How do power supplies draw current?

[xero52]

I am asking this question to make some decisions about buying a UPS. The question is if I'm running a computer on a 300 watt power supply but the components in the computer arent drawing that much, does the power supply still require 300 watts to operate or does it just need as much as is being drawn from it by the components? Thanks.

 

[SCSIRAID]

The supply will consume as much power as required to power the system. Power supplies are not 100 percent efficient so it will actually burn about 15-20 percent additional power as heat. So if your system is consuming 200W from the supply then the supply is consuming 230-240W from the power company (or the UPS)(reguardless of the size of the supply).