Battery power/ inverter(s) running computers. HELP!

PrpleButerflyPrpleButerfly Registered Users Posts: 1

I am trying to run a laptop, computer monitor, and a LCD TV for a full day. I have a 29 DC marine battery and 2 410 W inverters. I don't know how to calculate for loss that is being pulled directly from the inverters themselves and how to calculate for other battery losses. The TV is 1.1 amp, the computer is 1.6 amp, and the monitor draws 1.5 amps.

I am just at a loss as to how to figure out if my battery/inverter setup will run all of this for at least 12 hours straight. Thank you for your help in advance!


  • BB.BB. Super Moderators Posts: 26,951 admin
    Welcome to the forum PrpleButerfly.

    Lots of questions... Hang on for a bit of math. Using your numbers:

    TV: 1.1 amps * 120 VAC = 120 Watts
    Computer: 1.6 amps * 120 volts = 192 Watts
    Monitor: 1.5 amps * 120 volts = 180 Watts
    492 Watts total of 120 VAC Power
    4.1 amps @ 120 VAC

    First problem, if this is reading from the labels on the back of the units--They can be dramatically different vs real life measurements. You really need to make your own measurements under your operating conditions.

    One easy way to do this is get a Kill-a-Watt or similar type meter (local hardware store, sometimes even a library will have them for check out).

    Next--Picking Energy Star rated devices, Modern LED TV's can use 1/3 or less energy. And a modern laptop can use 20-30 Watts (computer+monitor) vs the 120 Watts (possible) for a desk top computer.

    Some quick estimates of the power usage math--First, what is "all day for you"? I will use 10 hours--But that is another choice you will have to make.
    • 492 Watts total * 1/0.85 AC inverter eff * 1/12 volt battery bank * 10 hours usage = 482 AH @ 12 volt
    Your battery is (I guess) around 125 Amp*Hours @ 12 volts--And for lead acid batteries, you would normally not discharge a deep cycle battery by more than 50% (for longer battery life).
    • Suggested battery: 482 AH * 1/0.50 max discharge = 964 AH @ 12 volt nominal battery bank
    • Your battery: 964 AH / 125 AH your battery = 7.7 ~ 8 of your batteries
    We do a lot of the calculations for power and energy here in "Watts" and Watt*Hours vs Amps and Amp*Hours.... Watts is a "total energy unit vs Amp*Hours--Which is "missing" the voltage for energy calculations.

    For example, your devices draw ~4.1 amps at 120 VAC... But draw ~41 amps (or a bit more) at ~12 volts DC.

    Before I start throwing a bunch of math at you--Let me know how you would like to proceed. As you can see, even a "nominal" amount of energy usage turns into a big/expensive battery bank pretty quicly--And we have not even discussed how to properly recharge such a battery bank (or even how to choose the "right" batteries for your project--Which does vary based on the size of your project).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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