What solar panel specs are used for calculating voltage loss

If you are asking about cable losses, none. That is a separate and unrelated (to the panel) spec.

You can find wire resistance loss calculators all over on Google tho.

HERE's ONE

I believe he is asking about cable losses but most voltage loss calculators assume 12V, 24V etc. We all know our panels do not put out 12V or 24V. They put out 17-18Vmp and 27-35 Vmp. So is your question, Should we be using Vmp when sizing our cable? I don't know. I think most just 12V and size for 3% or less loss from PV to controller and 1% or less from controller to battery.

The link mentioned above allows for specific voltage/amperage so in that case, I would tend to use Vmp and Imp from the PV array.

If I am close to the 3% margin I'll lean to going one size larger to account for possible connection losses.

As was said, you need to be more specific with your question.

"Line loss" is directly related to the resistance of the conductor AND the current carried.

If you meant something else, let's start over.

Joel_T wrote:
Which solar panel specs are reasonably used for calculating line voltage loss?

The variable is the amps being carried on the wire. So two typical situations are 24-12 with MPPT and 12-12 with PWM

24-12 with MPPT has the panel to controller wire carrying Imp at Vmp while the output controller to battery wire will be at 12-12 and the amps will be output watts divided by battery voltage at the time.

12-12 PWM will have the panel to controller wire carrying the panel Isc and passing that through to the battery with the panel's IV curve showing that amps slight reduction as battery voltage rises to the controller's high set point of say 14.8v For two 12s in parallel you add their Iscs for the combiner-controller path amps

Its not worth even thinking about if only a few hundred watts on the higher voltage. Just run a 10 ga cable for every 10-12 amps. I ran the first 2 panels on one 10 ga cable. When I got the 3rd panel I ran a 2nd cable, which will be good for a 4th if ever.
Concentrate more on the total amps from the controller on their way to the batteries, that's where the loss will make a difference.

Use Vmp. Probably will not matter much if using the typical PWM controller because you will just get Isc into the battery.

I used expected total output current of the panels vimp..The working current
Voltage drop is line resistance times amps
At 30 amps 14v or 30 amps 120v the drop is the same for the same cable...
But 1.0v means almost nothing to120v but means a great deal to 14v

Line losses from panel to controller mean very little to PWM, because anything over the output set point is not used
But line losses between controller and batteries mean a great deal. , lower voltage charges the batteries slower
If the set point is 14.2v
You want to get 14.2 at the batteries not 13.8 Or 13.2 etc..
Even with MPPT the controller output side is more important than the input side, and the higher the voltage on the input side the less effect the drop will have

CWJ8, and Mr Wizard you understood without me being wordy.

For the more challenged, especially me.......I've got 2 160w panels (a 3rd is unlikely) I'll be wiring 22' 1 way to a PWM controller, then another 2' to the batteries. Their specs are VOC-22.4v, VOP-17.9v, ISC-9.87a, IOP-8.89a. These specs are likely inflated and come from a perfect world without temperature variables. (Depending on wire size) I've no idea what voltage or amps can REALLY make it to the controller or batteries. Bigger wire size helps and with a voltage drop calculator I might not have to throw darts at a reasonable size to use. I'm wanting to know the likely real world volts and amps to use with a voltage drop calculator.

With the 12-12 PWM, you will use battery voltage for both the panel to controller path and for the controller to battery path.

Your "expected amps" at STC with two panels is twice the Isc of one panel (with panels aimed at a high sun) when the battery voltage is between 12 and 13

When battery voltage is between 13 and 14, the amps are a tad less (see the panel's IV curve)

For wiring set up, pick say 13.5v and 20 amps. Anytime amps are lower, line loss will be less. Most of the daytime, panel voltage will be between 13 and 14, so that is why I would pick 13.5.

My calculator says 30 amps 22' #8 wire will lose less than 1 volt.

So you will have 16+ volts hitting the controller at full amp rating. Controller clips voltage to ~14.8 and you are good to go in all conditions.

http://www.genuinedealz.com/voltage-drop-calculator

Even #10 wire at 44' loses 2.7 volts putting 15 to the controller (17.9v - 2.7v =15.2v) and still good to go.
(I recommend #8)