If I have my computer running on a 120 volt power inverter connected to a 12 volt power supply, how do I calculate the draw on the 12 volt system based on a 12 volt current?
Do the amps stay the same? Do the watts stay the same? Do they both change...?
Readings from a "normal" in house outlet: 120.5 volts / 3.90 amps / 311 watts
12 volts / ? / ?
I assume the amps are * 10 so 3.90 would become 39.0 amps (12 volts / 39.0 amps / 311 watts?)
Or would this all change based on the type of inverter I use?
The power (watts) for your device is what stays constant. P=V*I; P is directly porportional to the voltage; so, if the voltage is reduced by a factor of ten the amps (I) have to increase by a factor of ten. I didn't understand where you got the 311 watt number 120.5 * 3.9 is 469.95 watts.
Practically, there are always losses when doing conversion from one system to another. This is a very heavy current draw on a 12 volt source. This is probably about a third of what a cars starter motor draws when starting your car.
voltage inverters are usually consume very high amperage (look how heavy they are).
about 120*3.9 =? 311 :
some products have ticket on the back, with the manufacturer power consumption parameters, usually the params are true for new device, with the minimal configuration.
thus, if your computer is quite old,
OR ,
you connected new cards to it (graphic card), or any UBS devices, network card etc...
even some PC gives currnt to the screen, all these changes , increase the power consumption of the PC.
basically , reducing 2nd order effects, 120*3.9 = P1 ~~ 12*39 = P2 , and yes, P1 should be quite similar to P2 (P2 would be probably from 0.9*P1 to 1.1*P2)
With a perfectly efficient converter, 120.5 volts * 2.58 amps = 311 watts in would equal 12 volts * 25.9 amps = 311 watts out.
In practice there will be some losses in your converter.
Another complication is that the input is probably alternating current, so 120.5 volts might be a RMS average, whereas the 12 volts output should be constant.
The power drawn by your computer will vary depending on what you're doing, I'm not sure whether your 311 watts is the average or peak wattage needed by your computer.
Even with the computer drawing a constant current, the 3.90 amps input would probably vary with the phase of the 120.5 volt supply, If 3.90 amps is also is a RMS average, the average input wattage could be anywhere beween +469.95 watts and 469.95 watts, depennding on the relative phase beween the voltage and the current.
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The basic fact is that the power (V x I) into a converter = the power out of the converter. Thus your calculations are correct (baring misprints).

Some comments.
40 amps is a LARGE curret to draw from a car battery.
P2(out) will never be less than P1(in)
A very good efficiency for a power converter is 80%. Thus P2 = 1.3 P1.
In general practice alternating voltage and current are quoted in RMS not peak with RMS being the DC equivalent.
The phase between voltage and current is usually taken as zero.
Measuring watts of AC is a tricky thing you can't just multiply the average amps times the average volts. that's so because in AC both the voltage and the current are varying, and with usually some phase angle between the two, which bollixes things up.
this is especially true of PC power supplies, which have a big capacitor in them. Capacitors draw maximum current when the voltage is crossing zero, and minium when it's at the peak, so the voltage and current peaks are wildly out of phase with each other. So if you multiply the voltage times the current you get a very misleading number of watts.
The correct way to measure watts is with a "true RMS" wattmeter. This is a device which either through oldfashioned analog, or newer digital sampliing, does the right thing makes many instantaneous products of voltage and current and displays the integrated result.
In any case, we can fall back on the Law of Conservation of Energy to calculate how well a car battery can power a PC. Let's assume your PC motherboard is drawing 300 watts. The power supply in the PC is probably about 80% efficient, so the draw from the power line to supply 300 watts is about 375 watts ( 375 * 0.8 = 300). The 12V to 120V inverter is likely to be about the same efficiency, so the input to the inverter has to be about 470 watts ( 470 * 0.8 = ~375). At 12 volts that's about 40 amps. A typical car battery has a capacity of around 50 amphours to full discharge, so the car battery can run your PC for about an hour.
There's worse news car batteries are optimized for very shallow dischargecharge cycles. They work best and longest if you don't tap more than a small percentage of the 50 amphours between charges. If you run them to full discharge, they're likely to wear out very quickly. You may only get a few dozen chargedischarge cycles before the battery refuses to hold much of a charge.
So you shouldnt use a regular car battery to power your PC. There are specially made batteries for jsut this puropse they're called "deepcycle" batteries, typically used in UPS's, golf carts, and floorscrubbers. They cost quite a bit more than a car battery though.
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Practically, there are always losses when doing conversion from one system to another. This is a very heavy current draw on a 12 volt source. This is probably about a third of what a cars starter motor draws when starting your car.
Hope that helps, dhs