Does a 500 watt power supply always use 500 watts of electricity? [closed]

No. A 500 Watt Power Supply can DELIVER 500 Watts, but it will ever use only as much as the components in your PC need (and of course that depends on Load and Activity, if Energy Savings Mechanisms like AMD's Cool'n'Quiet or Intel's SpeedStep is enabled etc.).

In Theory, with a 100% efficiency rating, which is impossible.

The usual Efficiency rating lies around 80%, but it can vary greatly between low quality and proper power supplies.

So with 80% efficiency, your power supply will use as much power as your components need and then about 20% extra.

Another caveat: Optimal efficiency is only reached at a "proper" load. If you have a 500 Watt Power Supply but then a super-low-consumption PC that only consumes 80 Watt, you're not going to reach 80% efficiency and could easily use ~120 Watt (~50% efficiency).

Due to the ~80% efficiency, you can also not use 500 Watt out of a 500 Watt Power Supply.

Those numbers are all estimates, as PSEs vary greatly, but a rule of thumb is that you should get a PSU with at least 80% Efficiency and get one that is not too big (but not too small either) for your PC.

No it doesn't. It just uses the power it needs plus a little overhead so the voltage doesn't go down if you plug something in there...

Just buy a power meter.. they cost 20 bucks and are always good to have around.. or borrow one from your neighbor, work, parents, friends etc..

No the power supply meets the load up to its rated wattage plus some overhead.

Energy Star computers power supplies must meet the following standard: Internal power supplies: 80% minimum efficiency at 20%, 50%, and 100% of rated output and minimum Power Factor 0.9

If you are purchasing a separate power supply and looking for a more efficient one look for the 80 Plus logo on the power supply.

What is the 80 PLUS specification?

The 80 PLUS performance specification requires multi-output power supplies in computers and servers to be 80% or greater energy efficient at 20%, 50% and 100% of rated load with a true power factor of 0.9 or greater. This makes an 80 PLUS certified power supply substantially more efficient than typical power supplies and creates a unique market differentiation opportunity for power supply and computer manufacturers.

http://www.80plus.org/

Nah, I have a 750 W power supply, and I've never seen my wattage go above 200.

Absolutely not. See http://www.codinghorror.com/blog/archives/000353.html For good details.