2 panels 24V or 3 panels 36V?

PhilS · July 2009

My six new Sun HS 130 panels are up, an MX 60 is mounted and ready. This weekend I connect the two.

I'm using 8 ga wire and it's almost a 50' run. My battery bank is 12V.

I've been mulling this over for a few weeks: hook two panels in series times three? Or three panels in series times two?

I'm leaning toward the three panel option but I seem to remember reading that better performance is achieved by having the array voltage one step above the battery voltage (24V array into 12V batts). Or am I misunderstanding the voltages of the panels? Open ckt 24V, max 19V. Seems like either way is still an option.

But the higher voltage of the three panel option should mean less line loss?

I have sufficient wire to do either, and will still have enough left to replace some decade old array-to-controller 12 ga wires... no problems and even tho those runs aren't as long (25'?) I'm probably losing electrons because of the wire size.

Suggestions and/or opinions please?

Phil

mike95490 · July 2009

Re: 2 panels 24V or 3 panels 36V?

Not using the wire gauge caculator, but just gut feel, I think the conversion loss in the MX60 would be less @ 36V, than wire loss @24V Even 8 ga, over a long run has resistance. And you could measure the MX60 efficency localy, with meters, V & A in vs V & A out. If it seems too low, then revert back to 24V.

PhilS · July 2009

Re: 2 panels 24V or 3 panels 36V?

mike90045 wrote: »

Not using the wire gauge caculator

I should be using 6 ga according to the calculator I saw. 8 ga is better than the 10 and 12 I have already on some of my panels.

And my 'gut feel' is the same as yours, probably the way I'll wire it tomorrow, but figured it'd be better to post the question BEFORE the work rather than after.

Phil

BB. · July 2009

Re: 2 panels 24V or 3 panels 36V?

Efficiency in the Charge Controller--nearer to the output voltage.

Efficiency in the wiring, as high as you can get without Voc violating Vmax of the controller (around 140-150 VDC on a cold day).

In reality--You are probably only typing about a 1-2% difference in losses in the controller. I believe Outback as some graphs in their manual that show the efficiencies.

Check with the solar wiring calculator on how much your I^2 losses will be between the two configurations... Realizing that I^2 * R losses go up with the power of 2... 2x the current, 4x the resistive heating losses.

Inside the charge controller--you have both I^2 heating losses and V^2 parasitic capacitive losses... Depending on how things are done--one may be larger than the other (in Outback, it looks like the V^2 losses have more effect).

-Bill

PhilS · July 2009

Re: 2 panels 24V or 3 panels 36V?

Thanks Bill. Your input is always appreciated.

I attempted the math but got lost somewhere.

Reverting to more simple thoughts: two panels in series times three would have 50% more copper to carry the load. That also would put the voltage to the MX at the more desirable level. I originally planned on doing it that way.

But thinking that the higher voltage requires less wire..... ?

Which is why I'm asking. It almost seems half-dozen for one way, six for the other. And as Mike mentioned, it's not irreversable.

Phil

BB. · July 2009

Re: 2 panels 24V or 3 panels 36V?

Copper wire wise--If you double the current (1/2 the voltage), you need have the "area" of wire to carry the load (and 1/2 the amount of copper).

So, if you had three panels in parallel--you would need X sq.inches of copper. If you had three panels in series, you would need ~X/3 sq.inches of copper. And your cost to run the wire would be ~1/3 the cost with the high voltage panels (or you can run high voltage/low current, with heavier wire and have less I^2*R losses). For sort runs, cost is usually not a big concern. For long runs, cost and voltage drop can be a very big concern.

By the way, same issues on the battery bank side of the system too... As your system power needs grow (larger inverter)--the higher voltage battery bank allows you to keep the wiring, switches, fuses/breakers relatively small. Heavy current devices (roughly over 100 amps) can get pretty expensive to to right.

Of course, the devil is in the details... Different types of insulation, wire in conduit or in air, On roof temperatures vs in wall, etc...

-Bill

2 panels 24V or 3 panels 36V?

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