which panel to choose, 18V or 36V?

[perceval]

How does one choose a panel?

I have a 400ah lithium battery, 13.3 resting voltage, 14.4 charging.
I was looking at the panels available. I would like 2 panels of 200W each (that's pretty much what fits on the roof).

Most panels come in 18V and 36V version.

I guess it's for PWM controller in 12V or 24V setups.

But, what about MPPT?
I have a Victron 100/30.

Should I get the lower voltage, but higher amperage wired in series, or the higher voltage, and lower amperage wired either in series or parallel?
Would there be an advantage to either?

Thanks!

 

[iamrich]

I would always choose the higher voltage panel (all other things being equal). I am running 250w panels (30.3v / 8.37a) in series sets of three to bump the voltage up to ~92-100vdc and then combining three sets (parallel) into a combiner box.

Ultimately you just need to make more voltage than 14.4v, so either style panel will work. With the Victron 100/30 you will be able to run your panels in parallel or series, so get the less expensive option?

 

[HRTKD]

With MPPT, you don't really care about the "voltage" of the panels, meaning you don't care if it's a 12v or 24v panel.

Instead, you are primarily concerned that the total voltage of your panels when combined in series does not exceed the Maximum PV Input Volts of the solar charge controller. If you are in a location where it gets cold, you need to leave room for the voltage to climb when it gets cold.

Panels in series, the voltage adds up and the amps remain the same. Panels in parallel, the voltage remains the same and the amps add up.

 

[fratermus]

How does one choose a panel?

For me, in order:

1. what the controller will tolerate. Usually not an issue, but some DC-DC chargers have very low input voltages (~23-25v); in that case the 24v would not be acceptable.
2. what fits the allotted space
3. $/watt

Most panels come in 18V and 36V version.

1. nominal 12v / 36cell / Vmp ~18v
2. nominal 20v / 60cell / Vmp ~30v <-- somewhat less common, but they do exist
3. nominal 24v / 72cell / Vmp ~36v

Would there be an advantage to either?

24v is more common in 200w panels than 12v, so I'd expect them to be a bit cheaper by the watt. I suspect 12v 200w is a niche product that exists to meet the needs of people running 12v systems and PWM controllers. Hopefully someone who knows more than me will chime in.

 

[chrisski]

I like paralleling all panels so if one panel is shaded, the others still push amps. This sounds like a great idea, until I balanced that agains wire size for a 50’ run with a 36 amp run for 3% loss and the fact that 10 gauge wire is easier to work with because among other reasons, you can use MC4 connectors, which are limited to 30 amps.

With my six panels, I ended up with a 3S2P set up with 100 watt panels. That’s just the way the math worked out for voltage loss on the wires balanced with high amperage. By going with three panels for a total of 48 volts in series, that made the voltage loss less than 3% for the length of the run.

Others are not that concerned about my 3% loss number for my panels, and find it better to add an extra panel or two to compensate.

I then came up with two other arrays with two separate charge controllers. The second was a portable array. The third was 350 watts of flat panels on the roof,

My second array I added to power my RV was four portable panels, 100 watts each, all in parallel. This works fine. I even tilt it into the sun and because the tilt angle allows more early morning production and evening production, these 400 watts of panels throughout the day can produce more power than the 950 watts of panels I now have on my roof.

 

[perceval]

alright.
on a voltage drop only stand point, it would seem that using two 36V panels rated at 5.5A each, wired in series will keep the amps at 5.5, and ramp the voltage to 72V, which my MPPT can handle.
That way I can use awg10 wire, with only a 0.2% loss over my about 4.5m of wire.

I'm usually parked at the beach, and not much shade, so both panels would always be in the sun, but not tilted.

Reasonable thinking?

 

[Hedges]

Not sure what voltage your panel really is. "36V"
We need the label or the data sheet; what is Vmp, what is Voc?

Summer, no tilt is good. But if you want power other seasons, consider a manual tilt.

 

[perceval]

My box is already at the highest legally possible, so, adding a tilt mechanism would make it too tall.
Also, it's almost perpetual summer here. 25C for about 8 months of the year, and above 30C for some 6 months out of them!

We got 38C today....

Here's a capture of the panels I am looking at. These 200W basically all have the same ratings.

 

[Hedges]

Although I have no idea what the Kanji characters mean, I'll assume that says 36Vmp, 41Voc
Two panels in series would be 82Voc at 25 degrees C.
In freezing weather, voltage might increase 15%, possibly 20% depending on panels specs (only found on data sheet)
But it won't reach 100V, so you're fine.

"Victron 100/30"
That's 100V max input, 30A max output.

30A into a 12V discharged battery is 360W.
You have 400W of panels, but that's fine because they'll rarely deliver more than 360W. Even if they do, the MPPT will ignore power over what it wants, will limit its output to 30A.

Looks like a good fit.

A 40A fuse or breaker would be good between battery and charge controller.
Since that's a big lithium battery, I suggest class T fuse or something similar with 20,000 AIC rating.
Or a class T for the inverter, and a lesser fuse branching off after it for the charge controller.

 

[perceval]

Thank you!
Very much appreciated.

Winters never dip under 10C, and usually hovers around 15C, so, no freezing temps around here.
It also tends to be cloudy/rainy in winter.

Thanks again!