How much power does a 500W power supply actually use?

The amount of power a 500W power supply (PSU) actually uses depends on its efficiency and the load it's currently supplying.

A 500W PSU is rated to provide up to 500 watts of power to the components connected to it. However, the PSU itself consumes a certain amount of power to convert AC electricity from the wall outlet to DC electricity that can be used by the components in your computer.

The efficiency of the PSU determines how much input power is required to produce the desired output power. For example, if the PSU has an efficiency of 80%, then to produce 500W of output power, it would require approximately 625W of input power (500W / 0.8 = 625W). However, this is a theoretical calculation, and actual efficiency can vary depending on the load and other factors.

At lower loads, the PSU may operate at a lower efficiency. Therefore, if the system is only drawing 250W from the PSU, the actual input power consumed may be less than 625W, but still higher than 250W due to inefficiencies in the conversion process.

A 500W PSU will use varying amounts of power depending on its efficiency and the load it's currently supplying. To accurately measure the actual power consumption, you would need to use a power meter or similar device.