i L_i \tag{1}$$ $$L_i = N_{inv}\,R_w\,(i_f\,I_{mpp,i})^2 \tag{2}$$ $$R_w = f_w\,\frac{V_{mp,array}}{I_{mp,array}} \tag{3}$$

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	—
$R_w$	the DC wiring resistance of one inverter's array	Ω
$i_f$	the current factor of the flat derates: the square root of the product of the quality, light-induced degradation, light-soaking and mismatch factors	—
$I_{mpp,i}$	the current of one inverter's array at maximum power at timestep $i$	A
$f_w$	the design wiring loss fraction at standard test conditions	—
$V_{mp,array}$	the voltage of one inverter's array at maximum power under standard test conditions	V
$I_{mp,array}$	the current of one inverter's array at maximum power under standard test conditions	A

Reading the number

A loss, negative. Because the loss grows with the square of the current, the annual loss is about two thirds of the design fraction, so about 1 % for the usual 1.5 %, on utility and rooftop systems alike. Long cables and small conductor cross-sections make it large.

API

loss.dc_wiring

Symbols follow the symbol registry.