Question: What is the difference with the power wattages on a computer, example 300 watts and 400 watts.
Answer:A 200 watt power supply should be more than sufficient for most computers. If you have lots of devices attached that don't have their own separate power supplies then your computer may need a 300 watt supply in order to be able to supply enough power to all of the devices. You would need to have a really large number of devices to need 400 watts.
Modern computers draw a lot less power than earlier ones did although Pentium 4s draw more power than other modern processors. A lot of USB attached devices can draw their power directly from the computer rather than having their own supplies. As attaching lots of USB devices usually requires a hub, you can use a powered hub instead of having to upgrade the power supply in the computer itself.
This article written by Stephen Chapman, Felgall Pty Ltd.