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Thread: 12V vs. 24V System for "Off-Grid Portable Solar Power"

  1. #1
    MidEastSolar Guest

    Default 12V vs. 24V System for "Off-Grid Portable Solar Power"

    We're currently designing a mobile solar power trailer (similar to photo included) which will have two 250W monocrystalline panels (500W total power), an MPPT charge controller, 1000W inverter, two 12V200AH GEL batteries, and BOS. I am not sure whether to use a 12V system or 24V system which will dictate the selection of my various components. Can you shed some light on this ? Thanks
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  2. #2
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    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    A couple of comments...

    If you where going more than 1,200-2,000 watts of output, would recommend 24 volt system. A 1,000 watt inverter 12 volt circuit needs to be rated:

    • 1,000 watts * 1/10.5 volts cutoff * 1/0.85 inverter eff * 1.25 NEC wiring/breaker derating = 140 amps minimum

    A 24 volt system will be 1/2 that.

    For a 12 volt system, you will need, at least, a 25 amp MPPT charge controller or ~35-40 amp minimum PWM controller. A 24 volt system would need 1/2 the rating of charge controller.

    What is the Vmp rating of the solar panels? For a 24 volt system, the arrayl needs to be rated a minimum of Vmp~35 volts.

    If you have lots of 12 volt devices--many people like to power those directly--Although, a 12 volt solar RE system will have a volt range of ~10.5 to 15+ volts--Many inexpensive 12 volt devices don't like that wide range of voltage.

    Inverter--a 1,000 watt inverter on a 500 watt array will only run around 2-4 hours at full load. I would suggest a small TSW inverter to run lighter loads. Usually more energy efficient and a TSW (true sine wave) inverter is easier on the loads (electronics, small motors, wall transformers, etc.). MSW can damage many small electronic loads.

    MorningStar makes a very nice 12 volt TSW 300 watt inverter with low power AC search mode (can make people forgetting to turn of the inverter and draining the battery bank with inverter's no load current less of an issue). Only available in 12 volts--Makes a very nice reason to choose 12 volts.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  3. #3
    mvrck Guest

    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    My vote is 12V. Then if you use a 7-pin plug of on the trailer connection, it could charge its battery bank from the vehicles alternator.

    Good luck!

    Maverick

  4. #4

    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    Quote Originally Posted by mvrck View Post
    My vote is 12V. Then if you use a 7-pin plug of on the trailer connection, it could charge its battery bank from the vehicles alternator.

    Good luck!

    Maverick
    Just to clarify (or perhaps make more confusing).

    It could partially charge the batteries. Vehicle electrical systems tend to be held at 13.8 Volts, whereas deep cycles need to be pushed up to 14.2 for complete recharging. It would be able to maintain the batteries at a "float" level, providing the alternator is capable of handling the load.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5
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    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    I will add that charging from the vehicle is not really going to happen unless you do something like put an AC inverter in the car and an AC battery charger in the trailer.

    Also, have you thought about a backup genset in the trailer? Gasoline or propane powered Honda eu2000i or eu1000i are pretty nifty.

    Also, you may wish to reconsider Gel Batteries... If you want sealed, look at AGM instead.

    Deep Cycle Battery FAQ

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  6. #6
    MidEastSolar Guest

    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    Thanks a lot for the prompt feedback guys !

    Just to elaborate, the typical loads for the mobile solar trailers are: two or three 12V dc lights (6 hr. per day), 30-watt 12V dc cooler (12 hr. per day), 13" TV 80-watt (about 5 hr. per day). We estimate about 1,500 watt.hour per day. The trailer will be used for mainly camp sites in the Arizona area (good sunhours !).

    We wanted to stick with a 12V system because as you can see above, most of the loads are 12V dc. The only problem is that most 200+ watt panels operate on a 24V platform and since we're limited by a 4'x5' trailer, we don't have the option of cascading multiple 100-watt panels due to size and mechanical limitations. I really would like to use two 220 to 250 watt panels which will force me into a 24V system unless there're 250W panels out there that spit out 18V dc !!.

    Thanks Bill for the link on Battery FAQ. After reading, I think I'll switch to AGM batteries.

    Thanks again. Mark

  7. #7
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    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    Quote Originally Posted by MidEastSolar View Post
    I really would like to use two 220 to 250 watt panels which will force me into a 24V system unless there're 250W panels out there that spit out 18V dc !!.
    I see you chose an MPPT controller. This should solve the problem of charging 12V bateries from higher voltage panels. I can charge a 12V bank from 44v panels.
    6-180W CETC's., Midnite Solar Classic 200, Exeltech XP-1100E, Victron 24V-350W Inv, Energizer 24V 450aH FLA. Honda EU2000i Tri-Fuel. Victron BMV-602. Meanwell PB-1000-24. In Reserve: 2-180W CETC panels, Rogue MPT-3024, Exeltech XP-1100E

  8. #8
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    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    As SteveK says, with the proper MPPT controllers--You can have a 12 volt battery bank without any problem.

