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Thread: How many solar panels do I need.

  1. #1

    Default How many solar panels do I need.

    I have 4 12v batteries with 140 amhr each I know I will connect these so it will make 560 amhr and still 12v. How many and how big of solar panels will I need to charge these in one day If I drain the batteries to 50-60%?

  2. #2
    Join Date
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    Default Re: How many solar panels do I need.

    The usual rule of thumb for charging a battery bank is 5% to 13% rate of charge. And we tend to recommend 10% or more (you can go as high as 25% or more if needed/makes sense for your needs with thermal management of your battery bank). So, basics:
    • 4 * 140 AH * 14.7 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 535 Watt minimum array
    • 4 * 140 AH * 14.7 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,069 Watt array nominal
    • 4 * 140 AH * 14.7 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,390 Watt array "cost effective maximum"


    You really did not tell us your daily loads or where you are located. But, taking a guess at 50% DC discharge per day. Assuming near Springfield Illinois using PV Watts, fixed array tilted to latitude:

    Code:
    Month    Solar Radiation
    (kWh/m 2/day)
    1      3.68     
    2      4.47     
    3      4.55     
    4      5.58     
    5      5.58     
    6      5.98     
    7      6.02     
    8      5.73     
    9      5.28     
    10      4.87     
    11      3.34     
    12      2.91     
    Year      4.83
    Toss out the bottom three months of winter (assume generator/backup power to make up for poor weather/sun) is February at 4.47 hours of sun per day (break even month):
    • 0.50 * 4 batteries * 14.7v batt charging * 140 AH * 1/0.77 panel+controller derating * 1/4.47 hours of sun = 1,196 watt array minimum


    So, the suggested array would fall between (roughly) 1,196 and 1,390 watt array.

    Note that it can be difficult to recharge a battery bank discharged by 50% in just "one day". The number of hours per day of sun is limited and depending on battery (flooded cell, AGM, GEL, etc.), some batteries may have charging limits that make it difficult to assume that you could discharge/recharge 50% per day for most of the year just using solar (you only have ~6-8 hours to recharge. It would take ~5 hours at 10% rate of charge + 2-4 hours of absorb to fully recharge--or ~7-9+ hours per day of significant sun--which you may not always have). We usually suggest for off grid / daily use that batteries are usually "happier" if discharged an average of 25% per day.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  3. #3

    Default Re: How many solar panels do I need.

    I live closer to St. Louis about 2 hours away from Springfield. I'm not quite sure my daily use. I doubt it will be that much. I'm using it for a green house. To power a fan and a light or two. the fan is 115v and 15 amps I believe and the lights I'm not sure I haven't looked that far ahead yet.

  4. #4
    Join Date
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    Default Re: How many solar panels do I need.

    If you only need the fans when the sun is up (keeping green house cool), you might look for DC fans that can connect to solar panels directly. No batteries or charge controllers needed (you might want a Linear Current Booster to get more fan action in morning/evening--and LCBs are not cheap).

    If you need a fan 24x7 to circulate air--Then batteries are needed.

    Lighting, are you looking for work lights or grow lights? Grow lights take lots of power and an off grid solar power system usually does not make sense.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  5. #5

    Default Re: How many solar panels do I need.

    I got the fan for free its a $200 greenhouse fan so I'd hate to not use it just because it ac. and the lights are work lights.

  6. #6

    Default Re: How many solar panels do I need.

    Unless that is a really big fan it probably does not draw 15 Amps: 15 * 115 = 1725 Watts (2 HP), the full capacity of a standard AC circuit.

    Do yourself a big favour and get a Kill-A-Watt meter and actually measure the loads in use. It can save a huge amount of money and frustration in designing an off-grid system.
    Four 175 Watt panels, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  7. #7
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    Default Re: How many solar panels do I need.

    In general, if the green house is anywhere near a power pole--It would be cheaper to trench a couple hundred feet of copper power (120/240 VAC split phase) to the green house.

    In general, mechanical devices use a lot of power (air / water movement) and are hard to reduce power usage--Other than getting (usually) more expensive devices (motors, controllers, etc.) that are designed for very efficient operation.

    A typical AC powerd greenhouse fan is probably going to use a lot more power (2x or more) than a very efficient alternative (which may cost 2-5x as much).

    -Bill "no free rides in solar" B.
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  8. #8

    Default Re: How many solar panels do I need.

    The fan it self has six fans and they are toyo fan tf120115a I know for sure that it is 115v the amps was a guess so I'm sure about that.

  9. #9
    Join Date
    Mar 2006
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    19,689

    Default Re: How many solar panels do I need.

    Those appear to be 15watt computer fans (AC versions with ball bearings). Now discontinued, so not much information with a quick search.

    If these fans are large enough for your needs--You might look around for used 12 or 24 volt DC computer fans instead. They can run from your battery bank or just wire them up direct to a solar panel. You probably would be much happier.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  10. #10

    Default Re: How many solar panels do I need.

    Its just a small 8x8 green house so It would work for my needs. I'm also trying not to spend much money on what I don't have to

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