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Thread: Basic portable Solar panel setup

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    Default Basic portable Solar panel setup

    Hi all.

    I'm looking for a solar panel that can charge up my 90-100amp deep cycle batteries. Will a 120watt panel be enough? If i just keep it going all day. Where we go away, there are trees & so what about the varying sunlight intensity? Does it just affect the time to charge?

    Also can I connect a cheapo 12v inverter off the solar panel so I can run some other misc 240v chargers for small stuff like, laptop, ipod & camera etc.

    please reply with any advice & input, thanks.



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    1st thing you need to work out is how much power you will be using. This will tell you how many watts you will need. As for charging the deep cycle batteries yes it will charge them and the solar panel should still produce some power through the trees unless the sun is completely blocked. Also connect the inverter to the batteries not the solar panel.
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    Quote Originally Posted by ageno2gen1 View Post
    1st thing you need to work out is how much power you will be using. This will tell you how many watts you will need. As for charging the deep cycle batteries yes it will charge them and the solar panel should still produce some power through the trees unless the sun is completely blocked. Also connect the inverter to the batteries not the solar panel.

    I estimate ill be using 300watts off the battery, does the solar panel go straight to a battery? Or is there a charger needed for these batteries?

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    If it is a portable panel it should come with a charge controller but check 1st. 300 watts for how long?

    120watt panel will equal 120/12V = 10 Amps at max power.

    10 * 1hr = 10 Amp hr
    so
    300/12V = 25A

    so running 300 watts for 1 hr will be 25 amp hrs.

    This should help you work out if it is sufficient for you, but remember these figure's are only an estimate. There are to many variable's to consider e.g. charge voltage is around 14.5V, amount of sunlight (summer days are longer).
    Last edited by ageno2gen1; 03-02-12 at 03:23 PM.
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    Quote Originally Posted by ageno2gen1 View Post
    If it is a portable panel it should come with a charge controller but check 1st. 300 watts for how long?

    120watt panel will equal 120/12V = 10 Amps at max power.

    10 * 1hr = 10 Amp hr
    so
    300/12V = 25A

    so running 300 watts for 1 hr will be 25 amp hrs.

    This should help you work out if it is sufficient for you, but remember these figure's are only an estimate. There are to many variable's to consider e.g. charge voltage is around 14.5V, amount of sunlight (summer days are longer).
    300watts for up to 4hrs as a long run if possible.

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    so
    25 * 4 = 100 amp hrs ( your estimated power usage ).

    10 * 8 = 80 amp hrs ( from the panel on a day with full sun for 8 hrs ).
    As long as the 4 hrs is a "long run" it should be ok.
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    Except that 120 watt panels don't put out 10 amps, cos they're not exactly 12 volts. They are calculated at 18V. When charging a battery, the most amps you could ever expect from a 120w panel is a bit over 6 amps. And that's with the Sun directly overhead, which doesn't happen unless you live in the tropics. Even on a direct short, they only put out 7 amps. Real world conditions after losses see more like 5 amps from them.

    With 5.5 hours of sunlight at 5 amps, it would take 3-4 days to charge a 100ah battery from flat. You could squeeze more in if you rotate the panel to face the Sun throughout the day but it would still take a couple of days.


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    I'm curios why you cant run an inverter off the solar panel directly? If they are only small wattage alliances like phone & camera charger, it seems like a waste to have to use the battery for that small stuff.

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    Due to shade and solar intensity, the voltage of a solar panel isn't constant. Under load it could vary from <10V to 22V. The inverter needs a specific voltage range to operate within, which the panel can't provide 100% of the time. The battery acts as a voltage regulator, keeping the inverter happy.

    About the only things you can run directly from a solar panel are things insensitive to voltage fluctuations, like water pumps or fans.
    Last edited by Learjet; 03-02-12 at 06:10 PM.

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    Quote Originally Posted by Learjet View Post
    Except that 120 watt panels don't put out 10 amps, cos they're not exactly 12 volts. They are calculated at 18V. When charging a battery, the most amps you could ever expect from a 120w panel is a bit over 6 amps. And that's with the Sun directly overhead, which doesn't happen unless you live in the tropics. Even on a direct short, they only put out 7 amps. Real world conditions after losses see more like 5 amps from them.

    With 5.5 hours of sunlight at 5 amps, it would take 3-4 days to charge a 100ah battery from flat. You could squeeze more in if you rotate the panel to face the Sun throughout the day but it would still take a couple of days.
    Sorry ageno2gen1 is right "120watt panel will equal 120/12V = 10 Amps at max power."
    A 120 watt panel will put out 10 amps at 12v under optimum conditions.
    The formula is Amps x Volts = watts
    The variables of course are the strength of sunlight, battery condition, wiring loses & regulator loses.
    It is very hard to see optimal power output from a solar panel as even on a clear day there is enough moisture in the air to reduce output by up to 30%.
    When testing new panels I find the time that they show their peak power output is on a day with a few clouds & deep blue sky, when the sun pokes out from behind the clouds the amp meter peaks at the rated output plus some with good quality panels.

