I am looking at converting a computer to run off of a 12V battery, which means replacing its PSU with a DC-DC converter.

So, assuming that the computer is currently using 600 Watt PSU, then I assume that rating applies to the INPUT side of the PSU, which is a 120V. So, that would mean its max rated load is approximately 600W/120V = 5 Amps. Is that right?

If the DC-DC converter takes a 12V input, then it would seem the equivalent the rated load for the converter would be 5 Amps x 12 Volts = 60W. So, I would apparently need only a 60 Watt DC-DC converter.

However, when I look at typical catalog listings of DC-DC ATX converters, they seem to offer WAY more power than this. For example, if you look at PowerStream's DC-DC converters, the smallest one they have is listed as 320W and seems to have the same form factor as a typical small AC-DC ATX power supply. That does not really seem possible. So, is PowerStream listing their converter wattage as a 120V equivalent, not as a true 12V wattage? In other words is their "320W" PSU actually a 32 Watt PSU, and they are just calling it 320W to imply that it can replace a 320W 120V AC-based PSU?

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    \$\begingroup\$ Are you trolling or actually asking? ”So, I would apparently need only a 60 Watt DC-DC converter.” That would violate the laws of physics. To replace a 600 W whatever input voltage PSU will require a 600 W 12 V PSU. \$\endgroup\$ – winny Nov 1 '20 at 8:50
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    \$\begingroup\$ Does this answer your question? Choosing power supply, how to get the voltage and current ratings? \$\endgroup\$ – winny Nov 1 '20 at 8:52
  • \$\begingroup\$ A switch mode computer power supply isn't some linear regulator. current in != current out. 600W in means 510W out, assuming a reasonable 85% efficiency. If that is all 12V it would make over 42A. \$\endgroup\$ – Unimportant Nov 1 '20 at 8:55

A power supply converts power from some voltage to another, so regardless of input voltage, the input power does not change (assuming there are no conversion losses).

If the PC needs 600W, it needs 600W from 120VAC and 600W from 12VDC.

600W / 120VAC = 5A 600W / 12VDC = 50A

  • \$\begingroup\$ Oh, ok, so the amps changes, but the wattage does not. I had it reversed. Thanks. \$\endgroup\$ – Tyler Durden Nov 1 '20 at 9:31

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