watts AC watts DC

conntaxman
conntaxman Solar Expert Posts: 125 ✭✭✭✭✭
in Solar Beginners Corner #1
For sizing up batteries,im puzzled on this.On a AC ele appliance and you take a Watts meter and will say you get 2 amps. Now on a battery 12vdc, you have 115Amp hrs. I read that you can draw 25amps from that battery for 1hour.So that run time would be 2.5 hr.Is that correct so far? Next how do i figure out for the AC stuff, from the battery. I was always thinking of divieding 115 by 2 amp = 57 hours. But I don't thind that is right anymore. HEEEEELLLLLLPPPP. LOL.
Is their a calculator on the net for this?
John
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: watts AC watts DC
    conntaxman wrote: »
    For sizing up batteries,im puzzled on this.On a AC ele appliance and you take a Watts meter and will say you get 2 amps. Now on a battery 12vdc, you have 115Amp hrs. I read that you can draw 25amps from that battery for 1hour.So that run time would be 2.5 hr.Is that correct so far? Next how do i figure out for the AC stuff, from the battery. I was always thinking of divieding 115 by 2 amp = 57 hours. But I don't thind that is right anymore. HEEEEELLLLLLPPPP. LOL.
    Is their a calculator on the net for this?
    John
    I am a little lost--But will take a try. Note, I am assuming that Power Factor is equal to 1.0 ... If not, it will change the answers below:
    • 2 amp * 120 vac * 1.0 PF = 240 watts AC
    • 240 watts * 1/0.85 inverter eff * 1/12 volt battery bank = 23.5 amps DC
    12 volt battery at 115 Amp*Hours should be operated around C/8 for max continuous discharge (you can do more--but really only recommended for short term loads):
    • 115 AH * 1/8 rate of discharge = 14.75 amps max continuous recommended
    Note that your battery is rated for 115 AH at 20 Hour Rating, your usage would be:
    • 115 AH / 23.5 A = 4.9 ~ 5 hour discharge rate
    So, for the sake of argument, your battery's 5 hour rating is only 100 AH. And since we recommend a maximum of 50% discharge for longer battery life (and to have a reserve as the battery ages):
    • 100 AH * 50% max discharge = 50 AH recommended discharge
    • 50 AH / 23.5 Amps = 2.1 Hours of Load recommended
    Remember that Power is equal to Amps * Voltage...

    120 VAC is 10x the voltage of a 12 volt battery bank. So, for the same wattage, a 12 volt battery bank will have to output 10x the 120 VAC current (i.e., 2 amps at 120 VAC is the same power as 20 amps at 12 VDC).

    Watts is sometimes less confusing--as you can have 240 Watts on your AC side or DC side--it is a "complete" description.

    When people talk about Amps--we always have to ask at what voltage are those amps measured.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #3
    Re: watts AC watts DC

    conntaxman - sounds like you are looking for a Puerkert calculator.

    www.smartgauge.co.uk/calcs/peukert.xls

    Info on what the Peukert effect is: http://www.smartgauge.co.uk/peukert2.html
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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  • SolaRAC
    SolaRAC Solar Expert Posts: 52 ✭✭✭✭
    #4
    Re: watts AC watts DC

    I was working on a way to ask this question.. I'm glad someone else brought it up. :D
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
    Re: watts AC watts DC

    I am not sure watt you are asking, and if Bills response is not what you are looking for, please clarify.

    Watts are watts are watts. Watts is the measure of power, watt hours is the power consumed, so 100 watts consumed in one hour is 100 wh, irrespective of voltage. a 100 watt bulb using 120vac will use the same wh as a 100 watt 12vdc bulb.

    In the former case, the same 100 watts will draw ~ .83 amps,, for an hour will draw .83 amp/hours. A 12 vdc bulb will draw 8.3 amps (assuming 12 volts even) and will draw 8.3 amp hours.

    To put a different twist however, when you are talking about batteries, the net net of it, is that batteries do not convert input power to output power 1 for 1. In fact, FLA batteries take ~ 1.2 ah of charge power to replace 1 ah of draw.

    So, a 100 ah, 12 volt battery could (for the sake of argument) provide 100 amps for an hour, or 1 amp for 100 hours. (and take 120 ah to charge) In the real world it is not that simple, for a number of reasons, some technical, others practical. For example, a 100 ah battery could never deliver 100 amps for very long, due to the fact that it just isn't big enough to deliver that much power. (sort of like horse power) A lawnmower engine might deliver x amount of work, but a semi truck will do it much faster.

    Additionally, one would never (I hope) draw a battery down 100% and expect it to recover very often.

    I hope this helps,, perhaps it makes it clear as mud,

    Tony

    PS. As an added note, when you convert (invert) DC to AC there are conversion loses, the size of which depends on the efficiency of the inverter and it's loading. A good inverter loaded nearly fully might convert in the 95% range. A cheap inverter, loaded with only a small load might have loses in the 50% range.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #6
    Re: watts AC watts DC

    I am guessing the 115 AH rated 12 vdc battery is a Marine battery.

    They have likely played games with the 115 AH rating using a lower discharge, for longer then 20 hours.

    Guessing it is about 105 AH's at 20 hour discharge (5.25 amps). 20 hours is a standard commonly used.

    At a 10 hour discharge (9 amps) it will yield about 90 AH's
    At a 5 hour discharge (15 amps) it will yield about 75 AH's
    At a 2.8 hour discharge (25 amps) it will yield about 70 AH's.

    This is full discharge down to about 10.5 vdc. You don't really want to discharge it that deep as it will degrade battery longevity.
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  • conntaxman
    conntaxman Solar Expert Posts: 125 ✭✭✭✭✭
    #7
    Re: watts AC watts DC

    hi RC.
    I read about the 25 amp draw some were on the internet.Not form the battery co. So it could be what you said. Im just getting into this figuring out how long stuff will run on dc vs. ac.And im sure learining alot form all the guys here. And thank them for all the ideas.
    John
    Its alot better learning for some one else's mistake.
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