Power usage and battery question from a newbie

CskiCski Registered Users Posts: 11 ✭✭
I just recently bought an off the grid home. I am not living it in full time as of yet. I have a 960 watt solar array and a 1100 ah 24 volt battery bank. I have a few questions. 1. How much kWh can I use in a day and have enough reserve power for at least 2 or 3 days of no sun? The home is in AZ. My research showed an average of 5.5 hours of sun. 2. What should I let my battery voltage get to before I start my back up generator? 3. How would I know when my batteries are fully charged so I can shut down my generator? Any other advice you might have for a new off girder would be appricated.
Thanks in advance.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,523 admin
    Roughly, where in Arizona is the home... Trying to figure out how much sun you get by season.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 8,380 ✭✭✭✭✭
    Cski wrote: »
    ...3. How would I know when my batteries are fully charged so I can shut down my generator? Any other advice you might have for a new off girder would be appricated.
    Thanks in advance.

    When you start your generator, and initiate charging of the batteries, note the amps the batteries are consuming. Be sure you are looking at battery amps, not a mix of the amps going to the inverter and it's loads (there is a nice $60 sears clamp on DC ampmeter that works great for this) When you see the battery amps start to taper off, you have reached the end of the BULK stage of charging, and are now starting the ABSORB portion, which is a much slower stage, and it's likely safe to skip in bad weather, but you should complete Absorb at least 1x a week.

    I'll let lead acid folks figure the 24v bank recharge setting.

    If you Bulk in the AM, you may take advabtage of any sun breaking through to perform some of the Absorb for you,

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • CskiCski Registered Users Posts: 11 ✭✭
    BB. wrote: »
    Roughly, where in Arizona is the home... Trying to figure out how much sun you get by season.




    -Bill

    It is in the eastern part is AZ. In the white mountings, Near Show Low and Snowflake.
  • Mountain DonMountain Don Solar Expert Posts: 494 ✭✭✭
    mike95490 wrote: »



    If you Bulk in the AM, you may take advabtage of any sun breaking through to perform some of the Absorb for you,

    Exactly! Saves on fuel.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
  • BB.BB. Super Moderators, Administrators Posts: 29,523 admin
    OK, not a lot of information out that way in PV Watts... Lets try Prescott as a starting point. Fixed array tilted to 35 degrees from horizontal:
    Month    Solar Radiation (kWh/m 2/day)
    1      5.28    
    2      5.71    
    3      6.15    
    4      6.78    
    5      7.06    
    6      7.07    
    7      6.33    
    8      6.51    
    9      6.59    
    10      6.52    
    11      5.58    
    12      4.99    
    Year      6.22
    

    Lots of sun on average. Lets look at the design you have right now.
    Cski wrote: »
    960 watt solar array and a 1100 ah 24 volt battery bank

    A 1,100 AH 24 volt battery bank is pretty large (assuming you have the AH rating correct for series/parallel connected cells???)... I would be suggesting you look at reconfiguring to a 48 volt battery bank (some day) if you need one that large. Based 2 days of use and 50% max discharge (for longer battery life):
    • 1,100 AH * 24 volts * 1/2 days of storage * 0.50 max discharge * 0.85 inverter efficiency = 5,610 Watt*Hour of AC power per day
    And then there is recharging the battery bank... We use two calculations... One based on the size of battery bank and 5% to 13% rate of charge and a second based on load and hours of sun per day... First based on rate of charge:
    • 1,100 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,595 Watt array minimum (weekend/seasonal usage)
    • 1,100 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,190 Watt array nominal (full time off grid)
    • 1,100 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,147 Watt array "typical cost effective" maximum
    And based on Watt*Hours used and hours of sun per day. Use the battery daily nominal usage as a starting point:
    • 5,610 Watt*Hours per day * 1/0.52 end to end system efficiency * 1/4.99 hours December typical = 2,162 Watt array minimum for December "generator break even" month
    So--Without knowing anything about your daily/existing loads--Your present 960 Watt array is on the "too small" side and should be upped to at least 1,595 Watt minimum (for a flooded cell battery bank).

    in general, "we" recommend a 10% rate of charge as being very nice for full time off grid power--Much less "power management" trying to keep the batteries from getting over discharged based on how sunny today is and what needs to be done (washing, pumping water for garden, hot day and refrigerator/freezer).

