How to size a battery bank based on total panel watts.

bethbeth Solar Expert Posts: 32
If you were to size a battery bank based on the total wattage of your panels, how would you do it.

Thanks for everyones help, Beth


  • BB.BB. Super Moderators, Administrators Posts: 29,994 admin
    Re: How to size a battery bank based on total panel watts.

    There is a range of current recommended to charge a battery bank... 5-13%

    So, if you had a 500 AH 12 volt battery bank, roughly the range of solar panels would be:
    • 500 AH * 14.5 volts charging * 0.05 charge rate * 1/0.77 system derating = 470 watts of solar panels
    • 500 AH * 14.5 volts charging * 0.13 charge rate * 1/0.77 system derating = 1,224 watts of solar panels
    Remember the battery bank is sized on, again roughly, 3 days of no sun and 50% maximum discharge, or ~6x the daily load.

    And, lastly, you still need to size the solar array based on what your daily loads are and how many hours of "full noontime equivalent sun" you get per day (by season).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bethbeth Solar Expert Posts: 32
    Re: How to size a battery bank based on total panel watts.

    Thanks Bill, Ok got the charge rate formula, but why the 3 day of no sun. Is this for ideal conditions where in general you have enough sun to recharge your batteries after the clouds dissipate? or where you have enough sun to harvest to cover your usage and keep a big battery bank charged. If on the other hand you will have to use a generator to recharge your batteries any way then I am wondering why under lets say conditions where you know the sun is some times not going to recharge you batteries, under those condition why would the battery bank not just be sized jut to fill in for shorter periods of cloudiness during the day and to cover night time usage?
  • BB.BB. Super Moderators, Administrators Posts: 29,994 admin
    edited December 2017 #4
    Re: How to size a battery bank based on total panel watts.


    Good question... I will give you lots of hand waving answers ;):
    • Batteries are more efficient at low charge and discharge rates... The ratio of Amps:Amp*Hour (rate:capacity) seems to work out nicely at around C/20 or 5% charge/discharge rates. You can run a typical lead acid storage battery fairly efficiently at C/10 or C/8 or 10%-12.5% discharge rate too (note, maximum surge current should not exceed C/2.5)
    You can see a AH vs Discharge Rate chart here (note this is for AGM--which have a "lower spread" than flooded cell):

    NAWS Concord AGM page
    • Batteries last longer if they are cycled less deeply. Cycling, often, below 50% dramatically lessens their life. Never discharge a battery to less than 20% state of charge (80% depth of discharge) as you can "reverse charge" a weak cell and ruin the cell/battery.
    The above statement is true--but not really useful in some ways... If you look at a charge of typical cycle life vs depth of discharge, you will see that a battery that is discharged to 50% will last ~4 years. But if you cycle it to 75% state of charge (25% discharge), it will last 8-9 years (made up numbers for example here).

    So, you by 2x as many batteries, and your bank will last a bit longer than 2x as long. Is this a good deal? Difficult to say. Certainly, changing out batteries 1/2 as often is a good thing. Plus you have the extra capacity if it is ever needed in an emergency (broken solar system, generator failed, no fuel, etc.). Down side is you have to spend 2x as much for a battery bank, and you may need more solar panels (or generator fuel) to make up the 5% recommended minimum charge rate of a larger bank. And--if somebody "kills the battery bank" visiting family/weekend renters or such--leave the lights on until you comeback the next month--they probably have killed the bank and you now have 2x the amount of batteries to $pend and replace.

    There is a thread here where we discussed the 3 day rule to death:

    Time to Question the 3 day Rule ?

    You can certainly go with a smaller battery bank (1-2 day + 50% maximum discharge). Especially may make sense for an emergency backup system where you have generator+antiquate fuel supply/storage.

    For an emergency backup system--perhaps a "minimum sized" battery bank is the better "investment". The batteries are going to be on float 98% of the time and they are going to die (mostly) by age (and possibly poor maintenance) instead of cycle death.

    For RV's and Weekend Cabins, a "minimum" sized bank cycled even to 20% state of charge is probably fine too. More than likely, the 20-50 weekends a year is not going to cause a "cycle death" but more age (and/or maintenance issues) death.

    In the end, sizing a battery bank for peak current and 50% maximum discharge (over X days--whatever works for you) is probably the place to start for a "minimum" battery system. Plan on using a genset (or AC mains) power to run the drier+washer+vacuum cleaner+shop tools during the day, and use the battery (+small inverter) for quiet evenings.

    You can plan to run the battery bank to 80% discharge--but the difference between 50% and 80% maximum discharge is the "cushion" that allows for the battery to age (and slowly) lose capacity. So--6+ years out the battery bank won't meet your needs instead of 3+ years out (as the battery lost XX% capacity over time.

    I will stop here--probably getting too long anyway. And, as always, a couple of Battery FAQ links:

    Deep Cycle Battery FAQ

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Whisper0Whisper0 Registered Users Posts: 1

    I am always amazed at how much information becomes available just skimming threads answering other questions by random people. Thanks to all in advance - a lot of times my questions get answered before I need to post them lol.

  • myocardiamyocardia Solar Expert Posts: 118 ✭✭✭
    edited December 2017 #6
    Indeed, this site is a veritable gold mine of information on pretty much all things solar. Welcome, BTW. Any things that you would like to search this site for, just type "____________", with the underlining replaced by what you are wanting to search, and without the quotation marks, into Google. Every link provided will then be only to this forum.*

    *This works with all websites, btw. Just type in the entire web address of the site, minus only the http://. If the site has "www", then all 3 Ws must be included, for instance. IOW, to search only Yahoo for articles about Johnny Depp divorce, you would type "Johnny Depp divorce", without the quotation marks.

    DoD= depth of discharge= amount removed from that battery   SoC= state of charge= amount remaining in that battery
    So, 0% DoD= 100% SoC, 25% DoD= 75% SoC, 50% DoD= 50% SoC, 75% DoD= 25% SoC, 100% DoD= 0% SoC
    A/C= air conditioning AC= alternating current (what comes from the outlets in your home) DC= direct current (what batteries & solar panels use)
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