    Quote Originally Posted by Learjet View Post
    Due to shade and solar intensity, the voltage of a solar panel isn't constant. Under load it could vary from <10V to 22V. The inverter needs a specific voltage range to operate within, which the panel can't provide 100% of the time. The battery acts as a voltage regulator, keeping the inverter happy.

    About the only things you can run directly from a solar panel are things insensitive to voltage fluctuations, like water pumps or fans.
    You have this bit almost right, however, the battery will not act as a regulator, it will influence the system voltage depending on its rated voltage versus condition, age & state of charge.
    The battery acts as a buffer & storage medium for the system.

    A regulator is an additional part of the system, that controls the way the power from the input (solar panel or other input) is transferred to the battery. Some have other functions as well.
    Last edited by Tiny; 03-02-12 at 08:01 PM.
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    Yes poor wording on my part calling a battery a regulator, although it acts as a pseudo regulator of sorts up to a point.

    Ohms law aside for a minute, I'm hard pressed to believe you can squeeze 10 amps out of a 120 watt panel, when a dead short will only do 7.

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    Tiny what you are saying is wrong.
    "Sorry ageno2gen1 is right "120watt panel will equal 120/12V = 10 Amps at max power."
    A 120 watt panel will put out 10 amps at 12v under optimum conditions.
    The formula is Amps x Volts = watts"

    WRONG!

    Panels dont work like that. I install them , have a look a the specs on the back

    120 watts is the maximum power point ratings for volts and amps multiplied, the amps is NOT 120 watts/12 !!!

    if it says 6.5 amps output at the maximum power knee point then derate that to about 6 amps over batteries in peak sun conditions directly overhead.

    get out a current-clamp meter and go measure some yourself!, i have been offgrid on solar with battery bank since 2000. no power bills.


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    I'm not a professional of any sort, but I own 16 solar panels of various wattage ratings, all of which I have tested.

    I find a good general rule of the thumb for practical purposes after losses is 20 watts per amp for charging 12V batteries. So 6 amps for a 120 watt panel sounds about right.

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    I will agree with learjet aswell. I was just trying to help siles and not get to technical. He is only using it for when he is away, Thinking that he will have his 100AH batteries fully charged then when he is using it there will only be 2 (max) outlet on his inverter and he is charging 3 things? A laptop charging is around 100 watts for 2hrs. I still think the 120watt panel will be enough.
    Stop being so stupid.. itís my turn!!

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    Quote Bluedog, "120 watts is the maximum power point ratings for volts and amps multiplied, the amps is NOT 120 watts/12 !!!

    if it says 6.5 amps output at the maximum power knee point then derate that to about 6 amps over batteries in peak sun conditions directly overhead."

    Bluedog, you may be a little confused here, the formula Amps x Volts = Watts still applies to the panel ratings on the back of all panels, just get your calculator out.
    Since we already have an example in Learjets post we'll use that,
    Maximum Power: 120 W
    Max Power Voltage: 18 V
    Max Power Current: 6.67 A
    6.67 A x 18 V = 120.06 W
    It also works the other way as Ageno2gen1 put it
    120 W/18 V = 6.6666 A

    Now, the maximum power voltage of 18 V will not be seen when the panel is connected to a battery, unless you are killing the battery by over charging without a regulator, in which case it had better have large vent holes or it will explode.

    If you only get a peak of 6 amps per panel at 12 V then the panel is not 120 W or you have undersized wiring for the distance from panel to battery &/or pour connections. For 12V systems connections should be soldered to prevent excessive loss..
    I have 7 x 80 watt panels connected to a 12V battery system & have seen 42A at the meter in peak conditions when the system voltage is around 12 V to 13 V, that's 6 A per panel
    80 W / 6 A = 13.3333 V

    When I say peak conditions I don't mean a clear day with the sun overhead, there is usually too much atmospheric moisture at that time which dissipates the suns strength, so you would probably only see around 6 A per 120 W panel at that time. On my system during that time I only see around 28 A out of my 7 x 80 W panels, that's 4 Amps per panel or around 60% of peak output.
    Peak conditions for sun maximum strength are
    on a day with a few clouds & deep blue sky, when the sun pokes out from behind the clouds then you will see the Amp meter hit peak as long as the sun is at a good angle to the panels. You can maximise this with a solar tracker on your panel mount.
    Due to varying weather conditions, sun angle, clouds etc. peak output is only seen occasionally, so figures well below peak are the norm for solar & of course as both ageno2gen1 & I said, there are many variables that affect peak output to the batteries.

    My system has a data log & shows amongst other data the peak Amps in.
    Has 2 meters 1 for solar & 1 for hydro.

    Also have a Micro Hydro that produces 240 Watts continuous, that's
    240 W / 12 V = 20 A
    And you probably wont believe this, however, when the battery voltage rises during low load & high net input conditions the amp meter shows a corresponding reduction in amps. Eg; 240 W / 14 V = 17.14 A

    BTW, Bluedog I've been running my house on stand alone power since 1994. I installed, monitor & maintain the system.


