To clear up what redly1 said...
A "1000W" power supply is supposed to be able to power 1000W worth of internal PC components. Your components may not add up to that much, so you'll use less. Also, components don't use the same amount all the time. For instance hard drives use a lot more power when they're first powering up. Video cards use more power when in 3D mode. CPUs... well, you get the idea.
Additionally, power supplies are not 100% efficient. This means some of the AC power it takes in from your wall outlet is converted to heat instead of to DC power for your components. Most ATX power supplies range from 60-80% efficient. This means if you manage to build a system that really does draw 1000W from a 1000W power supply*** you are actually drawing closer to 1200-1400W from the wall outlet.
*** I want to note that not all power supplies are capable of really running at their advertised wattage. Most computers (not counting super high end SLI/Crossfire) don't use much power at all, however you see people and video card companies tossing big wattage numbers around. For people, it's just a matter of e-penis size. For companies, it is a matter of CYA - Cover Your Ass. If a computer with one particular video card really needs only 300W of power, but most power supply manufacturers overstate their power output by 20-30%, then to avoid problems the video card/GPU manufacturer will state "Use only with 400W+ power supplies." Additionally, the wattage is added up using three different voltages... but that's for another discussion.
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