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Off-grid roof rack solar array

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  • Off-grid roof rack solar array

    Hi folks,
    After coming across this forum and reading though a lot of posts i think i have finally worked up the courage to ask for advice for a proposed off-grid solar setup (although it could be considered RV i guess).

    From my reading it seems that when you are looking at solar on a budget you need to look at the trinity of size, storage, price and then pick any 2. I have picked size and price.

    The whole setup would be mounted on a roofrack of a 4WD so weight, portability and efficiency are a consideration in design.

    After looking throughout the forum regarding inverters i have decided that i would like to avoid using the word "BOOM" to describe what i have in mind and will be going down the 24v route.

    I am looking at a 2000w inverter to run a load of 1400-1700 watts (240v @ 6 or 8 amps AC). The load does not need to run all of the time and i would like it to run according to the health of the batteries - for example, Solar panels charge the battery to full - inverter kicks in to run the load and discharges the batteries to 80% (or whatever the ideal DOD is for the battery type) then disconnects the load until charged again. It would also be utilised in emergency situations as a backup power source however this is not a huge item as the load will be far less.

    For the batteries I am looking at either lithium or AGM but would like them not to exceed 50-60kg - as i said before the load does not need to run for a long time but as/when it can. From my understanding for the load i am running it is between 66-88 amps DC so i am hoping having 200 (or even 300) ah batteries will be able to do the job safely (the load runtime is not a huge thing but i would like it to be more then 20 seconds and also to avoid the BOOM factor keeping batteries healthy for as long as possible).

    The next part i am not sure if it is necessary or not but i would like to put in a low voltage disconnect between the inverter and the battery so it does the disconnect.

    A solar controller would sit between the battery and the solar panels (MPPT charger).

    There would be 2 permanent solar panels mounted with as much wattage as i can get out of them for their size (about 1600x1600mm is the space/size).

    A future secondary consideration is that it is expandable on demand - for example when going camping i can plug in extra panels into the array and add to the charge, then once the trip is finished disconnect the extra panels and go home. The future panels will probably be a different wattage so i am not sure if i add them as a different string or use a combiner of some description (or another controller?).

    Probably the biggest reason i am considering this is because we get a lot of sunlight where i live (northern end of Australia - minimum of 5 peak hours per day but can be up 7 depending on the month). The downside is that temp can be high (30-37 C).
    The whole setup would be well ventilated and a couple of DC fans would help with cooling the batteries and inverter if needed).

    Sorry for the longish post but i would like as much accurate advice as i can get and the help is very much appreciated.
    Last edited by dmaca667; 01-31-2018, 08:53 PM.

  • #2
    Welcome.
    1) systems are designed around watt hours of load
    2) not many choices to cut load off at 80% of battery capacity, that may require some custom hardware / software to interface to battery SOC & Load control
    Powerfab top of pole PV mount (2) | 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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    • #3

      I live in Northern Australia and yes we do have good sun hrs but we also have plenty of overcast days.

      They are pretty big numbers you are throwing around for a stand alone system when you are specifying that the battery storage and panels are limited.

      I would suggest you post what you are trying to run as there may be other alternatives.

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      • #4
        Your space limitation would allow two Sunpower 225 watt panels to nearly fill the space. These being the most efficient panels available in that size and being 61.4" x 31.4" or 1559.56 x 1595.12mm for two panels, side by side. They would fill your space and give the highest output available. Now, These panels would be difficult to find and if you could they would be used. My whole point here is you will most likely end up with less wattage than the possible 450 watts described here.
        Generally you would start designing your system by doing a load assessment including peak surge power. You are being vague about your daily load numbers but Whatever they are you can use the above best case panel output numbers to see how they work with your proposed system once you narrow down some of the other parameters.
        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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        • #5
          Thanks for the reply folks.
          Primarily the load that i want to run is to recharge a plug in hybrid vehicle. The max load that the charger draws is around 1750 watts (240v @ 8 amps, however there is a 6amp version that should be a bit more gentle).
          For a while i was considering an almost direct solar to drive battery DC-DC connection. But this may need some vehicle modifications and high voltage DC scares me a bit.
          After looking around a fair bit running from inverter should be doable, a bit safer and easier to implement (have seen people charging a leaf using the GoZero 1500 lithium).
          The plug connection is through the J1772 connection (the vehicle has a Chademo port as well for DC but finding the specs on the min input for this is impossible). I know that i will lose a bit through the DC-AC-DC conversion however the upshot is that it is fully reversible and i can run other things if i need to (grid power can be a bit unreliable sometimes).
          I know that there will be some weight added to the vehicle (and wind resistance). However even if it gets to 100kg+ this is still a very small % compared to the vehicle weight to start with (1800kg). Wind resistance would not be a huge issue as speed would rarely exceed 100km and 90% would be around 80km or so.
          The general idea is still a proof of concept.
          Once again, thanks for the replies folks it is helping

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