Wire Gauge Required Question

Almot wrote:

BFL13 wrote:
Mex makes a good point about the context where small losses matter. I think you can be a bit more sloppy when camping for a couple weeks in an RV, but still, you don't want to waste good solar coming in.

Depends on how much you want/can lose. Mex is using annual KWH because he is living there permanently. I was using daily AH because living there seasonally, and tried to stay within 2.5 AH loss a day.

BFL13 wrote:

PT, you are the one who says voltage drop array-controller does not matter for PWM 12v, but it does matter a lot for MPPT 24v.

If he says, then it is :)
However, voltage drop does not matter - too much - for MPPT with 24V array. Whatever voltage over 14.4 you feed into MPPT, it will down-convert extra V into extra A. Less extra volts to convert = less extra AH and WH. As long as it is 14.4 on the battery end, it will work.

voltage drop between the panel and input of a MPPT controller DOES matter. Why?? because voltage drop means power loss between the panel and controller. The power is disipatted in the cable, can't get it back. Power lost to the input of an MPPT controller will result in the SAME power loss at the output of an MPPT controller. Suppose your wiring loss is 12 watts in the cable. that 12W on the input results in a 12W loss on the output which is close to 1A reduction in output current.

What would result in a 12W power cable loss. Suppose the panel is a 160W 12V panel. Imp is about 9A. Suppose the wire run is 40ft of #12 wire. (0.127ohms) Power loss in the cable is Isquare x R = 9*9x.095= 11 watts.

Well, now your max output current from the MPPT controller just when down 1A from the panel rated 160 watts as max output current. Or think of it this way your panel is NOW a 150W panel.

Now the advantage of a 24 V array is that the Imp is 1/2 that of a 12V array of the same power (watts) and since power loss is related to the SQUARE of current, for the SAME wire size, a 24V panel will result in 1/4 the current loss.
In the example above, using 4.5A for a 24V panel, now Power drop is 3.3 watts which is about 1/4 A.

24V panels clearly win out and in most cases if you can hook 12V panels in series to get 24V it's an advantage over hooking them in parallel at 12V with an MPPT controller. Just stay within the input voltage range for the controller.