Frombergsa: Sizing solar array to battery bank size

This discussion was created from comments split from: How Many 12v Solar Panels Needed To Charge 24v Battery?.


  • FrombergsaFrombergsa Registered Users Posts: 1
    edited October 16 #2
    I am in kinda the same boat.  I have 24 - 2v 320ah batteries (that is not a typo - its is correct). 
    I am looking at building 2 - 24v systems as it appears to be the most efficient.  How do I figure out how many panels I need to charge the batteries?  
  • BB.BB. Super Moderators, Administrators Posts: 31,188 admin
    Formbergsa, welcome to the forum.

    I have split this to your own thread/discussion. That way we can focus the Q&A on your needs and system and not get confused about other folk's systems.

    Personally, I highly suggest that a system be designed to support your loads. Pick your most efficient devices, size the battery bank for those devices. Then size the solar array to support the battery bank charging needs, and your loads (based on your daily usage, hours of sun per day, seasonal needs--Summer cabin vs full time off grid).

    I will go through a very quick set of math here--Just to give you an idea of the process... But please ask questions and tell us more about your needs.1

    Since we are starting with a battery bank--Nominally, a good design is for the bank to support 2 days of "bad weather", and 50% maximum discharge (this is for a flooded cell lead acid deep cycle battery bank--But close enough for many other Off Grid cabin/home designs. For RV/Campers/etc. need more discussion).
    • 320 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge (longer battery life) =  1,632 WH per day from battery bank
    Next, charging the battery bank from solar... The typical range is 5% to 10% to 13%+ for solar charging (10% of 320 AH = 32 Amps charging current)... 5% can work for summer/sunny weather cabins. Highly suggest 10%+ for full time off grid living. Over 13% can make sense for places with limited sun (like winter in Canada)...
    • 320 AH * 29.0 Volts charging * 1/0.77 panel+controller derartings * 0.05 rate of charge = 603 Watt array minimum
    • 320 AH * 29.0 Volts charging * 1/0.77 panel+controller derartings * 0.10 rate of charge = 1,205 Watt array nominal
    • 320 AH * 29.0 Volts charging * 1/0.77 panel+controller derartings * 0.13 rate of charge = 1,567 Watt array "typical" cost effective maximum
    Then there is sizing your array for your daily loads... I will use 1,632 Watt*Hours per day (from nominal battery sizing). Assuming your system is somewhere around Burnaby BC, Canada, fixed array facing south:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 41° angle from Vertical:
    (For best year-round performance)


    Toss the bottom three months (not much sun in winter, assume either not used or backup genset and lots of fuel):
    • 1,632 Watt*Hours per day * 1/0.52 off grid system eff * 1/3.05 hours of sun per day (February) = 1,029 Watt array for February "break even" harvest
    And for a 320 AH @ 24 volt battery bank (FLA battery type)--Suggest an inverter of 800-1,600 Watts maximum (basically 500 Watt max inverter or array per 100 AH @ 24 volt battery bank)... Over ~1,600 Watts, the battery bank will not be able to reliably support the inverter+loads.

    There is the math... You could justify an array from 603 Watts (summer weekend usage) to 1,567+ Watt array, with at least 1,205 Watts minimum recommended for a full time off grid system.

    Your thoughts?

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