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Ive got hold of a Epson C80 but am baffled as to why it has a USB port AND a power socket. Having tried it out I assumed it would be powered through the USB port but it was'nt,I had to plug it in to the mains as well. Is this right? I have a scanner with a USB port and its powered through that.

the usb port can only provide 5 volts of power at mediocre current. Your scanner only has one motor, which is likely not very powerful/fast, so it can draw power directly from inside your computer.

a printer has at least two motors. One for feeding the paper and another for moving the print cartriages back and forth (extremely powerful). In gerneral these moters are very beefy and require 24 or even 48 volts to run on. If 2 motors aren't challenge enough, the logic to convert the print job to something the printer can actually use draws power along, and so does the actual print head to "spit" ink down to the paper.

The USB port was simply not designed to send that much power. Future revisions of USB may provide a full 12v line plus a 5v line, but I higly doubt if many printers will actually use ONLY usb power... it would simply be too much of a draw on the computer's power supply.

Wow,thats a pretty comprehensive reply! Pretty much answered my question and more. Thanks Test.

I like to give comprehensive answers. When someone gives me an answer like "Not all USB devices are powered through the USB port." I wonder to myself "why not" Technically the origianal answer is completely correct, but I like to give the whys and hows