If i have a computer that has a 500 watt power supply is this the consent use of power or will the useage depend on whats conntected to the power supply??
500 watts is simply the maximum. I doubt you'd ever get anywhere near that. The higher the wattage the more componants you'll be able to have in your PC.
You'd only use anywhere near the full 500 watts if your PC contains a couple of NVidia G80 based graphics cards, dual core CPU, a CD/DVD drive, a bunch of hard drives and you run something that makes the computer really work like FarCry or FEAR.