need help to configure 12v off grid system

pirate
pirate Registered Users Posts: 3
in Off Grid Solar & Battery Systems #1
Hello everyone! I have a cottage and I use 3000 watt/h per days,I already have 6x 145w mono panels, 6x us L16xc 420amp battery (3 string of 12 volt) and a xantrex 60AMP charge controler. I am planning to move there for ever,in the winter I will get about 3 hour off direct sun per days.
So my question is: is my system is big enought for 3 or 4 days of bad weater? if not ,what should do?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: need help to configure 12v off grid system

    welcome.

    power outages have plagued people since it was first distributed. things have improved since the first days of distribution with the largest leap due to tesla, but we still find little gremlins are taking our power away at times. you are one who wants to try and do something about these interruptions just as many in this forum have. that was my incentive that led to my interest in solar too.

    you will find that you need to know more accurately what kind of loads you have to power and for how long. if they are dc then a battery monitor can determine the amp hours drawn at whatever battery voltage you have to come up with watts as watts = volts x amps with watt hours as the watts drawn in reference to an hour. if the loads are ac then you could use a killawatt meter to determine those loads to be supplied by an inverter. that meter can measure kwh so no calculating would be needed. keep in mind that the inverter itself will draw power too as it is not 100% efficient in transferring power to your ac loads. once your loads are known we can make better comments on your needs to back you up.

    as to the pv wattage you have with the ah capacity of the batteries, i can say it is mismatched as that is insufficient to keep the batteries properly charged. generally batteries like that should be at about a 10% charge rate of the battery capacity. you have 1260ah capacity at 12v and at 10% should be 126a. it need not be an exacting 10%, but it should be in that neighborhood give or take a few %. also keep in mind pvs don't give their full stc ratings all of the time and we typically derate them by multiplying by .77 or 77% of the original stc wattage ratings. this isn't exacting either as installs can vary somewhat, but it is fairly close in general.
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  • pirate
    pirate Registered Users Posts: 3
    #3
    Re: need help to configure 12v off grid system

    Here is my load.
    fridge:1.8AMP ac ( 216w) total run time per 24 hour is 6 hour.
    tv :1.2 amp ac(145w) total run time per 24 hour is 6 hour.
    lighting:4 x 13w compact fluo total run time per 24 hour is 6 hour.
    water pump 5.75 amp ac (690w) total run time per 24 hour is 0.5 hour.
    total of 2828W per days.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
    Re: need help to configure 12v off grid system

    My general rule of thumb, is take the name plate rating of the PV, divide by 2 to account for all cumulative system loses, then multiply that number by 4 to account for the average number of hours of good sun one might logically expect, per day, on averge over the course of the year. In your case, you have already determined that you should only use 3 for the winter.

    Since you know your loads to be ~2.8 kwh/day, let round up,, and call it 3 kwh/day

    To keep even with this load, you would need a system like this:

    2000 watts/2=1000*3=3000 watt hours.


    Since you have under 900 watts of panel, I suggest that you are deficit by about 2/3!

    May I also suggest that all off grid loads grow with time, so make any design choices keeping that in mind.

    Tony
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: need help to configure 12v off grid system

    working backwards you need 2823wh/d x 4 days is 11292wh.

    now for the batteries to supply this you must double the capacity over what is needed to prevent drawing the batteries below 50% dod. with 11292wh at 12v this is 941ah capacity. doubling it gives 1882ah battery capacity. charging at 10% is at least 188a. you could stay with the same battery bank you have if you do one of 2 things or both,
    1> cut back on the power usage and/or time of use.
    2> allow for a smaller amount of time to be backed up for as 1260ah is about 2/3 as much as what i calculated for 4 days of backup. .66 x 4 days = 2.64 days. maybe you could also use a generator to account for the extra 1.36 days?

