What solar panel specs are used for calculating voltage loss

controller set point is currently 14.4
currently measuring 14.04 out at 20.59 amps
input voltage is 15.71

output is being read on a turnigy power meter
input is read with a Fluke 77 at the input wiring to the controller
the controller is the xantrex C-40
the wire run to the batteries is 4ga

voltage at batteries is 13.2v yes there is drop
my battery compartment is behind the rear wheels on passenger side and exposed to the elements, batteries get very dirty
controller is in storage compartment before the rear wheels
that was close as i could get and have it accessible, did nor want it under the bedroom closet behind the drawers, and that is where the wire goes to get thru the floor to the batteries

and there are loads, this PC and 5 small 120v fans and Fantastic vent fan on low (and ol yeah the fridge cycling)

when batteries are charged/nearly charged and only the fan loads are present voltage at battery is 14.1v
these fans have been running non-stop (except for dusting) for over a month, they are on, except for cleaning, until Fall weather arrives

as a side note the system hit 31.15 peak amps yesterday, 436.3 peak watts 1624.5 wHrs

the solar weather has been great this week NOT too hot
when its too hot (like it had been) all we can do is hide in full shade and run the Generator and A/C

sun has shifted past STC and just to the west on descent
panel voltage is reading 15.78v
control volt output .. 13.85v and 19.48amps , i think the fridge is cycling

MrWizard,
I hate to get off track but I think you could easily go 14.8 volt as a set point. And/or get the controller closer to the battery.
Especially considering the heavy usage.

I believe this is an example of what the OP wants to avoid.

yes your right it is to be avoided if possible
there is NO closer more convenient place for the controller, considering the layout of this RV
i did not want a hole all the way through the roof into the bedroom
and the Dw did not want the solar controller on the wall in the bedroom (two strikes against the shorter wire run and moving the controller
i have to live with what it is
not perfect but it works
i might raise the set point come winter time, but its high enough right now ,,there will be more Hot weather like we had for the two weeks prior to this week

this past monday was 82F, the Monday before was 101F
and the weather report says warming this week end into the 90's

today was 76f
today was great Solar weather, clear skies and a breeze to blow out pollution