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Thread: Help understanding Watts and Amp hrs..

  1. #1

    Default Help understanding Watts and Amp hrs..

    I've been googling around and can't seem to find some answers I need to learn. also ordered a kil-o-watt the calculate my wattage later too.

    I'm trying to figure out a few things here. and here they are.

    Can't seem to grasp this watts to amp hrs thing to easy. say I have a 200 ah battery thats (dod just for example). I have one 200 watt panel that puts out say, 19 volts and 11 amps. also say I have 5 hours of good clear sun on a given day, knowing I'll need a charge controller for the battery.

    So in a perfect world it would pump 1000 watts out to the battery in 5 hrs? Also say there is no draw at all, just charging the battery bank.

    Am I thinking right that since this battery is putting out 11 amps, that It would take it around 4 days just to charge the (Dod) 200ah battery @ 5 hrs per day?

    Also , does It make more since to buy 2 100 watt panels than one 200?

    thanks..

  2. #2

    Default Re: Help understanding Watts and Amp hrs..

    What voltage is the battery?

    Watts = Volts x Amps
    Old house - 10 Evergreen 205W panels, Xantrex 4548, XW charge controller, 8 Deka 8L16 batteries (48V 370AH). Trimetric battery monitor. Off grid since June 09 added Grid Support May 2011.
    Current house - only 85 gallons of solar hot water and a Honda EU2000i generator.

  3. #3

    Default Re: Help understanding Watts and Amp hrs..

    Quote Originally Posted by jeffkruse View Post
    What voltage is the battery?

    Watts = Volts x Amps
    Sorry, I knew I left something out....It will be a 12 volt.

  4. #4
    Join Date
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    pittsburgh, pa
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    10,239

    Default Re: Help understanding Watts and Amp hrs..

    the answers are here on the forum and here they are again in bold just for you.

    Quote Originally Posted by Sparky View Post
    I've been googling around and can't seem to find some answers I need to learn. also ordered a kil-o-watt the calculate my wattage later too.

    I'm trying to figure out a few things here. and here they are.

    Can't seem to grasp this watts to amp hrs thing to easy. say I have a 200 ah battery thats (dod just for example). I have one 200 watt panel that puts out say, 19 volts and 11 amps. also say I have 5 hours of good clear sun on a given day, knowing I'll need a charge controller for the battery.

    watts is volts x amps
    watt hours (wh) is volts x amp hours (ah)
    amp hours is amps in reference to 1 hour of time.
    examples; 200 amps for 1 hr = 200ah,
    1 amp for 200hrs = 200ah,
    50 amps for 4 hrs = 200ah
    dod is depth of discharge and is a percentage rating of how far discharged the battery is.
    0% dod is full and 100% dod is dead. if your battery has a capacity of say 220ah and is 50% dod then .5 x 220ah = 110ah
    state of charge (soc) is the opposite of dod and indicates the % of capacity in the battery.
    100% soc is full and 0% soc is dead.


    So in a perfect world it would pump 1000 watts out to the battery in 5 hrs? Also say there is no draw at all, just charging the battery bank.

    yes, in a perfect world a 200w pv could give 1000w in 5hrs if under a solar intensity of 1000w/^2, but pv ratings are at 25 degrees c and pvs get much hotter and they get derated, plus many other losses and efficiency factors so forget perfect.

    Am I thinking right that since this battery is putting out 11 amps, that It would take it around 4 days just to charge the (Dod) 200ah battery @ 5 hrs per day?

    you are confused in using the terminology as the pv would be outputting the charge current in amps to the battery and i already explained what dod is. now if you are charging a dead 200ah battery, which would be 100% dod, then 4 days at 5hrs per day would be 20hrs of charge time at your specified delivered current rate of 11a and would be 220ah and is 20ah over the charge point in your perfect world. in reality, it would taper the charge current some near the end for a longer timeperiod.

    Also , does It make more since to buy 2 100 watt panels than one 200?

    it soesn't matter unless the 2 100w pvs would cost more than the 200w pv or that the 200w pv is larger in size and the space to mount it in isn't configurable somehow to the 200w pv. an wxample may be for a boat or an rv.

    thanks..
    NIEL

  5. #5

    Default Re: Help understanding Watts and Amp hrs..