    Because you have so much sun in the region (unless you get the summer monsoons which can kill production for days at a time). the 960 Watt array and low daily loads may have gotten away with OK.... However, when it becomes time to change the battery bank--I would be looking at either a 1/2 size battery bank or a 2x or larger solar array.

    And if you are going to be reconfiguring--And if you need this size battery bank, a 48 volt system would be nice--When you get much more than an 800 AH battery bank, it starts making sense to go with a higher voltage battery bank to keep the current and copper wire sizes "reasonable" (Power = Voltage * Current, double the voltage and 1/2 the current for same power).

    You should get a good quality Hydrometer to start measuring/logging the temperature corrected specific gravity for your battery bank... The gold standard for determining lead acid battery state of charge.

    Voltage can give you some indication--Very roughly around 24.0 volts under light loads, and ~23.0 volts minimum battery voltage under heavy loads would be the normal maximum discharge before load shedding/starting the genset would be a good idea.

    To conserve generator fuel, charging from 50% to 80% state of charge for flooded cell lead acid is the most efficient. With the efficiency dropping as the SOC rises--Typically, 90% SOC would be the maximum you would need to recharge "a couple of times per week" for a happy battery bank.

    Normally, run the genset in the morning to 80%, then let the solar array finish charging to 90+% SOC.

    Generator wise--Matching the generator+AC battery charger to the size of battery bank can help you with fuel costs... Typically, gensets are more efficient around 50% to 100% of rated load, at 25% and less rated load, fuel economy goes into the dumps (possibly using 2x or more fuel per kWH of battery charging). Gasoline fuel flow tends to flatten out at 50% of rated load (i.e., same fuel flow at 50% electrical load or 0% rated load). Diesels tend to be more fuel efficient but don't like running at light loads (40% or less of rated output)--or they can wet stack/carbon buildup/glaze cylinder walls.

    Off grid solar power systems--We try to design for "balance" between Battery bank / Solar array / Genset / Loads... Without knowing your loads, you are flying blind.

    Anyway, a start. Questions/corrections to my guesses?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CskiCski Registered Users Posts: 11 ✭✭
    Thanks for taking the time and giving me a detailed response. I have to read this over a few times tonight to fully grasp it, but it seems like my battery bank is to big and my array to small. I would not have thought a battery bank could be too small, but it makes sense. You need more power to charge them. When I bought the house I knew I would have to replace the batteries. I had a solar contractor come out, he recommended 12 6 volt 370 ah batteries. I went with his recommendation and had him install the new batteries. Would you recommended removing one bank for a total of 740 ah? Would this be more efficient with the solar array I have now? I am only using the house part time for now (vacations). I do plan on living in it full time with my wife after I retire in 5 to 10 years. In the mean time I would like to slowly upgrade the system. I figured my array was to small. Thanks to everyone for responding. I got some good info.
  • BB.BB. Super Moderators, Administrators Posts: 29,523 admin
    Can you give/sell the batteries to some one else?

    How much power so you plan on using?

    Can you justify going to 1,600 Watt total array?

    Solar panels have never been cheaper. Batteries are expensive. Good charge controllers are not cheap.

    And theft is not getting any better.

    If you were starting from scratch. I would have suggested a Honda eu2000i genset, a 300 Watt 12 vdc inverter and 2 or 4 golf cart batteries (220 ah at 6 volts). And a smallish solar array. For a weekend use system without too much exposure to theft.

    Next choice is to build out 1,600 Watt array (especially if you use a fair amount of electrical power on the weekends).

    Last go with a smaller bank (if you cannot make use of the extra batteries).

    Choices. ....

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 4,672 ✭✭✭✭✭
    Triple the solar if you are going to live there!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Triple the solar if you are going to live there!
    Full time, that is.
    Some would be happy with higher power use on weekends and charging the battery bank back slowly over the rest of the week. Not the best thing for the batteries, but an option just the same.

    SMA SB 3000, old BP panels.
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