Solar Array

My array consists of 6 Kyocera 120-1 solar panels. 3 banks of 2 panels each wired in series. The 3 banks are wired parallel.
The electrical characteristics of each panel is as follows: Current at rated power 7.10 amps. Voltage at rated power 16.9 volts.
My system is 24 volts.
Given that Watts=Volts x Amps it would on the surface that I have 720W=24V x 30Amps. However, in the real world I understand that I have something less!
At the array's optimum generating capacity what should the figures be in the above equation?
My battery bank consists of eight (8) S460 Surrette batteries wired at 24 volts and 700AH. (350AH at C/20)
Thanks...Lone Ranger


  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Solar Array

    am i missing something here in your question as 120w/16.9v=7.1amps? the max voltage point (vmp) of the pv is 16.9v and not 12v. the series/parallel arrangement changes nothing on this as it just rearranges to be 720w/33.8v=21.3amps. this is also at the illuminance of 1000w/meter^2 for STC. you typically won't get the stc rating and the power between the battery voltage and the pv voltage is usually all lost unless partially recovered with mppt.
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Solar Array
    However, in the real world I understand that I have something less!

    Right you are!

    In short, for modules wired in series, add the voltages. For modules wired in parallel, add the currents. The nominal output specs for your array as configured are 33.8 V x 21.3 A = 720 W STC.

    However, these specs are based upon lab conditions that simulate what would be a fairly cold ambient environment (~ -10 C, or ~14 F). In the real world, PV modules typically operate in warmer conditions, and the output voltage suffers accordingly.

    If you’re using a PWM type controller (i.e., a Morningstar TriStar), you could expect to see something like 28.8 V x 21.3 A from the charger to your batteries at the end of the Bulk charging stage around Noon on a clear, mild day using a well-aimed array. That would be ~613 W, or ~85% effective efficiency.

    If you’re using an MPPT type controller (i.e., an OutBack MX60), you might see a slightly higher net output in the summer (perhaps 5% more, so 28.8 V x ~22.4 A = ~644 W). You should see a noticeably higher net output in the winter (cold PV modules; perhaps ~12% more, so 28.8 V x ~23.9 A = ~687 W).

    [All calculations assume a battery temp of 77 F (25 C). Colder batteries require a higher charging voltage, so the MPPT controller with a remote battery temperature sensor would increase the charge voltage -- and therefore decrease the charge current -- accordingly.]

    Once in the Absorb or Float modes, either controller would maintain battery charging voltage by limiting charge current, which would be gradually reduced as the battery state of charge increases. Regardless of controller type, the array will not operate anywhere near its nameplate rating.

    Jim / crewzer