Mixing Solar Panels

I am currently running (12) KC130TM panels wired in series of (3) for 52.8 volts (parallel of 4) (21.9 open circuit voltage / 7.39amps) with an Outback FM60 (150vdc max / 60amps max / 1800w max).

I have (4) Siemens SM50-H panels (19.8 open circuit voltage / 3.15amps) that I would like to wire in series with the existing series circuits of 52.8 volt plus the 19.8 volts equals 72.6 volts. I am wondering if this will increase performance by dropping resistance and allowing more amps for charging.

Roughly 70 foot run from panels to charge controller using number 4 (may be 6) wire in conduit. I figured this would cause a 2% voltage drop.


  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Mixing Solar Panels

    If I understand your proposed setup correctly--No, you should not connect 1x 50 watt panel in series with your 3x 130 watt panels.

    The 50 Watt panel is Imp=3.2 amps and the 130 watt panel is Imp=7.4 amps. The 50 Watt panel would try to limit the entire string to 3.2 amps -- and most likely would simply drop to near zero volts in such a setup.

    The best way I see... Try placing 3x of the 50 watt panels in series, and place that string in parallel with the existing 130 watt panels (5x parallel strings overall). That would probably work best... And unfortunately, that will leave you with one "extra" 50 watt panel (wire locally directly to the battery bank, behind a diode, or with its own small/cheap charge controller -- if this is a 12 volt battery bank).

    Very roughly, for optimum power, when placing panels in series you should match Imp to within 10%. For panels in parallel, you should match Vmp to within 10%... The farther you get from 10%, the more power loss (than 10%) you may get (limit overall production of the array, and can confuse the MPPT controllers with multiple power "power peaks" in the Power vs Current curve).

    Also, each series string of panels should have a fuse/breaker ~2xIsc of the series panels. This is "required" for arrays with three or more parallel strings to prevent panel/wiring short circuits from starting fires (other panels supply energy into shorted string).

    For the voltage drop, 3x7.39a+3.15a=25.32a total... For 70' of 4 or 6 awg wire (using a voltage drop calculator):
    • 6 awg > 1.7 volt drop; 1.7v/52.8v = 3.2% drop
    • 6 awg > 1.1 volt drop; 1.1v/52.8v = 2.1% drop
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhxsunuserPhxsunuser Registered Users Posts: 4
    Re: Mixing Solar Panels

    To Just some guy

    Thanks for the input, I did think of that confrigation, as well as adding the forth panel in parallel with the other three and than in parallel with the other four. I get much better production on that side of the house. Also the system is 24 volt.
    In addition to what we have allready discussed. in the same system I have another string of 4 sunteck STP175S (35.2 volts 4.95 amps) This string I run on a Xatntrex C-40 controller. All this goes in 12 Trojan L16- RE-B. So all in all it has been a very good system. I was just thinking maybe the 4 seimans would be better over there. And no left over panel. So it would be 2 seimans in series. In parallel with 4 of the 175's. I dont know what do you think? Just a little tweaking which would be close to maxing that controller.

    Sorry got carried away
    Thanks sunuser
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Mixing Solar Panels

    It should be OK to put the Seimens in the 2x2 setup (series x parallel) on the C-40 controller (assuming you have less than 40 amps total).

    You type about Voc (Voltage open circuit) but we really care about Vmp (voltage maximum power) when looking at matching panels and defining power output... Not a big deal if you talk about Voc between all panels--Vmp is just ~75% or so of Voc--so the ratios are all the same.

    Voc for the Seimens does sound a little low at 19.8 volts -- May make Vmp closer to 15.5 or so... On hot days when trying to equalize a battery bank--those panels may not supply as much power (Vmp-hot ~ Vbatt-equalizing).

    Still worth having the Seimens panels for regular charging 90% of the time--but you may not get the amperage you expect in some conditions (hot panels, high battery voltage, cool batteries).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Mixing Solar Panels

    hey just some guy,
    either those B's need to be bigger and bolder or you need to say "i'm just some guy.":roll::p
Sign In or Register to comment.