Trying to size stand-alone off-grid solar system for 12V consumption.

usaousao Registered Users Posts: 11 ✭✭
Basically, I need a continueous feed of 16A @ 12V for an stand alone purpose. Im trying to figure out the details. Here's what I have figured so-far.
1) Approx 200W continuous for 24/7 operation = 4.8 KW/day. Let's go with 5KW/day consumption.
2) Running with a constant load of 16A, I would need a 20A charge controller. Ive heard that MPPT is the best, these are going on eBay for around $10 on the cheap end for a CMPT02 unit. No inverter needed as the consumption is 12V.
3) Solar panels would need to supply 5KW/day into batteries. Assuming 5-hours sun/day (Phoenix, AZ) on average, that means I need the panels to generate 1KW/hr to reach 5KW/day???
4) Batteries... Assuming I want to cover 1 full day of operation, that means I need 12V @ 16A * 24H (384AH). Assuming that I don't want to go below 50%, then I would need 768AH or better, correct?

Overall, am I on the right track or have a screwed up the math?
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  • dennis461dennis461 Registered Users Posts: 108 ✭✭✭
    edited January 2017 #2
    This is actually a fairly large system you are going to need.
    Plug your numbers in here...
    https://www.batterystuff.com/kb/tools/solar-calculator.html


    CMPT02 is not a good choice, it's ampere rating is too low(the one I looked at) and it is not a MPPT
    Camden County, NJ, USA
    19 SW285 panels
    SE5000 inverter
    grid tied
  • usaousao Registered Users Posts: 11 ✭✭
    Thanks, I like that it shows you all the calculations. Ill save that site for reference.
    As for the controller, I was looking at cheap, and the eBay offer said it's MPPT (eBay item number: 131360246544).
    Guess you can't count on the Chinese to be accurate, they are trying to push stuff out quickly...
  • newestnewbienewestnewbie Registered Users Posts: 6 ✭✭
    I am just past that stage, so thought I chime in?
    1) First call is to, more than, try to reduce your daily power requirement well below 5kW when you are offgrid! That's "insane", but typical initially.
    2) Now, assuming you disregard 1) (???), and you really want to go with 12V because your distance to cover is really short(RV!), then:
    3) For 5kW/day, you'd effectively need to draw 5.8kW, because at 12V expect 86% inverter efficiency (always "err" on the safe side)
    4) For long life battery, expect 30% discharge max, thus 5814W / 0.3 = 19380W. This @ 12V = 1615Ah battery capacity required! That's quite a number of batteries(!) which for your desired 12V, you will parallel-connect.
    5) Next, you need to generate more than the 5.8kW/day, because there are also inefficiencies before the solar power even reaches the battery! If you have TOP panels and TOP charge controller and TOP short distance between, then expect to require to generate from your panels 5.8kW / 0.9 = 6.45kW/day! That's quite a number of panels(!) which almost certainly requires you to be stationary (won't fit on the largest RV at today's PV efficiencies).

    Then if that still doesn't frighten you and your bank account(??), note that you will need lots of (costly) safety equipment on top: breakers, switches, fuses, thick cables,... - which actually is my own question about that I wanted to post... lol

    I trust the above helps you to reconsider...


  • usaousao Registered Users Posts: 11 ✭✭
    My load is a 12V load and draws a fixed current 24/7, can't change those starting points.
    No inverter involved, DC only.
    I was going with 50% discharge, but from Reading at the Rolls site, 30% is a much better number.
    I was thinking along the lines of 6x of the 2V batteries.
  • just startingjust starting Registered Users Posts: 228 ✭✭✭
    384 amps @ 12 volts per day or 192 amps at 24 v or 96amps at 48 v per day * 4 for the battery bank. Makes A 10k $ system
    200ah LiFePO4 24v Electrodacus Sbms40 quad breaker chest freezer to fridge- Samlex PST 1524 - Samlex pst3024  - 1hp shallow well pump-Marey 4.3 GPM on demand waterheater - mama bear Fisher wood burning stove, 30" fridgarair oven ,fridegaire dishwasher  Unique 290l stainless D.C. Fridge-unique 120l portable fridge/freezer 
  • mike95490mike95490 Solar Expert Posts: 8,029 ✭✭✭✭
    I have about a 7Kwh daily load, and could not meet that with a 12V system.
    5Kwh load / 24 hrs = 208w average draw,   So it could be met with a massively parallel 12V system or one built up out of  2V, 1,000ah cells
    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 ,

  • littleharbor2littleharbor2 Solar Expert Posts: 1,109 ✭✭✭✭
    usao said:
    Thanks, I like that it shows you all the calculations. Ill save that site for reference.
    As for the controller, I was looking at cheap, and the eBay offer said it's MPPT (eBay item number: 131360246544).
    Guess you can't count on the Chinese to be accurate, they are trying to push stuff out quickly...

    Funny, just this morning I sent a very direct message to one of the larger solar controller venders (China based) on eBay. I made it very clear that they are breaking the law by advertising these junk controllers as MPPT. I might ad this isn't the first time I have done this. I doubt it has much effect on this situation but they need to know the only people buying these are unknowing novices and we know their bulls*#t

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators, Administrators Posts: 28,176 admin
    Lead Acid batteries take time to fully/properly recharge. There are not "enough hours of sun" in a day to fully recharge a flooded cell battery bank from 50%.

