How much AGM battery?

Ralph DayRalph Day Solar Expert Posts: 1,005 ✭✭✭✭
Hi all
A friend of mine has had bad luck (experiences) with his off grid system.  The original installer made a mess of things leaving the owner with a badly executed system.  Undercharged batteries for the first year, no way to find out why (no generator either at first).  The first set of L16's experienced a runaway charge event and many of the 16 batteries were toast.  He ran a little longer on some scavenged/salvaged L16's.

He then installed a 10kw system like mine, straight feed to the grid, no net metering possible (or use of any of the power).  At the same time an electrical service was installed for the house and the off grid system was de-commissioned.  Batteries sold for scrap value.

My system (with batteries still) allows me to use the grid when I want, charge or just have the utility carry the loads and let renewables charge and or charge and charry all the loads.  I think I've convinced my friend to get some batteries so he can still use the valuable MX60 and double Outback inverters etc, and use the solar power available to him.  

Here's the question:  How much battery capacity is required to really just have the system act like a big UPS?   Cost is a factor.  Enough battery to be off grid for 3 days...not needed, the propane generator is always available still.  1.8kw of pv, 2x 3600watt OB inverters and one MX60.  I'm thinking a minimum of 400amp hours (48 volt system).  AGM would be better than FLA's, time and inclination to work with FLA's is lacking on his part.

Contributions and suggestions are appreciated.



  • Dave AngeliniDave Angelini Solar Expert Posts: 6,444 ✭✭✭✭✭
    edited December 2015 #2
    Ralph, some KWH per day usage is needed here.  Short of that a 400 AH @ 20 hr rate 48 volt bank does really not have to last 3 days offgrid if there is a generator. 
    "we go where power lines don't" Sierra Nevada mountain area
    E-mail [email protected]

  • zonebluezoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    I dont really understand what drives these hybrid systems, they are usually just sinking money into large holes in the ground. If you want to use the gear you have as a UPS then the smallest battery that you can get away with makes sense. Id be looking at the high C pure lead thin plates, or going to lithium. 100Ah nickel maganese will produce a heap of amps like in the Tesla model S. LFP can also do a reasonable C rating.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,

  • BB.BB. Super Moderators, Administrators Posts: 33,140 admin
    Looking at some math as a starting point.

    First 5-13% rate of charge:
    • 1,800 Watt array * 1/59 volts charging * 0.77 panel+controller derating * 1/0.05 rate of charge = 470 AH @ 48 volt bank maximum (weekend/seasonal/emergency off grid)
    • 1,800 Watt array * 1/59 volts charging * 0.77 panel+controller derating * 1/0.10 rate of charge = 235 AH @ 48 volts nominal (full time off grid)
    • 1,800 Watt array * 1/59 volts charging * 0.77 panel+controller derating * 1/0.13 rate of charge =181 AH @ 48 volts minimum "cost effective" battery bank for array
    Roughly, a 3,600 Watt AC 48 VDC input inverter would be a 360 AH battery bank minimum (to run inverter at rated power).

    So--Back to what will "work" for your friend... Does he want to run an AC inverter at 100% rated power (and 2x surge power)? Or does he want to run closer to 2,350 Watt maximum?

    My suggestion would be a string of "golf cart" batteries in flooded cell at ~220 AH... That would be a nice sized system and ~2,200 Watts maximum. The 3,600 Watt inverter would be "over sized" for that bank--But would give him 3-5 years to experiment.

    Assuming >4.0 hours of sun ~7 months of the year (Ottawa?):
    • 1,800 Watt array * 0.52 off grid system eff * 4.0 hours sun per day minimum average = 3,744 Watt*Hours per day average minimum
    That would run a home "off grid/emergency power" pretty nicely (one energy star refrigerators, lights, computer, cell phone charger, washing machine, possibly a well pump).


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • zonebluezoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Except hasnt he got a two stack of Outbacks. That 6 (or 7.2)kW needs at least 600Ah of FLA lead. Thats why i was thinking something capable of a higher C discharge rate.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,

  • Ralph DayRalph Day Solar Expert Posts: 1,005 ✭✭✭✭
    The master/slave setup of the inverters means only one is operating most of the time.  Maybe well pump would be the 240v high amp load.  More to investigate.

  • stmarstmar Solar Expert Posts: 370 ✭✭✭
    I use my system in a similar way as a UPS. I went with Lifeline AGMs, pricey but good quality. My system is 24 volt and I have 4 X 6 volt GPL-L16 which are 400 amp hour. I have had good luck with the Concorde brands (Lifeline and Sunxtender), my last bank was the Sunxtender 12 volt and they lasted 12 years.
Sign In or Register to comment.