First Question on Solar System Design

lzhome
lzhome Registered Users Posts: 39 ✭✭
in Off Grid Solar & Battery Systems #1
This is my first post here but I have been running a very small solar setup for outside 12v DC lights for a couple of years now. I'm expanding my load to include some AC so the upgrade I'm planning now is somewhat significant to me.

Have purchased a couple of books and a lot of Internet research. So not to sound too naive, here is my first question.
My planning shows Daily Watt Hrs of 1032. This includes a few 12v Lights and a small Refrigerator including the inverter self consumption. So, does this mean I need at least 1032 watts to solar panes to fully replenish the batteries each day? I do understand that I will only realize about 85% efficiency from the panels.
I'm planning on 4 x 100w panels and 3 x 155ah AGM batteries.
TriStar MPPT, 8 x 100w PV, MNPV6 Combiner, 4 x 12v 155ah VMAXTanks AGMs and GoPower 2000w PSW Inverter.
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  • 706jim
    706jim Solar Expert Posts: 520 ✭✭✭✭
    #2
    1032 watts for one hour or 516 watts for two hours; you see where I'm going with this.....
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
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  • lzhome
    lzhome Registered Users Posts: 39 ✭✭
    #3
    706jim said:
    1032 watts for one hour or 516 watts for two hours; you see where I'm going with this.....
    Don't quite get it. When I calculate amp hrs 1032/12=86 Ah. 
    Not sure what you call Amps generated by PV but it is 4 panels x 4.5a x 5.5 hrs sun = 99 Amps. generated / day.
    Appears system is maxed out with 86 Ah load and can only generate 99 Amps/day. Guess I'm still missing something here.
    TriStar MPPT, 8 x 100w PV, MNPV6 Combiner, 4 x 12v 155ah VMAXTanks AGMs and GoPower 2000w PSW Inverter.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #4
    ~1kwh sounds a bit low for a fridge & lights, but let's go with that. In general, you should figure on pv providing about 75% of STC rated output in full sun in most places. You should also figure on putting about 1.5 ah into a battery for every ah you take out. DC/AC inverter losses will be at least 10-15%, but can be much more if the inverter isn't well matched to the load. An inverter could concievably use ~1kw/day just being on.

    So to put some numbers to this, 1kw out needs ~1.5kw in. 1.5kw ÷ .75 = 2000w pv. Full sun hours depends a lot on location, tilt, and time of year, but say you get 4hrs of equivalent to full sun on average for all but deep winter months. That means you would need ~500w in pv to generate ~2kw/day.

    But wait, there's more... If you have cloudy day(s), you need to generate enough to make up the deficit, plus today's load, when the sun comes out. So the question is, how many cloudy days do you want to be able to go on solar alone?

    Going back to your example, 450ah@12v of battery would give you roughly 2 cloudy days using ~ 100ah/day to ~50% SOC. You would then have to generate about 200ah deficit + 50ah current load = 250ah@12v= 3kw ÷ .75 ÷ 4hrs = 1000w of pv.

    In other words, 400w of pv might cover a day's load (or not, depending), but likely won't cover deficit charging. For that, it's a trade-off between more pv and running a generator (or turning off the fridge).

    The trade-offs are very personal decisions, but it's wise to understand them up front.
    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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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #5
    The fridge itself, I'd expect 1KWh per day consumption.
    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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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #6
    I would agree with the refrigerator being more of a load than using the nameplate as a guide, actual demand may well be 1.5 Kw for a small unit, don't underestimate, I'm sure we are all speaking from experience, it's often said a refrigerator is what turns a small system, into a medium or large system, the refrigerator rarely sleeps, and the inverter has to stay awake to meet its demand, consuming more energy, in some cases equal to the refrigerator itself. If you  have grid power, I would suggest using a Kill-a-watt meter to get the consumption of the refrigerator   over a week, to more accurately, estimate  its demand.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • 706jim
    706jim Solar Expert Posts: 520 ✭✭✭✭
    #7
    lzhome said:
    706jim said:
    1032 watts for one hour or 516 watts for two hours; you see where I'm going with this.....
    Don't quite get it. When I calculate amp hrs 1032/12=86 Ah. 
    Not sure what you call Amps generated by PV but it is 4 panels x 4.5a x 5.5 hrs sun = 99 Amps. generated / day.
    Appears system is maxed out with 86 Ah load and can only generate 99 Amps/day. Guess I'm still missing something here.
    I simplified my example as much as possible. One could sum it up as "you can only take out what you put back in"....plus an assortment of other things including inverter idle current, battery and cable losses and of course, the capricious nature of daily sunlight.
    My brother was the first to attempt a solar powered fridge. His unit draws 5 amps at 12 volts and runs about 40% of the time. He tried running it from a single 8D and 100 watt panel and the results were disastrous. Eventually, he went to 500 watts of panels and 4 L16's and the fridge works well. Guidelines given in earlier posts in this thread are pretty realistic IMO.
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
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  • lzhome
    lzhome Registered Users Posts: 39 ✭✭
    #8
    Estragon thank you. A light just came on in my head. In very round numbers, my 400w of PV will generate 400w/hour less the inefficiencies such as clouds.
    TriStar MPPT, 8 x 100w PV, MNPV6 Combiner, 4 x 12v 155ah VMAXTanks AGMs and GoPower 2000w PSW Inverter.
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  • lzhome
    lzhome Registered Users Posts: 39 ✭✭
    #9
    Like to expand my question some; as I read more and look at the Morningstar String Calculator it appears that going to a 24v battery bank allows more future expansion opportunities. But correct me if I'm wrong, going to 24v would not allow direct wiring of 12v devices such as lights and fans and I would also have to have a more expensive 24v inverter? 
    I'm planning on using VMAX 12v 155Ah batteries, Tristar MPPT 60 and initially 400w of PV and wanting to go to 1000w in next year or so.
    TriStar MPPT, 8 x 100w PV, MNPV6 Combiner, 4 x 12v 155ah VMAXTanks AGMs and GoPower 2000w PSW Inverter.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #10
    You are correct that 12v(only) loads can't be connected directly to a 24v bank. Some 12v loads can be run at either 12 or 24v (eg some fridges and heaters), but many can't.

    It's possible to tap 12v off a 24v bank, but this is a bad idea as the bank will develop balance problems. A DC/DC converter would be the right way to do it.

    24v does give more room for future expansion. In general, I don't think a 24v inverter should be more costly than an identical 12v model.
    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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  • lzhome
    lzhome Registered Users Posts: 39 ✭✭
    #11
    Estragon said:

    24v does give more room for future expansion. In general, I don't think a 24v inverter should be more costly than an identical 12v model.
    Thanks for the reply. I did a quick search on Xantrex 24v inverters and the price is tripled, wow. What is a good inverter brand in the 1500 to 2000w range?
    TriStar MPPT, 8 x 100w PV, MNPV6 Combiner, 4 x 12v 155ah VMAXTanks AGMs and GoPower 2000w PSW Inverter.
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #12
    Pretty much any of the brands sold by our hosts should do the job. In that size range, I'd certainly be thinking at least 24v.

    FWIW, I've found the support on my Outbacks to be very good.
    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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