'Twas brillig, and the slithy toves did gyre and gimble in the wabe

They provide that, but they don't necessarily use it. I've got a spare machine set up as a server (first-gen Athlon 1.3GHz, 1GB ram, nVidia Vanta graphics, average IDE hard disk, zip drive, pair of optical drives, sound and modem), and it's fitted with a 430W Antec PSU. The actual draw, however, is only about 70W (measured with one of those inline power meter thingies).

For comparision, my laptop (first-gen Pentium M 1.5GHz, 1GB ram, Intel graphics, 60GB disk, 1024x768 LCD, 802.11g PCMCIA card) draws about 10W when idling according to IBM's battery utility. Charging is another matter: peak charging power is in excess of 30W and that's on top of running current.