I leave mine on all winter. Running a shared computing project and/or defragging overnight. But in summer it makes no sense as the heat generated just adds to the a/c bill. All my office machines are turned off if nobody's using them. Another fear factor in summer is thunderstorms. Even with a UPS it's still a heck of a risk. Not that I won't keep working while lightning crashes around me, but if we're e.g. out to eat and a storm moves through it's a constant worry if the computer's on. I think the theory about initial power surges is a leftover from the days of filaments, but I doubt it applies too much to solid state components. No question though that heating and cooling causes parts to expand & contract which isn't a good thing. But most mobos I've replaced have been due to upgrading, so I really don't see shortened lifespan as a good argument to never switch off. - Users can unsubscribe from this list by sending email to 24hoursupport-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field OR by logging into the Web interface at http://web.tampabay.rr.com/spider1/24hrsupport.htm.