Panel & Battery Sizing

fireflyfirefly Solar Expert Posts: 66 ✭✭
Greetings all, can anyone tell me if there is a formula for calculating the proper panel array to battery bank size?


  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    Re: Panel & Battery Sizing

    General rule of thumb, the solar panel rating (output current from solar charge controller to the battery bank) should be in the range of 5%-13% of the 20 hour rate of the typical flooded cell lead acid battery bank (as always, read the maintenance manual for your brand/model of batteries).

    You can go as low as ~3%--but you will begin to have troubles properly equalizing the batteries (if flooded cell lead acid), and that would imply you don't have much loads on the battery bank (or it would take you a very long time to recharge the bank if heavily used).

    You can go as high as 20% or more--but run the risk of overheating the battery bank. You need to check with the battery manufacturer's ratings... For example, a fork lift battery may be rated at 20% (or even more with active cooling support). Or some AGM batteries also can take very high charging currents--But AGM's can be quickly destroyed by overcharging. So having the proper controller/settings/battery temp sensor is vital for battery health and your safety.

    And for any battery bank, a Battery Temperature Sensor connected to the charge controller is very important to properly recharge your bank. Also, a battery monitor is highly useful to monitor your bank and prevent undercharging/over discharging.


    PS: Need to add sizing the actual array needs a few more bits of information... How much load you will use per day, how much sun per day / season. And if this will be a large or small installation.

    Typically systems with over 400 Watts of solar panels will use an MPPT type controller where the output current of the controller is not exactly related to the output current of the solar panels. Basically here, V*I (voltage * current) of the battery bank (say 50 amps * 14.5 volts = 725 watts worth of solar panels) for an MPPT controller.

    For smaller systems, typically a PWM solar charge controller is used, and the Imp (maximum power) current of the panels is what you will use to define the array's output.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fireflyfirefly Solar Expert Posts: 66 ✭✭
    Re: Panel & Battery Sizing

    Thanks Bill, I have 3-4volt Surrette's 20ah @ 1104 amp, if I understand your reply, it looks like I will need approx., 800 watts of panels to properly keep these charged. My daily usage is only around 120 amps total. I do have a MX60 and 4-51 watt 12 volt KC's and 2-165 watt 24 volt SW's. (SW's not hooked-up yet, but soon) hope to add two more this fall. Does that look about right?
  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    Re: Panel & Battery Sizing

    Using this link and picking Ottawa, ON--Using off-grid derating factor of 0.52, fixed mount, other defaults, and 1 kWatt of solar panels (program does not go any smaller), we get:
    Mth  kwh/m  kwh/d  AH/day @ 14 volts
    Jan    49    1.63    117
    Feb    72    2.40    171
    Mar    86    2.87    205
    Apr    74    2.47    176
    May    76    2.53    181
    Jun    75    2.50    179
    Jul    77    2.57    183
    Aug    69    2.30    164
    Sep    64    2.13    152
    Oct    54    1.80    129
    Nov    33    1.10    79
    Dec    43    1.43    102
    Year      771 kWhrs per year
    If I am understanding you correctly, you are using ~120 AmpHours per day at, I assume, ~14 volts.

    From the table above, we see some months you are well over 120 AH per day, and other pretty close, or even under with a 1kW fixed solar array.

    Any times you are charging less than 120 AH per day (average), you will need to make this up with a generator.

    So the answer to your question is a bit more complex... About 4 months out of the year, 1kW of solar panels is not enough (on average).

    And, for proper battery life, you would want to not let your battery sit below 75% State of Charge for more than one day. After 1 day below 75% SOC, you would want, through a combination of solar and generator, recharge that battery bank back up to 100% charge (typically, run the genset in the morning, and when the sun is up, let the solar charge controller make up the last 5-15% of charge (typically more fuel efficient to run genset under full load and let the final, lower current charging, be carried by the solar panels--if the sun is out).

    You have flooded cell batteries, so you can use a hydrometer to measure the current State of Charge (and cell balance) very easily--although you would not want to do that too often.

    If you don't have a battery monitor yet, take a look at these (XBM in particular), or the new Xantrex battery monitors which should be out in the next month or two.

    Battery Monitor - LinkLITE
    whitepixel.jpgBattery Monitor - LinkPRO

    Your battery bank itself, using our 3 days of no sun down to 50% capacity would be sized:

    120 AH * 3 days * 1/50% = 720 Amp*Hours minimum

    So, it is a good sized bank for the load you wish to support.


    Note: The 52% derating factor is based on using an inverter (~85% efficient) and flooded cell batteries (~80% efficient) plus the 77% losses present in the normal Grid Tied system (calculator default).


    Numbers used here are approximate and others may be substituted based on your individual needs.

    For better winter power collection, you can tilt the panels to near vertical, both to better shed snow (make sure panels are mounted high enough off the ground) and also collect reflected light off of the snow (cold weather and snow reflections really can help)--but I am sure you know this better than I from California. ;)
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fireflyfirefly Solar Expert Posts: 66 ✭✭
    Re: Panel & Battery Sizing

    Bill thanks for the help, would gladly trade snow for sun. Will see what happens when all is up and running. Thanks Again
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