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So a 500w PSU will always use 500w or just what other computer's components demand?

I myself thought it would be the latter. Can anyone confirm this?

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marked as duplicate by Hennes, Tog, Carl B, Mokubai, Randolph West Oct 2 '13 at 21:25

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The latter. Sort of. Any "Energy Star" power supply will have a very low "standby" power draw and will largely "pull" power from the electrical outlet in proportion to the power is must "push" into the system. But no PSU is 100% efficient. –  Daniel R Hicks Oct 1 '13 at 3:18
    

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A computer Power supply is converting energy for the use of it at specific voltages for the computer. This is much the same way that other ac power converters for DC devices work.

The more the computer or device draws in power, the more power is converted to use. Therefore a 500W power supply while capable of supplying up to 500W total, will usually not be using all 500W.

There is no reason to believe any differently based on the curcuitry and the fact that there is nothing in them to Waste all this unused power. There is every reason to understand that if it did use 500W always there would be much more heat comming out of the unit as it wasted power up to the 500W.
I have also tested both with ampmeter and voltmeter testing, plus readouts on UPS units, and have almost all my electronics marked with the power they consume per hour, to keep track of it. I can confirm that many different power supplies for computers and other devices , use power more or less on a Demand basis.

There is an ammount of power they use when just being on, and the computer is "off", there is an ammount of waste , which varies on the efficency of the power supply.

A computer that is set up proper with only needing a 500W power supply can be using a very small ammount of total power, when the computer is mostly idle. An Ex: ~100-150W

Notes: Some 500W power supplys are so bad, they cannot deliver the 500W anyway. Some are so good they can deliver more than that, and some of course are rated for the marked capability.
Some are way more efficient than others, and there is an efficency curve for even the most efficent, so the ammount of waste percentage varies with the ammount consumed.

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It should be noted that way, way, way back power supplies of the type used in computers were pretty inefficient, since they used a "series regulator" that would "burn off" a lot of power to regulate the voltage. But since the late 70s most computer power supplies have been "switching" supplies that regulate voltage by turning power off and on at a very high rate. If well-designed, these can be incredibly efficient. –  Daniel R Hicks Oct 6 '13 at 12:25

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