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Thread: Small Solar Off-grit setup - calculation check?

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    Default Small Solar Off-grit setup - calculation check?

    Considering getting a small solar off grid system for my house, and wanted to check if my calculations are correct. Just for clarity, I will still be connected to the Eskom grid, but will have a spate off-grid system.

    Wh needed for 24 hour period: 4868 Wh
    Hours of sunlight per day: 6 hours
    Watt to generate during sunlight period: 811 W

    300 Watt panels needed: 3
    Ah needed for batteries: 406 Ah (12 Volt batteries, deep cycle etc.)
    Batteries needed: 3 x (200 Ah @ 80% efficiency)
    Solar Charge Controller: 3 x (150Ah @12 V)
    Inverter: 1 x 1500 W

    I am not sure about the Charge Controllers though, if a battery is rated at 200 Ah (160 Ah if taking 80% efficiency into account), then 150 Ah might be too low.

    Thanks

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    Grandmaster lsheed_cn's Avatar
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    Some off the top of my head for you -


    -----

    Figure on 3hrs in winter @ rated panel rate.
    5hrs in summer.

    ALWAYS buy for your winter needs.

    If you need 5KW daily (which is low)

    I'd look at 6 x 325W panels - around R2000 each incl. vat so winter is catered for.

    eg http://www.solarsolved.co.za/index.p...607-641-detail

    6x 325 x 3 (winter) = 5850W / day


    6 Panels hookup = either

    8.6A / 1950W - Serial (Close circuit will be higher, check panel rating, inverter needs to be able to handle the open circuit rating)
    51.6A / 325W - Parallel

    I would suggest if you're going offgrid to get a hybrid inverter.
    The Axpert 3KW would probably suit you, although I'd get the 5KW for some headroom.
    Around R15,000- R20,000 depending where you order.

    Batteries - you will need 48V, so 3 isn't going to work. 4 minimum if Dead Acid..
    If you have money, I'd suggest rather spend it on Lithium.
    These aren't too badly priced -

    2.4KW - @ R14,720
    http://www.solarsolved.co.za/index.p...rackets-detail

    Could get away with two of those, but I'd suggest 3 to be safe, and then the batteries would last longer.

    You'll also need mounting and cabling, but thats cheap - R1-2k total
    http://goingsolar.co.za - My Solar Blog

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    Quote Originally Posted by Okty View Post
    Considering getting a small solar off grid system for my house, and wanted to check if my calculations are correct. Just for clarity, I will still be connected to the Eskom grid, but will have a spate off-grid system.

    Wh needed for 24 hour period: 4868 Wh
    Hours of sunlight per day: 6 hours
    Watt to generate during sunlight period: 811 W

    300 Watt panels needed: 3
    Ah needed for batteries: 406 Ah (12 Volt batteries, deep cycle etc.)
    Batteries needed: 3 x (200 Ah @ 80% efficiency)
    Solar Charge Controller: 3 x (150Ah @12 V)
    Inverter: 1 x 1500 W

    I am not sure about the Charge Controllers though, if a battery is rated at 200 Ah (160 Ah if taking 80% efficiency into account), then 150 Ah might be too low.

    Thanks
    As matter of interest, why you making the move to solar? I started looking it up myself, but in my use case, its cheaper to drop the money into the bond and save on interest.

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    Thanks Isheed, will have a look.

    One thing though, 3 hours sunlight seems a but rough for winter?

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    Quote Originally Posted by calypso View Post
    As matter of interest, why you making the move to solar? I started looking it up myself, but in my use case, its cheaper to drop the money into the bond and save on interest.
    My bond is almost payed off. Two main reasons, one to save on some electricity, eskom and municipality increases making a noticeable difference.

    Secondly, our municipality doesn't pay Eskom, so Eskom going to load shed us again, severly, ie punish those of us who actually pay. Randwater also droping our supply pressure due to non municipality payment.

    So yeah, want an off gridd system that keeps the lights, fridges, internet etc on.

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    Call it 5kWh you need as storage

    If going for Lead Acid, you want to use about 50% DoD. If lithium, 90% DoD.

    Therefore you need about 10 kWh of PbA storage, call it 5.5 kWh for LiPo/Li-Ion

    For PbA, you need (10[kWh:VAh]*1000[Wh/kWh]/12[V]) = 833 Ah total. 24 V is much better in my opinion, and depending on which inveter you can source even 36 V is better. 48 V is best. For compatibility's sake lets say you are using 200 Ah cells, arrange them 2S2P for 800 Ah which is close enough and at 24V nominal for the inverter.

    Check if you can source some e-bike or drone batteries as they may infact come out cheaper than going the PbA route.

    Winter weather call it about 4 hours equivalent sunlight normal to a correctly oriented panel face in Joburg, 3 hours for Capetown.

    Therefore you need about (5[kWh/day]/4[h/day]) = 1.25 kW worth of panels for Joburg and 1.67 kW for Capetown. To be honest panels are the cheapest part, if I were you I would get about 2kW in panels total.

    The inverter is the part you dont skimp on. Get a decent one that is pure sine wave and can charge the batteries from solar (via MPPT not PWM - what year is it??) and via grid. That way you can buy an el cheapo generator to charge your batteries if you need to if is really bad weather for more than a couple days

    I can do a proper analysis and simulation for your area if you want so you know what angle to point the panels and if it is worth the effort to make them adjustable, how many hours of sunlight you'll get per day each month etc. - you're welcome to drop me a PM
    I-Rah los Pruzah ahrk Mul
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    Just because I'm technically bloody and imperialist doesn't prove anything!

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    Thanks for the agentrfr, especially the inverter MPPT part.

    PM sent.

