epeters avatar image
epeters asked

3x200w 12v or 3x200w 24v for a 12 volt system?


I'm looking at the victron smartsolar 100/50 paired with 3x 200w 24v (in parallel, 600w total) newpowa solar panels paired to a 12v batteries and system. I'd need close to a 25-35 foot run so to save myself from getting 4-6awg wires(17v, 35.28A) I could use 10awg(34v, 17.64A) is going with 24v panels better than 12v? All the expensive high end panels are 12v and because I am putting the panels on my sailboat I prefer to wire more panels in parallel than series due to shading performance so it seems like 24v panels make the most sense. Thoughts?

My battery set up is 3x firefly G31's in parallel for 330Amps (12v), and my Average draw on anchor is 6amps(at night inside, anchor light, fans, possibly tv can increase it to 8-9 amps)

Also, what pv breaker isolator would people recommend in my situation?

newpowa 200w 12v: volt pmax 17.V, amps 11.76A, open circuit 20.23V short circuit 12.45A

newpowa 200w 24v: volt pmax 34.0, amps 5.88A, open circuit 40.46v short circuit 6.23A



MPPT SmartSolar
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klim8skeptic avatar image klim8skeptic ♦ commented ·

You might want to run your voltage drop calculations through this calculator.

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3 Answers
Alistair Warburton avatar image
Alistair Warburton answered ·

I cant give you an answer speciffic to that charger but I can tell you where to look for the answer...s

Your panels will have a maximum power point, likely around 18V if they are nominally 12V panels... An MPTT charger should track and optimise this by holding the panel string at an optimal voltage and current given the condiions. This will likely be nowhere near the required charging voltage so a good controller will use a buck circuit to match the voltages with minimal losses.

Some cheaper controllers, and I doubt this is one, only use PWM to resirict the energy drawn from the PV whilst managing charge current, and in so doing, voltage on th battery.

I am going to assume that the Victron is doing things well and should therfore be able to efficiantly handle a voltage on the PV hat is significantly higher than the required battery voltage at any point.

You need to check this in the spec for your unit... There will be a max PV voltage and a range in which it performs well when charging a 12V battery.

RE paralell or seriese...

Assuming the overall PV voltage is in spec you should go with seriese and if you need them add bybass diodes, most reasonable panels have them built in anyway, to alow current through a dark panel, or part panel.

The point is 600W is always 600W but using twice the voltage requires 1/2 the current and thus 1/2 thr cable loss. You amy even get a few W in poor weather that would not be available with a paralell array but to be fair it will not be much in real terms.

Pick your cable based on the anticipated volt drop not the rated current.

Tri-Rated cable, equipment wire, and prety much all YY / SY and the like is rated at either 70 or 90 C in 45C ambiant temp.

That is fine if you want to minimise copper and space but useless as a starting point for solar. You will be able to get an ohms/km figure form the cable spec sheet, wirk out your volt drop for worst case, bulk essentally, and size ghe cable based on acceptagble losses.

V=IR, W=IV so W=(I^2)R

10AWG for a 10' circuit will be circa 0.02 ohms which is about 38W at 44A (600W @ 13.5V)

Obviously there is a point at which buying thicker copper stops being justified but it is almost cirtaily not going to be at the rated current of the cable!

Hope that helps...

Sorry I cant comment on the speciffic kit but I dont like to guess.


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nebulight avatar image
nebulight answered ·

24v panels for sure. The MPPT will work better (turn on sooner) with higher voltage panels.

I like the Bussman 187 series breakers. They only support up to 48V so in parallel this breaker will work fine. However if you ever wire them in series the voltage will be over that limit.

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michelg avatar image
michelg answered ·

Hi Erik, in my home I was asking this kind of question. My installation runs on 108/120VDC coming from the panels arrays. The higher voltage, the less Amps to carry over the wires and reducing losses. I do use 6mm2 wires, which can handle around 32A/240AC, but loaded only with max. 16A/120VDC.

Better having a larger size for the wiring, if you need one day to increase your PV production, you won't have to reinstall it.

In my opinion, going with the 24V panels is a better choice for your needs, so you can keep them running in parallel.

Regarding my installation, I have 4 arrays with 5x140W panels on each south orientated, and 2 arrays with 6x140W panels on each west orientated. Before noon, I get around 108VDC with no load (everything disconnected = 2500 - 2800W), and in the afternoon around 120VDC with no load (everything disconnected = 3000W - 1600W when sun decrease)

By having the PV serial mounted in each array, I increase the VDC, and by having the arrays linked in parallel I increase the Amps.

May be you can add & distribute more PVs on your boat (even smaller), so you will have a longer production.


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