For a desktop Mac, Apple recommends sleep instead of power-down unless your Mac won't be used for "a few days."  For example, see page 16 here: http://manuals.info.apple.com/en_US/imac_mid2011_ug.pdf

For a laptop, they recommend sleep instead of power-down unless it won't be used for "a couple of days or longer."  For example, see page 17 here: http://manuals.info.apple.com/en_US/MacBook_13inch_Mid2010_UG.pdf

The explanations I've heard from electrical engineers are that the frequent "inrush" of current can cause premature wear on some components;  and frequent heat-to-cold cycles can wear others.

When you put your Mac to sleep, most everything including the disk and of course the display, are powered-down, but a "trickle" of power remains to certain components.

See About Energy Saver sleep and idle modes in Mac OS X for details about what does and doesn't run during Computer Sleep.

According to Apple’s Environment Reports, a sleeping recent iMac or Mini draws about 1.5 watts of power.  At the typical electricity rate of about 11 cents per kilowatt-hour, an iMac or Mini sleeping 8 hours per day will add about one cent per week to your power bill.  Even a Mac Pro (just under 7 watts) would be under a nickel.

In addition, the process of powering-up (a "cold start") takes considerable energy.  Depending on the particular hardware, and the sleep time involved, that can easily use more power than letting your Mac sleep.  And the frequent "inrush" of current can shorten the life of some electronic components.

And, of course, if one Mac in a hundred has a premature component failure, the energy involved in producing, shipping, and installing a replacement may be many times the energy "saved" by powering off for short periods.

7/12/12

Copyright © 2012 James Pond.  All rights reserved.