Wired reports that there's no longer a reason to keep your computer always on:
"IT folks used to advise leaving computers on all the time to avoid wear and tear. But components are much more reliable now, and new research shows there's no reason not to go green and turn PCs off when not in use. ...computers use about 110 watts of electricity while booting up and 60 watts when idle; a 17-inch CRT monitor uses about 75 watts. PCs in sleep mode draw about 35 watts, roughly equivalent to three clock radios. Multiplied by hours of standby time and millions of PCs worldwide, that adds up to a big drain. Infineon Technologies estimates that a 1 percent decrease in U.S. standby power use would save 360 megawatts -- equivalent to a medium-size power plant."
More data points:
My Powerbook draws between 20 and 30 watts when running (screen at full brightness) -- power draw depends on how hard the processor is being taxed (100% use = 30 watts). With the system idle and the screen turned off (a typical not-quite-sleep mode), draw was 12 watts. With the screen closed and the system asleep (full sleep mode), draw was 2 watts. All measured with the Kill A Watt I mentioned awhile back.
I work at a university, and my IT person recommended keeping my PC on only so that it would automatically update Windows immediately when such updates were available. I do lock it and turn off the monitor though.
Powerbook here too. OS X does housekeeping in the very early morning using its cron scripts. If I turn it off, I have to run the scripts manually and they take up about fifteen min of my day.
Of course, I don't leave my Windows boxes up unless they are being used. Leaving a Windows box unsupervised is asking for it. Actually I'm not using the server much at all anymore as I am sucked into the delights of *nix and my wife's Win ME laptop sucks up maintenance time like a sponge. ewwwww