PSU power consumption

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yaro137

If my PSU is 500W does it actually mean that when my computer is on it
takes 500W all the time?
yaro
 
yaro137 said:
If my PSU is 500W does it actually mean that when my computer is on it
takes 500W all the time?
yaro

No, it means it will provide up to 500 watts if necessary.
 
yaro137 said:
If my PSU is 500W does it actually mean that when my computer is on it
takes 500W all the time?
yaro

The power supply *transforms* the power, so only draws as much power
as is needed to power the load.

If you connect a 12V 1 amp light bulb to the output of the power
supply, the power supply in turn will draw 12 watts of power from
the AC wall outlet. The power supply is not 100% efficient, so in the
process of transforming the power, some is lost as heat. The
worst ATX power supplies might be 68% efficient. For the 68% efficient
supply, it will draw 12W / 0.68 = 17.6W

So if the power supply powers a 12W light bulb, it draws 17.6W from
the wall. As SC Tom points out, the "500W" is the overall limit,
as to how much power it is capable of transforming, without damage.
Above 500W loading, the power supply will shut off to protect itself.

A typical modern computer now, suitable for office work, might
have an internal load of 100W. With the 68% efficient supply,
that computer will draw 147W from the wall. 47W of heat will be
emitted by the power supply directly (which is why the power
supply has a fan on it for cooling). The other 100W of heat
is dissipated inside the computer cabinet, which is why the
computer cabinet must have a cooling fan to help it stay cool.

A good reality test of this, is to ask the question, "how much heat
is pouring out of my computer". Do you feel 500W of heat coming
out of the computer ? Or is the level of heat consistent with
150W of heating ? Reality tests like that, are great for
confirming any theories you might have. At least "feeling the
heat", doesn't cost you any money for test instruments.

You can buy a meter like this, to measure the consumption of the computer.
This is another way to confirm it is drawing 147W.

http://www.p3international.com/products/special/P4400/P4400-CE.html

The consumption of the computer changes with the activities the
computer is doing. If you start rendering a DVD movie with software,
the computer may draw 50W more than it would draw when idle. In the
idle state, the computer may have features to help drop the power
consumption. So when I give the "147W" example above, the actual
power changes from moment to moment, and maybe 147W would be a
long term average.

Paul
 
Might be of interest that the power-consumption figures given on most
consumer devices are wildly inaccurate. Usually on the high side.

A few tests with a wattmeter some years ago suggested that a typical desktop
system-unit might draw 150W, not including display. Laptops are typically
around 50W when mains-powered and maybe 30W when in low processor-speed mode
on battery. So if you want to save power, use a laptop.

As for the PSU, you can only draw 500W if all outputs are fully loaded. If
you drew 500W from (say) the 5V rail alone, you would cook it. At that, the
500W rating is an absolute max, and for a reasonable life expectancy you
should not expect it to run at that level continuously. A 500W (max draw) PSU
is suitable for an typical desktop, for a server or graphics workstation I'd
look at 750W or 1kW.
 
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