    this brings us to another possible problem in your system. the high amps will be hard to wire and use so a higher battery voltage would be warranted. going to a 24v arrangement will still keep the same wh capacity of all the batteries while halving the current due to the formula watts = volts x amps. so 12v at 1882ah is the same wh capacity as 24v at 941ah and as 48v at 470.5ah.

    that brings us to yet another point as you only state the controller to be a 60a xantrex without specifying if it is mppt or pwm. the pwm model will not reap as much power from a large array as an mppt type would and can make a huge difference on average of about 10% extra current delivered to the batteries. the 60a pwm cc by xantrex will not go to 48v battery banks either. we also don't know the specs on the pvs to know if they will need to be placed in series to account for the increase in battery voltage or not. we, in fact, don't know how compatible you pvs are for charging at 12v either.

    furthering on the pvs, i have already stated they won't be enough and i hope you just bought them because you got a deal on them as there are many higher power pvs out there going fairly cheap. if you can't get more of the same pvs to match this will complicate things when you need to expand the pv system, but in some cases it is possible to do this expansion. you may have jumped the gun here in buying all of this stuff before the real thought went into a workable design. maybe you thought you did, but it would've been easier on you to ask us before you bought anything.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #6
    Re: need help to configure 12v off grid system
    pirate wrote: »
    ....So my question is: is my system is big enought for 3 or 4 days of bad weater? if not ,what should do?

    No, the easiest fix, is a small generator, to power a charger.

    The Honda auto-throttle inverter generators (EU-1000, EU-2000) are the top of the line for inverter gensets, but there are other companies producing them for less $.
    Power the genset up for a hour in the morning on cloudy days, any sun will help top off the batteries.
    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 ,

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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #7
    Re: need help to configure 12v off grid system

    Even ann hour of bulk with a 30 amp charger, ( about th e biggest a eu 1000 will run with PF considered) is a drop I n the bucket compared to the defict he is already running, if my math is close! He will have to run the genny ~ 5 hours with 30 amps of charge just to get close,, plus whatever sun.

    He is using ~ 3 kwh/day, and generating 1 kwh/day. 5 hours @30 amps=150 ah, or ~ 1800 WH.

    Move up to a eu 2000, double the charge current, and one can halve the hours of run time.

    Tony
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  • pirate
    pirate Registered Users Posts: 3
    #8
    Re: need help to configure 12v off grid system

    Thank's for all of you'r honest awnser, so if I double up my panels I should be close , so if I install 6 more panels at 145w each (46 AMP DC total) I will divide my load into two system of 12volt, I will need 60ft of cable to my battery bank and I am a litle confuse on wire size,the only solar cable I can find is mc4 awg10.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #9
    Re: need help to configure 12v off grid system

    In fact you have to triple your PV to "get close".

    Tony
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  • LandKurt
    LandKurt Solar Expert Posts: 41 ✭
    #10
    Re: need help to configure 12v off grid system

    Triple doesn't seem necessary. Double the panels would be 12 x 145W = 1740W. If he needs 3000 watt hours in 3 hours of winter sunlight that means 1000 watts average output. That would require 58% efficiency from the entire system. That's not a 50% derate but it's close. The figures would be better outside of winter.

    Am I overlooking another factor such as bad weather days, or is the three hours of sunlight an average figure that incorporates that? I know with my shading issues three hours of sunlight is the best I can expect in the winter, the average is worse.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #11
    Re: need help to configure 12v off grid system

    By my rule of thumb,

    1000 watts of PV (Using the OP's 3 hour number, not my usual 4 hour number) might ok like this: 1000/2=500*3=1500 wh/day, average, so by my reckoning, he would need ~2000 watts of PV just to cover his loads,,, with no allowance for days of no or partial sun.

    That would be ~14/145 watt panels,,and he currently has 6,, so it is in fact between 2 and three times. One could argue that one would like to have enough capacity not only cover one's daily draw, but have some head room for bad weather, as well as to allow the loads to grow, as they always seem to do, hence my suggestion to triple the number of panels.

    Tony
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