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    Quote Originally Posted by siles View Post
    300watts for up to 4hrs as a long run if possible.
    Sorry siles, we have gotten a little deep in discussion here.

    Back to your situation, if you intend to run 300 watts continuous for 4 hours per day you will likely need more than 120 W solar input, as to the variables that affect panel output, weather being the most costly.
    You will also need to have a battery that will cope with the current draw you are proposing.
    At around 25 Amps/hour a battery that is too small will suffer from low voltage quite quickly & your inverter will cut out if it has a battery protecting low voltage cutout, which it should.
    If you want to draw 300 W continuously for 4 hrs on a cloudy day you will need to have around 500 to 600 Amp Hours of battery to cope with your demands.
    Cheers, Tiny
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    I am glad you have decided that 10 amps cannot be achieved from a 6 amp panel Tiny. 120 watts does not mean 10 amps.

    but that size panel with a charge regulator over Siles's batteries should be fine

    the temperature of the array also plays a part in how much you get out of it too, not just shadowing from trees.

    i started out with a few 50 watt BP's over a forklift battery in the weekender we had here back in 1995. still have them actually, but they have turned brown.
    the system evolved from then to now and incorporates a wind generator as well.
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    Today I have done some real-world testing for Siles on a similar setup to what he envisages

    I have 110 amphours of storage and a solar panel across it.
    the panel isnt 120 watts, its 30 watts, but you can get the idea of how to do power budget calculations based on the voltage and current measurements taken.

    right now the batteries are full. the on-charge terminal voltage is 14.1 and the panel puts in 1.55 amps in full, direct sun.
    this equals 21.9 watts from the 30 watt panel.

    now I also have a similar amphour rated battery thats flat. its terminal voltage when on-charge is 12.18 and the charge current is 1.71 amps.
    this equals 20.8 watts from our 30 watt solar panel.

    the short-circuit current of the solar panel is measured at 1.97 amps.

    lets now look at the specification sheet for this panel:
    maximum power point voltage = 16.8
    maximum power point current = 1.78 amps
    16.8 * 1.78 = 30 watts.
    Thats where the figure of '30 watts' comes from. you can immediately see in the real-world tests performed over the battery banks that we do NOT get 30 watts out of this panel!
    Hence calculations performed using the Rated wattage of panels will give a significant error compared to the ACTUAL wattage achieved from the panel.

    So, lets be clear : 30 watts divided by 12 volts does NOT indicate the correct amperage of the panel! if that were the case this panel would be delivering 2.5 amps into the flat battery. obviously as you can see it is not.

    Lets base our power budget calculations on the actual amperage of the panel and the voltage of the battery, NOT the manufacturers stated wattage for the panel....

    as Tiny also correctly points out you would require a much larger battery bank and more panels if you were to draw that sort of power day after day and account for bad weather.

    some pics:


    panel spec sheet

    the test setup

    charge current on the full battery bank , a pair of 6 volt 109 amphour batteries in series

    charge current on the flat battery

    short circuit current of the panel

    voltage of the full battery bank on charge

    voltage of the flat battery on charge
    Last edited by Bluedog; 05-02-12 at 03:20 PM.
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    Quote Originally Posted by Bluedog View Post
    I am glad you have decided that 10 amps cannot be achieved from a 6 amp panel Tiny. 120 watts does not mean 10 amps.
    120W@12v does mean 10amps this is basic electrical maths , but a 120W solar panel in it's raw form will not give you this.
    If the panel is producing it's full 120W and is wired to a 12v regulator ( not accounting for loses ) will deliver 10amps , it could even give you 6volts at 20amps.

    It's quite strange that people think you can use 12 volts to charge a car battery or deep cycle Gel , Agm etc , in reality any of these batteries sitting at 12volts is pretty much very flat, giving it 12volts will do nothing at all.
    Charging any 12volt battery is going to need between 13.8 and 14.4v , with this is mind the 120W panel mentioned connected to a proper charge regulator in perfect conditions ( this time accounting for loses ) should produce about 6amps maximum.

    The OP wants to be able to use 1.2Kwh per day 300W x 4 hours ,a normal 120w panel should produce about 80% of it's rating.
    So using the same average as a home PV system ,1KW of panels will average 4Kwh per day averaged over a year.
    Meaning you cant assume any more than 480Wh per day from a 120w panel , regulation and battery charging is at best only 70% efficient so only 366 watts of energy is going to be usable. Of course on clear sunny cool days this will be better and in winter with clouds much worse so 366W is based on the averages used for home PV calculations.
    The single 120W panel will only provide enough for just over an hour of what the OP requires , he needs at least 4 x 120W panels to cover the daily usage and to keep the batteries maintained.
    I won't go into battery sizing etc unless needed.

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    Bluedog , how old is that panel and battery rig ? , is that a lead acid battery or an AGM ? ( the Hawker )

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