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If I have a system running at ~500W of power draw, will there be any tangible difference in the outlet wattage draw between a 1200W power supply vs, say, a a 800W power supply? Does the wattage only imply the max available wattage to the system?

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One thing to keep in mind is that a PS that is MUCH larger than the system needs is not only inefficient but may not regulate as well. A power supply needs some minimum load to work properly. – Daniel R Hicks Jan 13 at 22:48

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The wattage of your power supply is what it could potentially supply. However, in practice the supply won't ever make that. I always count 60% of the capacity as the truly maximum capacity. Today however, there are also bronze, silver, gold, platinum power supplies which guarantee a certain amount (minimum of 80%) of efficiency. See this link for a summary of 80 PLUS labels.

Example: If your 1200W supply has a 80 PLUS label on it, it will supply probably 1200W but will consume 1500W. I think you 800W supply will be sufficient, but it won't guarantee you safety.

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It will supply at least 1200 Watt, not 960 Watt. If it is 80% efficient and supplying 1200 Watt then it will draw 1500 Watt from the wall socket. If a PSU fails to deliver its stated rating then that is fraud. – Hennes Nov 23 '12 at 7:49
@Hennes your right, I was still sleeping I guess :)... Edited my post – Mixxiphoid Nov 23 '12 at 8:12
@Hennes - "It will supply at least 1200 Watt" - No, a PSU will not supply "at least" its rating. The PSU is capable at most of delivering 1200 watts. The rating is often bogus, since it is a sum (often rounded up) of the rails measured independently, rather than simultaneously. Mixxiphoid - You corrected your arithmetic, but the first paragraph still misconstrues the meaning of 80Plus certification, which is for efficiency, not power levels. – sawdust Nov 23 '12 at 9:45
I should have written "Capable of delivering at least" – Hennes Nov 23 '12 at 11:08
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The wattage implies the maximum available wattage to the system.

However note that the PSU draws AC power from the wall socket, converts it to some other DC voltages, and provides those to your system. There is some loss during this conversion. How much depends on the quality of your PSU and on how much power you draw from it.

Almost any PSU is very inefficient when you draw less then 20% of max rated power from it.
Almost any PSU has less than peak efficiency when you draw close to the max rated power from it.
Almost any PSU has their optimum efficiency around 40% to 60% of maximum load.

Thus if you get a PSU which is 'just large enough' or 'way to big' it is likely to be less efficient.
[But note that your PC does not consume a fixed or constant level of power. At idle, when not much is happening, the DC power consumed will be low. Perform a lot of processing and I/O operations, then power demand goes high.]

A nice example of areal world efficiency graph is this:

enter image description here

will there be any tangible difference in the outlet wattage draw between a 1200W power supply vs, say, a a 800W power supply?

The 800 Watt PSU would run at 62.5% of max rating. That is a good value.
The 1200 Watt PSU would run at only 41% of its maximum rating. That is still within the normally accepted range, but at the low end. If your system is not going to change than the 800 Watt PSU is the better choice.

Note that even with a good (bronze+ or silver rated PSU) you are still loosing about 15% during conversion. 15% of 500 Watt means that your computer would use 500 Watt, but the PSU would draw 588 Watt from the wall socket.

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How ironic, you correct Mixxiphoid's arithmetic, but use bad arithmetic in your answer. A 15% inefficiency when delivering 500 Watts of DC power implies that the PSU consumes 588 Watts of AC power from the wall outlet. 588W * (100% - 15%) = 500W – sawdust Nov 23 '12 at 9:54
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For the most informed reading about PSUs go to this site and especially read the article "Firecracker or Power Plant" Very informative even for the lay person.

http://www.tomshardware.com/reviews/power-supply-oem-manufacturer,2913.html

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Welcome to Super User! Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. – Daniel Beck♦ Jan 13 at 21:17
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manufacturers of electrical power supplies often indicate the input power (the power which it consumes) because it is the higher value and can be marketed better. you should check if they also indicate the output power to be on the safe side.

regarding efficiency there is a big difference based on the technology used. the efficiency is calculated as efficiency = output power / input power.

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