Solar Strings which is better higher voltage or more strings Xantrex XWMPPT60-150

mnittler
mnittler Solar Expert Posts: 63 ✭✭✭
in Solar Beginners Corner #1
I have a Xantrex XW6048 using 48v battery bank Trojan T-105's (8) which is being charged by the xw6048 via the grid. I also have a critical loads electrical panel powered by the Xantrex 6048. I want to add a Xantrex XWMPPT60-150 solar charge controller (so Xantrex communication equipment is the same) and 6 each 300 watt panels to the system to be able to charge the batteries during a grid outage. Which is better higher voltage higher amperage strings or lower voltage lower amperage strings? My question is which is actually better 2 strings of 3 or 3 strings of 2 panels? Since I have other grid tied renewable energy sources I have considered AC coupling but that is an entirely different subject.
Using 6 each Conergy PE300P 300 watt panels (Because I already have 18 of these panels using Aurora Power One micro inverters and can get more) and want to add the same panels. I live in extreme South Texas so really cold weather is pretty rare.
VOC 46.08V
VMP 36.82V
ISC 8.53A
Temperature Coeff. VOC % -0.33%C
Is my math/real life correct?
VOC 46.08V * 3 = 138.24 Volt which is under 150V limit of Xantrex XWMPPT60-150
ISC 8.53A * 3 panels * 2 strings = 51.18 A which is under the 60 amp limit

My thinking was that 3 of these panels in series would reach greater than 48 volts quicker and stay above 48 V longer than 2 panels in series? Which would you use based on experience in real life?
19.76kw Solar/GT Enphase IQ7+ MicroInverters
 5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
 6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
18kw Kohler Propane Generator

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Solar Strings which is better higher voltage or more strings Xantrex XWMPPT60-150

    You can't have "higher Voltage higher Amperage" or "lower Voltage lower Amperage" really; to get the same amount of power from an array you pick between "higher Voltage lower Amperage" and "lower Voltage higher Amperage".

    The array Voltage has an effect on controller efficiency. It also factors in to Voltage drop between the array and controller (along with Amperage, distance, and wire size). As such you try to keep the Voltage of the array near the 'ideal' for the system, which in this case is 70, without going higher than 2X that and/or the controller's input max. Otherwise PV Voltage isn't the issue because they are a current source. They will hit Voc rating with any amount of light but not produce any current. If the light is intense enough they will produce current 'first' and then Voltage up to the Imp * Vmp power point.

    You've got one string of T105's? That's 250 Amp hours @ 48 Volts. It would only need about (25 * 48 / 0.77) 1558 Watts of array to charge properly under typical conditions. In multiples of those 300 Watt panels that would be five (1500 Watts) to six (1800 Watts). Were it me I'd pick the latter.

    The panel Vmp at 36 means you need two in series minimum to have enough to charge a 48 Volt system. That would be Vmp 72 and Voc 92. Three in series would be Vmp 108 and Voc 138. The first is better for controller efficiency and eliminates any risk of exceeding the input max. It is doubtful the Texas heat would significantly drop the Voltage to levels where it would be insufficent to charge. The second is better for a long wire run but may be a cold temp risk even in Texas (Voc only has to go 10% over to cause trouble).

    Were it me I'd pick three parallel strings of two. And yes that means combiner and fuses/breakers.
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