    All About Charge Controllers
    Read this page about power tracking controllers

    Beware that 1,500 WH per day is about the upper end of how much power you can expect from 500 watts of panels in summer.

    Around Phoenix, you are looking at ~1,700 watt hours on average for 8 months of the year. [note: that is assuming a 85% efficient inverter for 120 VAC power--You would expect around 1,700w/0.85=2,000watts for DC only output]

    With solar RE systems you should never plan on being able to use, on average near 100% of the power every day--It usually does not work out well without having a standby genset to help.

    So, back to conservation... TV's are all over the map--but if you look around (get a Kill-a-Watt and take it with you to the store):

    Quote Originally Posted by waynefromnscanada View Post
    Now here's something for you!
    I've been looking for a small TV for the bedroom and picked up one today at WalMart, a "GPX" 13 inch LCD HDTV. Made in China of course, but considering what it is, has an excellent picture, and get this- - - - - - plugged it into the Kill-A-Watt and - - - 9 whole watts! That's right, just nine watts! I nearly fell over. For anyone with a limited power supply, or anyone who wants to sit up close like you do with your laptop and save power, it's awesome! Best I've seen yet and was only $58.00 Cdn.
    It's AC only, but when I get a chance I'll open it up and see what it has for a power supply, just in case it actually operates on 12 volts. Unlikely that it does, but who knows for sure till we check it out.
    Other measurements from the same thread:

    Quote Originally Posted by vcallaway View Post
    The Plasma my sister bought draws 600 watts!!! I thought something was wrong with the kill-o-watt.

    Our 27" tube TV only uses 50 watts.

    The Vizio LED TV's we bought for the motorhome are 22" and 19". They are around 30 watts when on A/C. I suspect it is less now that I have them connected directly to battery. The transformers ran pretty warm which means wasted energy. I probably should put a meter on and see what the DC load is.

    There seems to be a dramatic change in TV power needs once you get over 27". We tested quite a few at Costco. Everything 27" and under was pretty low, once you made the jump it went over 100 watts and up.

    I also should share this little story.

    In December we were taking advantage of the sales to replace the TV's and DVD player in the motorhome. We wanted low power consumption and 12v if at all possible. We walked into a Video Only store and asked if they carried Vizio. I was greeted with a tirade on why I don't want one. They obviously don't carry them.

    Ok, so I asked if they had any Blu-Ray players that ran on 12v or at least used a wall-wart type power supply. I was informed that there was no way you could power a Blu-Ray player on 12v. That the laser drew so much power on startup that like a laser printer you could not even run it on a UPS.

    Wife and I just looked at each other with a *** is wrong with this guy look.

    We wound up buying the 22" TV at Costco ($180), the 19" at Target ($140) and the Blu-Ray from Wal-Mart($85). The Blu-Ray draws 20 watts max. The guy must use a really small UPS
    Regarding refrigeration--Usage makes a huge difference--Keeping things cool vs cooling warm stuff down, hot vs cool ambient temperatures, etc... Here is another thread with lots of discussion regarding small portable cooling:

    Quote Originally Posted by cfcw View Post
    I have collected data for the past few days on a Engel Mt-45 hooked to a 12V battery with a doc watson meter. Here's some of my data

    Unit draws ~35 watts while compressor running- only .3W when not running.

    Unit initially was placed outside in shade to simulate a non-power environment. High in 90s, lows in the 70's

    As mentioned earlier with the unit closed up, insulator zipped up it was quite efficient it used 460 watt hours in 40 hours without opening or about 275 watt hours per day. However, I felt this was unrealistic load because most people have every intention of using their refrigerator. So, every twelve hours I opened it up and placed two gallons of room temp water in there to give the refer something to do. As expected, the results were much different. To cool that water off in the Summer environment used 330 watt hours every twelve hour period, or about 660 watt hours per day.

    I had painters coming to the house, so I moved the unit off my rear porch back to my basement. Temps in the 70s. It used less power for the same task, around 250 watt hours per twelve hour period.

    So to recap-

    • Closed, insulated in hot environment 275 wh/day
    • Opened twice daily and used to cool four gallons water 660 watt hours per day
    Do you have any instruments to measure DC power usage? A couple of things to consider:


    The DC AH meter is great for measuring energy usage over a period of time (days, weeks, etc.) at 12 - 60 volts.

    The Battery Monitors are really a nice tool for watching what is happening to your battery (loads, charging, current state of charge)--And I would suggest if you go with AGM batteries--almost mandatory for monitoring battery state of charge (AGM/Sealed batteries, you cannot use a hydrometer to measure state of charge).

    The Trimetric is probably a good place to start looking for a "lower cost" unit.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  9. #9
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    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    Quote Originally Posted by MidEastSolar View Post
    two 12V200AH GEL batteries

    BZZZZZ

    Wrong.

    You want AGM batteries, not gel.

  10. #10
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    Default Re: 12V vs. 24V System for "Off-Grid Portable Solar Power"

    Quote Originally Posted by mike90045 View Post
    BZZZZZ

    Wrong.

    You want AGM batteries, not gel.
    CLANG--Late buzz.

    See post number 6.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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