Selecting panels and string sizing to feed MX60's?

HuckMeat Registered Users Posts: 24
I'm getting ready to install a 3-4kw (initially) solar PV system - I have MX60's and Outback GVFX3648's already (screaming deal) so I'm trying to figure out the best panel setup. Right now, I'm looking at

Evergreen 210's in strings of 4, Voc 23.10 * 4 = 92.4V, * 1.25 = 115V

RECSolar 225's, in strings of 3, Voc 36.8 * 3 = 110.4V * 1.25 = 138V

Kyocera 210's in strings of 3, Voc 33.2 * 3 = 99.6 * 1.25 = 124.5V

I'm in Colorado, and using a 1.25 multiplier for temperature, which I'm hoping is correct.

Because of line losses, I'd like the highest voltage I can do with my MX60's - If my math is right, each of these configurations would work, unless I need to derate for altitude or my multiplier is incorrect.

I'm having trouble comparing panels for this system, and was hoping a) Someone might give me a sanity check on my string sizing/math B) point out any other intangibles or preferences of these panels, or another panel. Any of the three fit my multi-panel polemounts.

If I could get the midnite classic, I'd change the strings higher... And save a lot on wire. :D But alas, probably not in the timeframe I want to get the project done.


  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Selecting panels and string sizing to feed MX60's?

    i think you may be ok with any of those arrangements and you may have figured the temp coefficients wrongly, but it looks to be in the neighborhood for a max low temp of -25c or -13f. i'm not sure as to what your actual max low temp is. if there's any doubt it would come from the recs 225s as they may encroach upon the controller's maximum area so lean either of the other 2 for peace of mind. the mx-60 has a 150vdc maximum (etl rating ul rating for 1741 standard) with an operational max of 141vdc temperature corrected voc.

    remember that line losses will also drop the voltage presented to the controller and losses we like to keep to a minimum with the runs from the pvs to controllers at under 2 to 3% with many leaning on under 2% due to the base costs of solar being expensive and the nec requires overall losses (source to load) to be at 5% or less. you can see the voltage drop calculator here to figure what your wire sizing needs would be.
    ps-do not count on designing around the voltage drop for if the batteries are charged there won't be any load to drop the voltage and the controller will see the full brunt of the pvs voc under the temps at that time.

    in keeping pv input voltages high you are also keeping the controller efficiency lower so keep that in mind in your designing of a system. sometimes there's no getting around needing thick copper wires.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Selecting panels and string sizing to feed MX60's?

    Array Voltage rule-of-thumb recommended by Outback is "one step up from system". In other words, a nominal system Voltage of 12 should have a nominal array Voltage no more than 24. This usually allows the Voc issue to solve itself.

    You're quite right to be concerned about over-Voltage on the input. Been there, done that, MX60 does not reset itself ... Six months later you come back and buy new batteries. In the cold of the Cariboo, my Voc goes way over the panel rating.

    A nice thing about connecting "12V" Evergreens is you can drop one if the Voltage is too high, or add one if it's too low. Using "24V" panels the change from doing this is usually too severe.

    Altitude only affects panels in that the skies are clearer up here. You get some nice blue at 3000 feet! :D
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
    Re: Selecting panels and string sizing to feed MX60's?

    If you do a search for outback you can get a string chart there. Also our sponser has a string chart if you search morningstar link under charge controler. I am waiting for the Midnight controler also. S:Dlarvic I decided to check outback string chart.
    MX-60 evergreen 210 panels = 1 string of 5 1050 watts
    or 2 strings of 3 =1260 watts
    flexmax 80 evergreen 210 panels = 2strings of 5 2100 watt
    Mx-60 kyocero 210 watt panels = 2 strings of 3 1230 watts
    flexmax 80 kyocera 210 watt panels =4 strings of 3 2461 watts
    Mx-60 rec 225 watt panels =2 strings of 3 1347 watts
    flexmax 80 rec 225 panels = 3 strings of 3 2021 watts
    or 5 strings of 2 2246 watt. lowest temp -26f highest temp 110 f.