Amp hours confusion

steel
steel Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
I'm trying to comprehend amp hours in a battery bank configuratoin.

Example Trojan T-105 225AH @ 6V

so formula A*V = W 1350 watt hours

Tie two trojans together into a 12v system you get 225a*12v = 2700 watt hours

I have a 120v appliance that draws 0.3 kw per day as measured on my kill-a-watt

is that 300 watts/24 hours = 12.5 watt/hr?

So two trojans in 12v system could power this appliance for 216 hours assuming inverter (100% efficiency)?
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Comments

  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #2
    Re: Amp hours confusion

    2x 6v batteries in series of 225 ahr each =12v 225 ahr
    an inverter at best over its usual working range is about 85% effecient
    MSW a bit more than true sine wave.
    The most you could draw from the batteries unless you want to give them a very short unhappy life is 100ahr so 100x12 = 1200whr
    your device draws 12.5 watts per hr your inverter will have a self use of about 6 watts per hour also total including all losses will be about 22watts per hr
    1200 divide by 22 =54 hrs run time for that device on the 2 batteries
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #3
    Re: Amp hours confusion

    First of all, there is no such thing as "watts per hour" or "amps per hour". That is like saying 40 miles per hour per hour. http://www.homepower.com/view/?file=HP88_pg106_WordPower

    As far as the original quesion goes, if you have 225 AH of battery @ 12 volts, that is 2700 watt-hours. Your appliance consumes 300 watt-hours per day, so roughly it could run for 9 days - under perfect conditions.

    However - Inverters, especially when used at a small percentage of their capacity are not very efficient. If you are running a 15 watt item from a 1000 watt inverter, your efficiency is probably more like 50% at best.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #4
    Re: Amp hours confusion

    Windsun you have it wrong..As far as the original quesion goes, if you have 225 AH of battery @ 12 volts, that is 2700 watt-hours
    He has 2 x6v batteries rated at 225ah each

    His appliance uses 300whrs a day plus the inverter will self use another 140whr a day
    total 440whr
    battery1350 divide by 440 = about 3 days at best.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #5
    Re: Amp hours confusion

    JohnP,

    Sorry, but you are wrong,

    225ahX12v=2700wh

    If you take your numbers, 2700wh capacity, and divide 440 wh/day of load (Load plus inverter loss) you end up with 6.13 days if you (could) draw the battery down 100%.

    Windsun number is correct, without taking inverter losses into account. 2700/300=9

    Tony
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  • steel
    steel Registered Users Posts: 5 ✭
    #6
    Re: Amp hours confusion

    I really just wanted to see if I had the theory/formulas correct. Looks like I did but it is still hard to figure. I guess in practice it becomes easier. I kind of knew there'd be loss thru inverter inefficiencies but didn't know the load was that high. Lots to learn about this stuff before I get serious about spending money on things.

    Thanks guys!
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