Extremely Long Run Solar Calculation

2 Posts
Oct 19, 2015 03:56 pm
Extremely Long Run Solar Calculation

Hello Everyone!

I have a small off-grid, low-amp (3-10) environment cabin, running 12v lights and charging cell phones only. No appliances, no radio...lights and charging only. So no major amp/power needs.

I'm currently using a 130w panel to charge 2 130 amp hour deep cell batteries..on site at the cabin. Works great! However, during the 4 winter months, I get no direct sunlight on the panels.

I do have a "sunny spot" on my property that gets sun all-year. However, the "sunny spot" is approx 400ft from my cabin (prob more like 360ft).

So,  I'm thinking that If I place 1 or 2, 24v panels in the "sunny spot" and run 10awg wire the 400ft, I figure I could get enough power. I have calculated an end voltage of 16.01 @ 10amps (this according to calculator.net/voltage-drop-calculator.htm).

I don't care about the loss of voltage, as I'm aiming for 12v anyway.

Question, does this seem right? AND could I then run that 16.01v through a properly rated 24v~12v charge controller to charge my 12v batteries on-site at the cabin?

47 Posts
Oct 19, 2015 04:27 pm
Re: Extremely Long Run Solar Calculation


Yes, running a higher voltage panel (or panels) and running it through an MPPT charge controller is the way to go. Ideally, if you can do two 24V panels in series, that's even better. We've got a 100W 24V panel that can ship by UPS to control the shipping costs. https://www.altestore.com/store/Solar-Panels/altE-Poly-100-Watt-24V-Solar-Panel/p10355/.With two of them in series, 10AWG wire gives you 3.3% voltage drop for 69V at the charge controller! Then with a Midnite Kid charge controller, https://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Midnite-Solar-Charge-Controllers/Midnite-Solar-The-KID-30A-MPPT-Charge-Controller-150V-Black/p11138/ , that'll drop it right down to where you need it. Also note that the current you should be using for your calculations is the Imp of the panel, not the draw of the load. So it'll be down around 3A. High volts, low amps is the way to travel the distance.

Solar Queen
altE Store
« Last Edit: Oct 19, 2015 04:33 pm by Amy Beaudet »
2 Posts
Oct 19, 2015 06:50 pm
Re: Extremely Long Run Solar Calculation

Thanks SO much!

3A will be enough to charge both those batteries?

I assume to get more amps (closer to 5) I just add more panels to keep upping the volts (too a point the wire can support, of course).
47 Posts
Oct 20, 2015 09:31 am
Re: Extremely Long Run Solar Calculation

The cool thing about MPPT charge controllers is that the input is low current, high voltage, but the output is high current, low voltage.
Watts = volts x amps.

So, if you have two nominal 24V 100W panels in series, that's 36Vmp x 2 = 72Vmp. One string is 2.78A Imp. 72V x 2.78A = 200 watts.

The MPPT charge controller takes that and reduces the voltage to about 14V to charge the 12V battery bank.  200W / 14V = 14.2A output into the battery bank.

You have two 12V 130Ah batteries in parallel (is that true, or are they 6V batteries?).  That's 260Ah at 12V. 260Ah / 14.2A = C/18 charge rate. That's a little slow, those batteries would be happier with under a C/10 charge rate, so if possible, 400W would be better.  However, you are currently charging the batteries with 130W, and you say it's working great. So while it is not ideal, it seems you have low enough loads that you are not using that much power. So jumping up to 200W is better than what you are currently doing.

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