Need opinion on panels

[Jafo]

I have seen these panels all over Craigslist in various locations and am about to inquire as to getting a couple. I was wondering what your opinions are on them? It almost seems a bit too good to be true:

https://newjersey.craigslist.org/mat/d/w-mono-solar-panels/6385721054.html

My current system is a 24 volt system at a camp, with two Sun Xtender PVX-2120L AGM batteries using a Renogy 20 Amp MPPT Solar Charge Controller. I currently have two, 100 Watt Renogy mono panels and thought this would be a good upgrade.

Thoughts?

[PNPmacnab]

Grid tie panels are a pretty much a commodity these days. That is the standard price these days for small lots, 60 cents a watt. I'm getting two delivered on Thanksgiving day for that price. Almost no one should buy 12V type panels these days.

[Jafo]

I am guessing I would need to upgrade my controller, but otherwise this should work? If I wire these two in parallel, that would work for my 24 volt system?

[sensij]

Those are 60 cell panels, too small to put in parallel on a 24 V battery. The Renogy Rover has a 100 V max input, so you would put two of these in series on that controller. No need to upgrade for that reason alone.

Those batteries will safely accept much more than 20 A charge current, so you could consider a much bigger expansion of your system.

[Jafo]

Those are 60 cell panels, too small to put in parallel on a 24 V battery. The Renogy Rover has a 100 V max input, so you would put two of these in series on that controller. No need to upgrade for that reason alone.

Those batteries will safely accept much more than 20 A charge current, so you could consider a much bigger expansion of your system.

Ok, so to be clear, if I wire the panels in series and send that to the controller, the controller will match the voltage to the 24V battery bank right?

[sensij]

Ok, so to be clear, if I wire the panels in series and send that to the controller, the controller will match the voltage to the 24V battery bank right?

Yes, correct.

[littleharbor]

Not sure about this budget MPPT controller, whether you can over panel it. My guess would be no. 560 watts may be too much for it. You didn't mention if you were going to keep the 100 watt panels but whatever you do don't mix these different panels.

[Jafo]

Not sure about this budget MPPT controller, whether you can over panel it. My guess would be no. 560 watts may be too much for it. You didn't mention if you were going to keep the 100 watt panels but whatever you do don't mix these different panels.

Not going to mix the current panels. According to the documentation of the controller, it can handle up to 100 volts and 20 amps. According to the docs on the panels, the max performance of voltage would be 31.37. My math says that should come to about 18amps max. Is that right? (280 * 2) / 31.37 = 17.8..

[littleharbor]

560 watts /24 volts is just over 23 amps. Considering you rarely get full STC output you should be ok. The 20 amp rating is the output, not the input. Mppt controllers are designed to take high voltage/low amperage input and step down voltage while raising amperage. This is why you look at the watts of the array. You could have 95 volt thin film panels putting out very low amperage. If you tried putting 20 amps worth of these into that controller you would definitely over panel it.

[sensij]

Not going to mix the current panels. According to the documentation of the controller, it can handle up to 100 volts and 20 amps. According to the docs on the panels, the max performance of voltage would be 31.37. My math says that should come to about 18amps max. Is that right? (280 * 2) / 31.37 = 17.8..

No, you are mixing up the input side of the controller.

Input side
Pmp = Vmp * Imp
280 = 31.37 * 8.93

With two in series
560 W = 62.74 V * 8.93 A

Output side
Power In * Efficiency = Power Out = Vout * Iout
560 W * 0.92 = 515 W = 24 V * 21.5 A

The Tracer "A" series very clearly states in the documentation that it will move off the maximum power point of the panels (intentionally not operate at Vmp) in order to keep the output current within the CC's rating. The "RN" series documentation doesn't state that so clearly, but I'd guess that it still does. Worst case, it looks like it would throw a "C" or "P" error.

In any case, as littleharbor is saying, the 560 W rating of the panels is very rarely reached in most systems, so you aren't likely to run into that 20 A limit.