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Thread: Practical application for a 200W panel with 1 kwh battery?

  1. #1

    Default Practical application for a 200W panel with 1 kwh battery?

    I am thinking of constructing a small and simple 200W system with a 1kwh batteries (rated at 8 hour discharge).

    Excluding the inverter variable (meaning, I could potentially install an inverter that can handle thousands of watts), what can this little system do?

    Given a 50% discharge / day, I can only use 500-wh.

    I want to be able to run the basic peripherals (meaning, not the heavy appliances - Washer/Drier/Vacuum/AC/Fridge).

  2. #2
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    Quote Originally Posted by solarnoobie View Post
    Given a 50% discharge / day, I can only use 500-wh.
    If you discharge that deeply each day your battery will be shot in a year. You do not want to go more than 20% in a day.

    So to answer your question, it cannot do much. At 200 WH per day it can power a laptop about 4 or 5 hours based on your battery restriction. However depending on your location a 200 watt panel deliver 350 to 500 watt hours in winter. So you have selected a mismatch in solar panel wattage and battery capacity.

    But I can tell you this, you are going about this all wrong. You need to determine how many watt hours you need in a day, then design the system to meet that objective.
    MSEE, PE

  3. #3

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    oooh - Sorry, I was under the impression that it's acceptable to discharge up to 50% and still retain 5-7 years of use. I suppose I'll need a larger battery bank to only discharge 20%.

    But I can tell you this, you are going about this all wrong. You need to determine how many watt hours you need in a day, then design the system to meet that objective.
    I completely agree. However, I am not out to be completely "off-grid". My limiting factor is the size of my solar panel. Then, I design the system to fit the limiting factor, and use it as much as I can, hence the question of what I could actually use a 200W panel, 1kwh battery bank system.

    I suppose I could double my panel size to 400W and rewire everything to 24V. I guess I'll be able to power TWO laptops then :-p. And with a battery bank of 2kwh (x2 12V 1kwh batteries), 20% of that capacity is merely 400W. Is my battery bank the limiting factor then?

    I've always thought that solar panels were the expensive, limiting factor - but it seems as though batteries are what sucks the wallet dry.

  4. #4
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    Yes, battery also become a limiting factor.
    It makes me begin to looking for an alternative, and i try to find micro water turbine powered by water from a tank.

  5. #5

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    Just did some calculations....

    I only use 60 kwh per month (about 2 kwh per day). My low energy consumption made me believe that I could potentially run "off-grid".

    But just calculating the battery bank size required was a complete turn off - I'll need 32 12V Type 27 batteries (~ 1kwh capacity each)!!

    Assuming 2 kwh consumption, I'll need 8 kwh capacity (25% Depth of Discharge (DoD) ).

    So 8kwh (8,000w) / 24V system = 333 Amp / day.

    x2 12V Type 27 batteries at 24 V with 25% DoD gives me 500w, or 20.8 Amp-hrs per day (500/24).

    333/20.8 = 16 24V systems (Or 32 Type 27 1kwh batteries).

    Is that correct?!

    It's hard to imagine the number of batteries a typical US home will require (avg usage of 20 kwh per DAY).

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    Quote Originally Posted by solarnoobie View Post
    I only use 60 kwh per month (about 2 kwh per day). My low energy consumption made me believe that I could potentially run "off-grid".



    Assuming 2 kwh consumption, I'll need 8 kwh capacity (25% Depth of Discharge (DoD) ).

    So 8kwh (8,000w) / 24V system = 333 Amp / day.

    x2 12V Type 27 batteries at 24 V with 25% DoD gives me 500w, or 20.8 Amp-hrs per day (500/24).

    333/20.8 = 16 24V systems (Or 32 Type 27 1kwh batteries).

    Is that correct?!
    No not really, you have not taken into account charge and discharge efficiency of the batteries. To find the battery AH capacity needed is really simple and straight forward. You need 3 pieces of iformation.

    Daily Watt Hour Usage
    Number of days reserve capacity
    Battery Voltage.

    Ok for the daily capacity you came up with 2000 wh or 2 Kwh. Now we need to adjust that or add in the fudge factor for charge/discharge efficiency. Multiply your daily usage by 1.5. So 1.5 x 2000 wh = 3000 wh or 3 Kwh.

    Next is number of days reserve capacity. That number will be 2.5 to 10 days. Then multiply that by 2 so as to never discharge more than 50%. So lets use the minimum of 2.5 days. 2.5 days x 2 = 5 days

    Last is to pick a battery system voltage lets use 12 and 24 to demonstrate the number of the cells is the same, just the AH changes, not the wh capacity.

    So now we have all we need to know. The formula is:

    (Daily adjusted WH x Number of Days) / Battery Voltage

    So at 12 volts = (3000 WH x 5 days) / 12 volts = 1250 Amp Hours

    At 24 volts = (3000 WH x 5 days) / 24 volts = 625 AH

    FWIW you would never use a BCI spec battery in most cases. You want to use batteries made for Renewable Energy that will last for 5 years with good care.
    MSEE, PE

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    Quote Originally Posted by solarnoobie View Post

    It's hard to imagine the number of batteries a typical US home will require (avg usage of 20 kwh per DAY).
    This is 1 reason why off-grid battery systems should NEVER be used, unless you live in a remote area where commercial power is not available.

    Here are two facts to consider.

    To go off grid, electricity will cost you 10 to 30 times more than buying it for the rest of your life. There are few rare exceptions if you use very little power and your utility charges you a large minimal usage fee every month. But for most folks that does not apply.

    There is no EROI on a battery system. That means you will never generate more energy than it takes to make the system. The reasons are simple; battery systems are extremely inefficient, and you have to replace the batteries every 5 years or so.
    MSEE, PE

  8. #8

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    Ugh -

    This is impossible for a small home owner.

    Battery option is obviously not feasible at this time.

    I wish I could just plug in my solar panels into my inverter and then the wall socket.

    WHEN will there be a UL rated 'plug-and-play' inverter?!

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    Enphase makes a <200w GT approved inverter, and comes attached to a panel. Still need approval, but it's very close to a plug-into-the-outlet solution.
    Since the dawn of time it has been mankind's dream to blot out the sun.
    Montgomery Burns

    "Always listen to experts. They'll tell you what can't be done and why. Then do it."

    spreadsheet based voltage drop calculator:
    http://www.solar-guppy.com/download/...calculator.zip
    http://www.solarpaneltalk.com/showth...oss-calculator

    http://www.mike-burgess.org/PVinfo_2.html

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,
    battery lugs http://tinyurl.com/LMR-BigLug
    Setting up batteries http://tinyurl.com/LMR-NiFe

    gear :
    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 || || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

  10. #10
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    Quote Originally Posted by solarnoobie View Post
    This is impossible for a small home owner.
    It does not matter what the scale is for battery systems, no one can justify them.

    Quote Originally Posted by solarnoobie View Post
    Battery option is obviously not feasible at this time.
    They will never likely ever be unless the minority government gets their way with Cap-n-tax legislation and makes conventional energy so expensive you have no choice. But that will not natter much, because if they do the USA will collapse and be ruined.
    MSEE, PE

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