If your PC has a 500-watt power supply, the amount of power it actually draws from an electrical outlet may be higher. Learn how PC power supply efficiency impacts electricity costs and how you can lower your energy bill with Energy Star products.
Low Energy Computing: How to Save Electricity
The efficiency of a power supply determines how much energy is converted from the wall outlet power to the internal power components. For example, a power supply with 75 percent efficiency that generates 300W of power internally would draw approximately 400W of power from the wall outlet.
When you plug your computer into a wall outlet, the power does not flow directly to the components inside the computer. The electrical circuits and chips operate at lower voltages than the power coming from the wall outlet. Therefore, the power supply must convert the incoming 110 or 220 volts to 3.3, 5, and 12 volts for the various internal circuits. The power supply must do this reliably and within certain tolerances to avoid damaging the device.
Changing voltages requires various circuits that lose energy as it is converted. This energy loss is generally transferred to the power supply as heat, which is why most power supplies have fans to cool the components.
