XW-MPPT60-150 question

dellsworth
dellsworth Registered Users Posts: 20 ✭
in Off Grid Solar & Battery Systems #1
Hi,

I got an XW-MPPT60 recently from someone cleaning out their garage. Don't ask. it's a gift... Anyway I thought I'd upgrade my system from the xantrex 60 I currently have. I also some spare 120W 12v panels with total panels available of 18. I'd like the best configuration for the panels to work with a 24V bank. What's the maximum panels I can put together to make this work? I've 18 -120 watt panels available.

I was think 4 x 4. 9 x 2 will be too many volts and I suspect 6 x 3 too many amps. What do you think? Obviously I'd like to use all 18 but that doesn't seem feasible. I realize with the mppt I'm going to get more amps than with the old PWM I've got so I'm not sure how to figure it.

Thanks,

Dan
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: XW-MPPT60-150 question

    Pretty simply, 60 Amps is the maximum output the controller can handle. At 24 Volts that's 1440 Watts. Allowing for the typical efficiency of panels that's about 1870 Watt array.

    If you divide that by 120 Watt panels you get about 16, which would be 1920 Watts. All eighteen would be 2160 Watts, which would be a bit over. All this really means is that under sunny conditions the controller will be 'clipping' the extra Watts off.

    Now another issue. The Voc of those panels is probably around 22, and the controller's maximum input is 150, so the most in a string would be 6. If you have cold weather there you'll want to lower that so it doesn't exceed V-in max when cold.

    All things considered probably 3 strings of five would be the right configuration.

    Or you could use Xantrex's string sizing tool I suppose. :p
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  • dellsworth
    dellsworth Registered Users Posts: 20 ✭
    #3
    Re: XW-MPPT60-150 question

    I forgot they make these sizing tools now :blush:

    So I looked at it says the controller limits the amps to 60 on other over-sized array's. So if I put in the 4x4 I was thinking about would it really hurt anything? I've got the panels anyway and I get a bit more boost on cloudy days. They'll be under the VI max and under the 60 amps on input. If it never goes over 60 on output because the controller limits it I can size wiring and breaker for 60 (125% over makes it 80) and I'd think I'd work fine. Is there a downside from doing it like that aside from wasting some panel capacity on sunny days? Someday I can add another 60 and split the array and use all the panels but meanwhile I'd get a boost on cloudy days I normally wouldn't have. Three more months to rainy season....
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: XW-MPPT60-150 question

    Not really any downside to over-sizing the array. On days with less than perfect sun you get a bit more output. On days with great sun the controller simply doesn't make use of the full capacity. Most of the time this is what occurs with a "proper" sized array anyway, as by the time the sun is full-on the panels the batteries are mostly charged and don't need that maximum current.

    A 4 x 4 array would work fine, I'd say. Voc of about 88, Vmp of about 70, Imp about 27-28. 1920 Watts and a maximum output current probably around 30 on a 48 Volt system or 60 on a 24.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #5
    Re: XW-MPPT60-150 question

    If I were you I would go with 6 strings of 3. That would give you about 66V Voc and 50A Isc. Xantrex specify 60A limit for Isc. Since you're in southern Mexico, you're unlikely to get very cold conditions that would increase Isc, so 50A shouldn't be a problem. It's a good idea to look at exact Isc number for your panels. If it's at or below 8A, then there's no problem. If it's higher than 8.5A, you cannot do that.

    When batteries are charging at 27V, 60A output will give you 1620W, which is 75% of nominal. I those rare times when your panels produce more, your output will be capped at this amount. All other times, you'll get the whole output from all 18 panels.
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  • dellsworth
    dellsworth Registered Users Posts: 20 ✭
    #6
    Re: XW-MPPT60-150 question

    Hi NorthGuy,

    Thanks. That had actually been my original plan but then I got nervous about the 60 amp rating and didn't know how it worked exactly. The ISC is 7.45. They are Kyocera 120's. At the moment I can't see the point in getting another mppt to make use of the full array during full light conditions. I actually don't need that much generally but it'll be nice to have use of the full array during the rainy season. Mainly I"ll use it to drive my half horse pump at times other than only after the batteries are charged like I do now.

    Thanks again to you and Cariboocoot. As always this is the greatest forum. So many knowledgeable folks are on here.

    Hasta luego.

    Dan
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
    Re: XW-MPPT60-150 question

    that 60a limit is the output current limit and no matter the input configuration, if the watts are the same in each configuration and in keeping the same overall number of pvs, the output will be about the same. i do agree with the 6 x 3 configuration as it is more efficient for the cc on the input output ratio. higher voltages are usually needed when wire run losses become high.
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