    AGM may be a bit better, or possibly LiFePO4 would recharge much more quickly (lead acid batteries have an "absorb" phase from ~80% to 90%+ charge that takes a few hours). As said above, planning on 25% to 30% discharge per day would be a better plan (i.e., battery bank is 4x larger than daily load).

    Is the load all year round, or just during "non-winter" time (i.e., only when you have lots of sun)? Where (roughly nearest major city) will the system be installed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,114 ✭✭✭✭
    And you have to know that that 12V device will get between ~ 11.0 V to ~17.0 V or more during the day if you are charging during the day, because of the function of the charge controller...  the voltage range needs to be tolerated... it's not 'just' 12V..!
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • usaousao Registered Users Posts: 11 ✭✭
    edited January 2017 #11
    @mike95490, my load is 5KW total, not per day. It adds up to 5KW * 24 = 120KW/day.
    If I draw 120KW/day from batteries, I need to replace that in about 5 hours of sun.
    120KW/5h = 24KW/hr. So, I would need about 100 of the 250W panels.
    If I assume that 5KW @ 12V is approx 400A, then I would need 9600 AH / 30% depth-of-discharge, or a total of 32KAH capacity.
    This is sounding like it's not feasable.
  • PhotowhitPhotowhit Solar Expert Posts: 4,768 ✭✭✭✭
    Correct!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • mcgivormcgivor Solar Expert Posts: 2,421 ✭✭✭✭✭
    edited January 2017 #13
    .usao said


    usao said:
    Basically, I need a continueous feed of 16A @ 12V for an stand alone purpose. Im trying to figure out the details. Here's what I have figured so-far.
    1) Approx 200W continuous for 24/7 operation = 4.8 KW/day. Let's go with 5KW/day consumption.
    2) Running with a constant load of 16A, I would need a 20A charge controller. Ive heard that MPPT is the best, these are going on eBay for around $10 on the cheap end for a CMPT02 unit. No inverter needed as the consumption is 12V.
    3) Solar panels would need to supply 5KW/day into batteries. Assuming 5-hours sun/day (Phoenix, AZ) on average, that means I need the panels to generate 1KW/hr to reach 5KW/day???
    4) Batteries... Assuming I want to cover 1 full day of operation, that means I need 12V @ 16A * 24H (384AH). Assuming that I don't want to go below 50%, then I would need 768AH or better, correct?

    Overall, am I on the right track or have a screwed up the math?



    @mike95490, my load is 5KW total, not per day. It adds up to 5KW * 24 = 120KW/day.
    If I draw 120KW/day from batteries, I need to replace that in about 5 hours of sun.
    120KW/5h = 24KW/hr. So, I would need about 100 of the 250W panels.
    If I assume that 5KW @ 12V is approx 400A, then I would need 9600 AH / 30% depth-of-discharge, or a total of 32KAH capacity.
    This is sounding like it's not feasable.
    So, I'm a little confused , which one is your load? Seems to have increased somewhat.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • usaousao Registered Users Posts: 11 ✭✭
    edited January 2017 #14
    The  initial post stated 5KW/day (which was in error).
    This is why I wanted to go through the math and make sure I was doing things correctly.
    I have a 5KW static load. Im drawing approx [email protected] 220V going a pair of 2880W IBM 12V power supplies... The target equipment is all 12V.
    My bullet-item 1 was incorrect. I mixed up daily load with instantaneous load.
    I wanted to see if I could get off the AC circuit and go strictly on DC to avoid grid charges for the fixed load. Im paying $0.09/KWH, which works out to around $10.80 per day.
    I wanted to see if I could go with DC battery/solar to avoid this charge, but it looks like it's going to be way too expensive. It would take YEARS to make up the purchase costs.
  • mcgivormcgivor Solar Expert Posts: 2,421 ✭✭✭✭✭
    With that size of load, grid tie would be the logical solution, if it is an option, supplementing day time use. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    900W  3 × 300W No name brand Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal as a backup system. 
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergencies and welding.
  • Raj174Raj174 Solar Expert Posts: 648 ✭✭✭✭
    Sounds like your running some very powerful bit miners. Something close to 20 TH/s, hopefully, since their using that much power.
    I believe that even if you could manage to put such a system together it would not work, because as westbranch said, the battery voltage will range between 11 and 17 volts and would not be acceptable to most equipment that needs a regulated 12 volt supply. A good idea. I had considered it myself, however it would only work on an existing system as an opportunity load. And even at that, the extra power produced would be quite limited. 
    3600W PV, MNE175DR-TR epanel modified, MN Classic 150, Outback Radian GS4048A, Mate3, 54.4V 207AH LiFePO4 no BMS, 4500W genset.
  • EstragonEstragon Registered Users Posts: 3,034 ✭✭✭✭✭
    No way you can beat 9c/kwh with a solar/battery solution. Overall cost for that is going to run on the order of 10x that.

    Depending on location etc you MIGHT be able to offset some daytime grid power with solar sort of cost effectively.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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