How much electricity will a power supply draw?

Question

I'm in the early planning stages of building my first PC. I know in terms of making the computer work, too much power is better than too little. But in terms of energy efficiency, is the power supply going to constantly draw its maximum wattage from the power outlet, or just what it needs at any given time to power the various hardware components up to its maximum?

Smokies Hiker · Answer

The power supply will only draw the watts needed by the components you have installed. A 650 watt power supply may only be drawing a maximum of 500 watts if that's the total watts the components could draw.

Adrian · Answer

A PSU will only draw from the wall what power it needs to run the components, plus any efficiency loss.
A 750W PSU for example, can provide 750W to the internal components. If it is 80% efficient, it will draw about 950W (937.5W) from the wall when running full 750W loads.
However, most computer desktop internals may only use 350-400W or maybe even 500W. Say your system uses 480W internally, your 750W PSU, 80% efficient, will thus draw 600W from the wall, lose 20% to internal heat/inefficiency and provide that 480W to the system components.
All PSU are rated for what they can supply to the system, not what they take from the wall outlet (in general). Efficiency for cheap PSU can be 70% or lower. Better PSU can reach 90% efficiency. The more efficient, the less power is wasted in the PSU itself (mostly heat)

steve_loir · Answer

Modern switched mode power supplies are very efficient (70% or more) and they also vary the amount of power they consume with the amount of power they supply.

Izzy · Answer

however many watts it says on the box

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How much electricity will a power supply draw?

4 Answers