How to calculate panels and batteries

higginshiggins New UserPosts: 56Solar Expert ✭✭✭
I would like to know if there is a ratio of panels per battery for charging or does more panels mean enough power to offset losses or does it not matter if you have a solar charge controller,would i still need to calculate to match them up?


  • BB.BB. Just some guy Posts: 23,746Super Moderators admin
    Re: How to calculate panels and batteries

    Physically, some basic rules of thumbs (just a starting point--your needs can modify). Battery Charging:
    • 20 Hour Battery capacity * 0.05 = minimum recommended charger current
    • 20 Hour Battery capacity * 0.13 = maximum recommended charger current
    • Battery self discharge runs from a few percent per month (AGM's) to 1+% per day (old forklift batteries). May need additional solar panels to keep up old batteries.
    Recommend that battery 20 Hour capacity rating be sized to average daily loads:
    • Daily load * 3 days of no sun * 1/0.50 max discharge = 6x daily loads
    Real power out of an off grid system can be estimated by:
    • 0.77 * solar panel rating + charge controller losses
    • 0.80 to 0.90 battery charging efficiency (flooded cell to AGM type)
    • Call an AC inverter 0.85 efficient
    • 0.77 * 0.80 * 0.85 = 0.52 end to end efficiency (120/230/240 VAC useful power out)
    Sun is highly variable by location on globe and local weather conditions...
    • 2 hours of full sun per day (winter, coastal/island locations)
    • 4-5 hours of full sun--rough yearly average good sun
    • 5-6 hours of full sun--good summer sun in south west US
    • 6+ hours of full sun in Africa/Middle Eastern deserts
    To figure you your power needs... Say 9 months of the year, you get at least 4 hours of sun (assume generator power for other 3 months to make up for bad weather). Assume 1,000 watts of panels for example:
    • 1,000 watts of panels * 4 hours of sun * 0.52 derating = 2,080 Watt*Hours per average sun
    Assume 2,080 WH average load per day (you should never plan on Solar power = load used unless you have back up power or can shed loads). Assume 12 volt bank, 0.85 eff inverter:
    • 2,080 WH * 1/12 volts * 3 days no sun * 1/0.50 max disch * 1/0.85 eff inverter = 1,224 Amp*Hours @ 12 volts (20 Hour Rate)
    Normally, try to limit your DC loads to a 100 amps or so nominal maximum:
    • 12 volt bank * 100 amps = 1,200 watts max load/charge rate
    • 24 volt bank * 100 amps = 2,400 watts max
    • 48 volt bank * 100 amps = 4,800 watts (people do larger current--because 48 volts is typical maximum bank voltage for solar RE equipment because of NEC safety rules)
    Regarding generator design... Ideally, you want to run the generator around 50% maximum load when charging your battery bank... Generators typically use 50% fuel flow (gph, lph, etc.) when the electrical loads are 0-50% of rated power (Honda euX000i series gensets are pretty fuel efficient down to 25% or less of rated load).

    Also, Diesel gensets should be operated at ~60% minimum load for clean burning and long mechanical life.

    So, to design a charger/generator setup for the above 1,000 watt solar system. Assume 50% minimum generator load, assume 0.80 charger efficiency, assume 5-13% battery charging rate (note, generator size based on 50% load factor):
    • 1,224 AH * 14.5 volt charging * 1/0.80 chrgr eff * 0.05 min chrg rate * 0.50 generator load = 555 watts minimum generator size
    • 1,224 AH * 14.5 volt charging * 1/0.80 chrgr eff * 0.13 min chrg rate * 0.50 generator load = 1,442 watts maximum generator size
    Notice that this gives you some pretty small generator for a relatively sizable off-grid solar system.

    In general, it is difficult to find a "prime mover" rated power plant in the 500-1,500 watt range... Yet, charging with a 5-10kW diesel genset is ridiculously over-sized and--if done correctly--would need dump loads to operate the genset at a minimum 50% load.

    In the end--the battery is the "heart" of your off-grid system... And, is the only part (besides the genset) if abused, will have a very short life (sometimes measured in months).

    How you "treat" your battery pretty much defines it ultimate life. Will you get the 5-10 years stated by the manufacturer, or 1-2 years (or less) as you have read here from folks that did not look at "all the variables" -- and in the end, simply did not keep the battery in the 75-90% state of charge operating range day in and day out.

    Over charging (over equalizing) and boiling batteries dry are probably the other two major battery killers.

    Over-sized generators/undersized charge controllers/running geset to fully charge-equalize battery bank too often probably cost many people 2-4x as much fuel as they really need to spend.

    Is this sort of the laundry list you were looking for?

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