New on this and have questions

casanova
casanova Registered Users Posts: 4 ✭
in Solar Beginners Corner #1
Hi, I just installed myself, in my house, a backup system to support the 120V load in the house only. I live in Puerto Rico and since hurricane Maria hit the island the power in my area is not stable. The system is working and is supporting the loads required during power outage. I have 4 panels with this caracteristics. Peak power (Pmpp) = 240W, Rated Voltage (Vmpp) = 30.4V and Rated Current = 7.9A. They are installed in Series/Parallel two panels by strings. Based on this I calculated that the panels must provide 961W in total, But I never see that amount coming down from them, normally I see about 760W maximum sometimes, more or less. 
This is my equipment:
     Schneider Conext MPPT 60-150 Controller
     Schneider SW4024 Inverter
     Schneider DC Switchgear
     Schneider AC Switchgear
     2 Batteries Deka Solar 8A8D, 250 Amps
     4 Panels with the specs mentioned above
I am using 10AWG wire from the panels to the controller, the distance between them is about 20 t0 25 Ft.
Instead of a generator I have the system getting AC power from the power authority and the setup have active the AS Support Mode and the Enhance AC support too, so when the power drop the system provide power the load needed.

Questions:
It is normal to have an imput less than 961W?,  Is there any setup that I have to do to get the maximum power? Is the series/parallel the best option or must be parallel?
 
Let me know if ny other information is needed in order to ansewr my questions

Thanks in advance,

Gabriel Hughes
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Comments

  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #2
    casanova said:
    Based on this I calculated that the panels must provide 961W in total, But I never see that amount coming down from them, normally I see about 760W maximum sometimes, 

    Questions:
    It is normal to have an imput less than 961W?,  Is there any setup that I have to do to get the maximum power? Is the series/parallel the best option or must be parallel?
    Perfectly normal, in fact, I wouldn't expect 760 watts. If you look at your panels spec sheet they might give you NOCT values for the panels. These are the Normal Operating Cell Temperature values. So manufacturers even put the information on the back of panels. Panels are rated for output based on Standard Test Conditions, STC, most put out about 75% of STC at NOCT.

    Here is an example give by the Manufacturer Silfab.


    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #3
    In general, it appears that your system is configured/operating correct... For a whole bunch of reasons, assuming the battery is something like ~80% state of charge or less (and/or you have >1,000 Watt loads on the battery bus/AC inverter), the typical "best case average" peak wattage for a solar array (using "marketing specifications") is around 75-77% of rated array power for a cool/clear day, relatively clean/dust free panels, near solar noon (panels pointing to within ~10 degrees of the sun)...
    • 4x 240 Watt panels = 960 Watts
    • 960 Watts * 0.77 "typical best case array power" = 739 Watts (expected minimum "best harvest" a few times a year near solar noon)
    For your system, the series/parallel connection of your array is probably optimum too.

    Regarding your battery bank, assuming those are sealed AGM batteries--Generally, it is is recommended to not charge above ~14.4 volts (~25C room temperature, if hotter, battery charging voltage should be decreased by charge controller automatically).

    For a 24 volt @ ~250 AH battery bank (2x 12 volt @ 250 AH batteries in series), the recommended rate of charge is around 5% to 13% for solar power systems. 5% is good for weekend/backup use... 10%+ is recommended for full time off grid. Your system would "like" a solar array of:
    • 250 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 470 Watt minimum array
    • 250 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 942 Watt nominal array
    • 250 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,224 Watt "typical cost effective" maximum array
    So--Your system seems to be a very nicely designed "nominal/optimum" system for the components you have... You did not say how much energy you used per day (Watt*Hours or Amp*Hours @ 24 volts).

    Assuming you do not have problems with marine layer/shadows fro trees/power lines/buildings for most of the day, your daily Hours of sun per day should be something like (fixed array, facing south, average tilt, near Carolina PR):
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Carolina
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 72° angle:
    (For best year-round performance)

    JanFebMarAprMayJun
    5.56
         		6.03
         		6.49
         		6.39
         		6.23
         		6.66
     
    JulAugSepOctNovDec
    6.64
         		6.20
         		5.90
         		5.83
         		5.27
         		5.24
     

    So, looking at least ~6 hours of sun per day (a lot) during non-winter months:
    • 960 Watt array * 0.52 off grid AC system end to end efficiency * 6.0 hours of sun min per day = 2,995 WH per day....
    That is a good amount of power (should supply an efficient refrigerator, LED lighting, eff TV/laptop computer, small water pump, etc.) pretty nicely (or at least very closely). (note 2,995 WH / 10 hours of usage per day = 299.5 Watts average supported AC loads).

    Of course, never use 100% of your predicted energy every day (some days more harvest, some days less).

    For your battery bank, the average (dark day) usage (2 days of no sun, 50% maximum discharge):
    • 24 volts * 250 AH storage * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum battery discharge (for longer life) = 1,275 ueWH per day suggested average "dark day" energy usage from battery bank
    The above numbers are rough rules of thumb (cookbook) we use to design and predict system performance. Your usage may differ. The above is fairly conservative.

    Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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