Hey guys I was just wondering if someone could please explain everything about a power supply and why you get certain amounts e.g 650 w or 850 w. I'm just curious about all this stuff and I'd like to know a little bit more about computers before I go buying anything. Thanks.

Wattage is the amount of power that the supply can provide to the parts of a computer. Each computer part needs power in order to run. The right sized power supply is determined by the hardware in your computer, eg: graphics cards. If you don't meet the right power supply for the computer as a whole, it wont have enough electricity and wont be able to run the hardware in your computer.

Example:

Say you have a GTX 570 card. I believe it needs a 650W power supply minimum to run. Therefore you need to grab a 650W power supply or larger in order for the card and the rest of your computer to run. People sometimes get way over the power supply requirement, this is mainly due to future proofing their build, incase they want to add something new in, that way they wont need a new power supply when they upgrade.

http://en.wikipedia.org/wiki/Power_supply_unit_%28computer%29