Is power in equal to power out (approximately)?

[les45]

OK, based on a prior related post, I've about decided to use a computer power supply for my cabin DC requirements. Now I'm trying to balance my AC loads. I'm looking at a 650W power supply that will produce 12 volts DC at 28 amps. Will my AC draw for this power supply be approximately equal to the DC power output. Power out = 12VDC times 28 amps = 336 watts; AC draw = 336 watts / 120 volts AC = 2.8 amps (?). Is this in the ball park?

[Jim.Johnson]

Yes it's in the ballpark. There will be losses in the power supply and other circuitry.

[MtnDon]

Depends on the unit. Intel’s ATX specification only requires that a power supply is 60% efficient at 50% load. Most decent quality power supplies made in the last decade are around 70% efficient at 50% load. If the unit in question is an "80 Plus Gold" model it would have 80% or better efficiency at 20%, 50% and 100% capacity. Big difference in pricing too.

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In the field of battery chargers there are many that waste up to 2/3 of the input energy due to cheap design. California has adopted rules that will require all battery chargers to become at least 90% efficient.

[noseoil]

It's close, but there's a "fudge factor" which should be taken into account here. With any systems, you need to add a percentage for loss, slop, inefficiency, etc. An efficient inverter draws the load, plus whatever energy it takes to make it run (between 10% & 20% depending on the type). Any time you are figuring on wires, loads, current, use, etc. I would add about 25% for the "slop factor" in the system. Always better to have it & not need it than need it and not have it.

Conversions from one voltage to another always seem to draw a bit more than we think.