    Quote Originally Posted by niel View Post
    the answers are here on the forum and here they are again in bold just for you.
    Thank you. I'm not trying to be lazy and have everyone do my work for me. I really do appreciate all of everyone's input.

    I did a search on it here and couldn't find what I thought I wanted.

    After I made my first thread, I've realized I have a ton to learn and don't want to keep making threads day after day....

    And I was going to use a 975 watt coffee pot too..

  6. #6
    Join Date
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    Default Re: Help understanding Watts and Amp hrs..

    no problem as i didn't make you search for it all, but it was only a reminder to all that it is out there. i admit that it is allot for somebody new to it to grasp.
    NIEL

  7. #7

    Default Re: Help understanding Watts and Amp hrs..

    Quote Originally Posted by niel View Post
    no problem as i didn't make you search for it all, but it was only a reminder to all that it is out there. i admit that it is allot for somebody new to it to grasp.
    I've copied your post to notepad. It will be of a big help to me. I had two choices on my thread, either make one a get the answer or my have wife strangle me for sitting at the PC for countless hours. I got the answer and saved my Marriage too in one thread.


    EDIT......Also, sorry I meant 11 amps that the PV panel was putting out, not the battery...

  8. #8
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    Default Re: Help understanding Watts and Amp hrs..

    Just to be clear in your first post... you typed about 1,000 watts (5 hours * 200 watts). That should be 1,000 Watt*Hours instead.

    Watts is like Miles per hour or Gallons per hour--it is a rate. When you run the load/generator for 5 hours... then it becomes an "amount" (Watt*Hours, Gallons, etc.).

    Watt*Hours=200 watts * 5 hours = 1,000 Watt*Hours = 1.0 kWatt*Hours

    Note that for many battery system designs, they talk about Amps and Amp*Hours instead of Watts and Watt*Hours.

    The conversion is Amp*Voltage=Watts; Amp*Hours*Voltage=Watt*Hours; etc.

    -Bill

  9. #9
    ZipSnipe Guest

    Default Re: Help understanding Watts and Amp hrs..

    Hello all newbie here!!!

    So let me see if I got this right, if I have a battery at 12v 51ah which should = 612watts and if I want to run 250 watts of power basically I have about 2 hours worth give or take?

    Also Sears has a battery I'm looking at it say Amp hours at 20hr rate is 115, does this mean its 115ah?
    Last edited by ZipSnipe; June 26th, 2009 at 6:31 PDT.

  10. #10
    Join Date
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    Default Re: Help understanding Watts and Amp hrs..

    To be clear:

    12v * 51 Amp*Hours = 612 Watt*Hours (0.6 kWHrs)

    612 WH / 250 W = 2.448 Hours run time

    However, there are some limits to that calculations.

    Do not run the battery to dead. It will not last very long at all (a few weeks at most). Typically, discharging to 50% capacity will give a longer battery life.

    Batteries are not perfect energy storage devices. They have internal resistance and limits to their chemical processes. Normally, battery capacity is rated at a "20 Hour Rate". So--if a battery is listed as having a capacity of 51 Amp*Hours at a 20 hour rate, it may have a capacity of 35 Amp*Hours at a 2 Hour discharge rate.

    So, putting the above into an equation:

    12 volts * 35 Amp*Hours * 50% (for longer battery life) = 262.5 Watt*Hours of usable capacity

    262.5 Watt*Hours / 250 Watts = 1.05 Hours of useful load

    There are other limitations too... Such as batteries are temperature sensitive.

    • A very cold battery will have much less capacity vs a 77F battery.
    • A very hot (100F) battery will "age" faster than a very cold battery.
    • As a battery discharges, the electrolyte specific gravity falls (Acid+water becomes mostly water). A fully charged battery will not freeze until -90 F. A 40% state of charge battery will freeze around 16F. A 0% state of charge battery will freeze at ~32F.
    • Hot batteries (100F+) self discharge very quickly. Cold batteries (32F and below) will self discharge much slower.

    A couple of battery FAQs to read:

    www.batteryfaq.org
    Deep Cycle Battery FAQ

    -Bill

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