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    For where Okty stays he asked specifically for winter (June-Aug)

    How to use: Expected daily insolation * size of panel W is how many Wh you can genreate: Eg. 4 hours a day for a 1kW panel is 4kWh generated per day on average (panels only run at 80% efficiency when they get hot, so take that into account too not done above)

    Bruh, it's moerse hot where you stay.

    Here are results:
    Data is normalized about the 21st of each month (so you can compare to winter/summer solstices) per m^2 of a flat panel. I suggest panels are adjusted on the 9th of each month to the optimal value(s) for that month. Or for winter just use July and point it that way at the start of winter

    June: Optimal Azimuth: Due North. Optimal Elevation (Degrees above horizon): 32.5 degrees. Clear sky kWh: 8.0097. Weather clearness index (22 years avg): 0.7. Expected daily insolation: 6.36 hours equivalent
    July: Optimal Azimuth: Due North. Optimal Elevation: 35.75 degrees. Clear sky kWh: 7.9656. Weather clearness index: 0.71. Expected daily insolation: 6.55 hours equivalent
    July: Optimal Azimuth: Due North. Optimal Elevation: 46 degrees. Clear sky kWh: 7.7877. Weather clearness index: 0.67. Expected daily insolation: 6.58 hours equivalent
    Last edited by agentrfr; 15-05-2018 at 06:04 PM.
    I-Rah los Pruzah ahrk Mul
    IMA CHEMICAL ENGINEER WEO
    Agents are people too.
    Just because I'm technically bloody and imperialist doesn't prove anything!

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    Quote Originally Posted by agentrfr View Post
    For where Okty stays he asked specifically for winter (June-Aug)

    How to use: Expected daily insolation * size of panel W is how many Wh you can genreate: Eg. 4 hours a day for a 1kW panel is 4kWh generated per day on average (panels only run at 80% efficiency when they get hot, so take that into account too not done above)

    Bruh, it's moerse hot where you stay.

    Here are results:
    Data is normalized about the 21st of each month (so you can compare to winter/summer solstices) per m^2 of a flat panel. I suggest panels are adjusted on the 9th of each month to the optimal value(s) for that month. Or for winter just use July and point it that way at the start of winter

    June: Optimal Azimuth: Due North. Optimal Elevation (Degrees above horizon): 32.5 degrees. Clear sky kWh: 8.0097. Weather clearness index (22 years avg): 0.7. Expected daily insolation: 6.36 hours equivalent
    July: Optimal Azimuth: Due North. Optimal Elevation: 35.75 degrees. Clear sky kWh: 7.9656. Weather clearness index: 0.71. Expected daily insolation: 6.55 hours equivalent
    July: Optimal Azimuth: Due North. Optimal Elevation: 46 degrees. Clear sky kWh: 7.7877. Weather clearness index: 0.67. Expected daily insolation: 6.58 hours equivalent
    Thanks very much for the info - seems my initial estimate of 6 hours was not far off

    However, I took 4 hours of equivalent sun hours into my calculations (lets say it includes panel inefficiencies etc).

    So, as a 'starter pack', I am thinking of getting the following:
    6 x 260W Solar panels http://www.solarsolved.co.za/index.p...r-panel-detail
    5kVa Inverter/Charger http://www.solarsolved.co.za/index.p...charger-detail
    3 x 2.4 kW Li-Ion battery http://www.solarsolved.co.za/index.p...rackets-detail

    Can always later on add another battery pack to the system.

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    Grandmaster agentrfr's Avatar
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    Quote Originally Posted by Okty View Post
    Thanks very much for the info - seems my initial estimate of 6 hours was not far off

    However, I took 4 hours of equivalent sun hours into my calculations (lets say it includes panel inefficiencies etc).

    So, as a 'starter pack', I am thinking of getting the following:
    6 x 260W Solar panels http://www.solarsolved.co.za/index.p...r-panel-detail
    5kVa Inverter/Charger http://www.solarsolved.co.za/index.p...charger-detail
    3 x 2.4 kW Li-Ion battery http://www.solarsolved.co.za/index.p...rackets-detail

    Can always later on add another battery pack to the system.
    Yeah looks close enough for a start. Just check with them that the inverter is happy and tested with those batteries

    Just to check, you are aware that your selected inverter is Off-Grid, right? I.e. you are not grid tied with your panels, and any excess power you generate will be "wasted"

    Also those panels are 30 VMp, so you would connected them as 3S2P to match the inverter's optimal MPPT range of 60-115 V

    Otherwise post some pictures when it's up
    I-Rah los Pruzah ahrk Mul
    IMA CHEMICAL ENGINEER WEO
    Agents are people too.
    Just because I'm technically bloody and imperialist doesn't prove anything!

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    Get the 325W panels.

    Will last longer for you. Both in lifetime and additional future usage needs.
    http://goingsolar.co.za - My Solar Blog

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    Quote Originally Posted by agentrfr View Post
    Yeah looks close enough for a start. Just check with them that the inverter is happy and tested with those batteries

    Just to check, you are aware that your selected inverter is Off-Grid, right? I.e. you are not grid tied with your panels, and any excess power you generate will be "wasted"

    Also those panels are 30 VMp, so you would connected them as 3S2P to match the inverter's optimal MPPT range of 60-115 V

    Otherwise post some pictures when it's up
    Hi

    Yes, already did the calcs for the panels, i.e. 3S2P, same principle as hooking LEDs

    I'll double check the inverter, but from its data sheet it can receive a solar and AC input, and you can priorities which input charges the batteries.

    Yes, don't want to add any energy back into the grid (I believe it is actually 'illegal' here to do that).

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    *breathes*...

    I started investigating this again after the last price hike, and the 3rd random power outage in my area...

    And it started to look feasible to at least take the lights off the grid... until I saw those battery prices
    Quote Originally Posted by Korn1 View Post
    I have been called a retard my whole life

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