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If one bought a 700W power supply, just for the sake of argument, would it simply convert 700W of power from 120V AC to DC or is this dependent on demand? I have a couple of puny (<300W) PSs on my desktops and see no reason to UG despite having video cards and 2 HDs in each as I leave them on and don't want to use more power than I have to.
 

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PSU converts only the power that is required.
So if you're using 10% of the rated output, it's only generating 10% of the rated output. (though you will be using more than the calculated number of watts due to inefficiency in conversion)

You should size a PSU to roughly 30-70% of its output in power requirements
This is because the middle is generally the 'sweet spot' or most efficient.

E.g. a 750W PSU powering a 150W computer may only be 70% efficient (20% load) (at this efficiency the computer is using ~215W
While a 300W PSU powering a 150W computer may be 90% efficient (50% load)
That same computer with a properly sized PSU is using ~167W.

The efficiency curve is basically parabolic - low utilization and high utilization is the least efficient. If your 300W PSU is being used at max, it may be worth upgrading.
Not only for a reduced power bill, but a quieter and cooler system. And most likely longer life of the PSU as it isn't being stressed 24/7.


I'd recommend looking up the TDP for each component and adding them up. You can then use that value to determine how large of a PSU you really need.
There are calculators on the internet to do this as well.
 
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