How Does Power Consumption Compare Between AC and DC in a Single Device?

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Discussion Overview

The discussion revolves around the comparison of power consumption between AC and DC in a single device, specifically focusing on how to calculate the current draw when using a power inverter to convert DC to AC. Participants explore the implications of efficiency in inverters and the theoretical calculations involved in determining power draw from a 12V DC battery compared to a 120V AC outlet.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant asks about the correlation between AC and DC power, specifically how to calculate the current draw on a 12V DC system when a device draws 500W from a 120V AC source.
  • Another participant provides a calculation indicating that a 12V system with a maximum of 50A could deliver up to 600W, but notes potential limitations in battery performance.
  • A third participant agrees with the initial calculations but emphasizes the importance of considering inverter efficiency, suggesting that if the inverter is 70% efficient, the current draw would increase to 59.4A.
  • There is a hypothetical discussion about what the numbers would be if everything were 100% efficient, questioning whether the calculations would yield consistent results.
  • One participant confirms that the calculations are straightforward but warns that real-world conditions may significantly affect the outcomes.

Areas of Agreement / Disagreement

Participants generally agree on the basic calculations involved in determining power draw but express uncertainty regarding the efficiency of inverters and the actual performance of batteries. There is no consensus on the typical efficiency values for inverters or how they affect the calculations.

Contextual Notes

Limitations include assumptions about inverter efficiency, potential discrepancies in actual voltage output from the inverter, and the performance characteristics of the battery under load.

pmpstyles
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Hello, I just came across this site when looking for some info. Not sure if this is a stupid question but I would like to know what the correlation between AC and DC power is when comparing one device. For example if i had a a device connected to 120vac and it drew 500w then the amps would be 4.16. So if i use that same device and connect it to a power inverter on a 12vdc system, it would draw that same power through the inverter, but what would be the draw on the 12vdc side? Is it just simply 500w/12v=41.6amps. I am basically trying to figure how much AC power can be drawn from a 12vdc battery where 50a is the max current the can go through a dc meter. Thank you
 
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12 V x 50 A = 600 W. An upper limit (some losses in the DC-AC converter)
But my guess is the battery can't deliver that for very long ...
 
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You are quite correct in your calculation of the power needed, but, as BvU indicates, the inverter is not 100% efficient. So you need to divide your calculated result by the efficiency.
So if the inverter were 70% efficient, the current draw would be 41.6 / 0.70 = 59.4 A.
But I don't know what a typical efficiency would be. (I have found a reference that suggests it can be in excess of 90%, which would mean 41.6/0.90=46.2A)

Also, the inverter might not deliver the 120V AC you expect. If it delivers less, then your device might not draw its full 500W, then the 12V current could be less.
 
Sorry, I was just going hypothetically. If everything was 100% efficient and a device plugged into a home outlet drew 500w over 120v, then the amps would be 4.16. In theory, if the same device ran 500w over 120v on a 12v battery. What are the numbers for the voltage, amps, and watts coming from the battery and going to the inverter? Is it simply calculating the amps because we know the voltage of the battery (12v), we know that the device used 500w over A/C, then would that give the answer or is it some random number? Hopefully that makes sense.
 
pmpstyles said:
Is it simply calculating the amps because we know the voltage of the battery (12v), we know that the device used 500w over A/C, then would that give the answer

yes, as stated in your OP

but remember, as you have been told, it's a long way from the real world, so don't try and use
the figures to produce good infoDave
 

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