For many years now, I've been shutting off my computer at night. But I'm now convinced you can leave your computer on at night and still conserve as much energy. (by Monte Enbysk)

If you're a Windows user (Windows 98 Second Edition or beyond), just set up your PC to "hibernate" overnight. "Hibernate" powers down your monitor to about 5 watts of energy and your PC to 2.3 watts — virtually the same as turning your PC off (your monitor uses zero watts when turned off; more on this below). Either way, you save as much as $90 a year in power costs compared to a PC left on with a 3D screen saver running.

"Well, duh. Welcome back from the Disco Era," many of you are thinking. You already knew all this.

Maybe so, but the question keeps coming up, year after year: Should you shut your computer down at night or leave it running? Some time ago, I touched on the issue at the end of this column. I essentially passed on the recommendation of the good folks at Energy Star, a product-labeling program sponsored by the U.S. Environmental Protection Agency, that "if you are going home for the day, turn it off."

"Andy in Austin" recently triggered my interest in revisiting the subject by raising the question in tech guru Kim Komando's weekly e-mail newsletter. "Should I shut my computer down at night? Or is it better to leave it running?" he asked. Komando's response, in a nutshell: "The truth is, it really doesn't matter."

The truth is, if you use the "hibernate" feature of Windows XP (and previous versions from Windows 98 Second Edition on), or even the "sleep" feature of most new Dell PC models, it really doesn't matter much. Even the folks at Energy Star agree you save almost as much energy as you do turning off your computer for the night (minus unplugging it). And you won't have to endure a lengthy "re-booting" process the next morning; your computer should "wake up" in 30 seconds or less.

Again, I may not change my habits. I like the security of having it off (though locking your system or logging off is just as secure), and I like the ability to shake the cobwebs from my system on a daily basis. But, with every minute I spend booting up in the morning, I can see why someone would rather leave their machine on.

Turn it off, they still say
Full disclosure: Those at Energy Star still prefer that you turn your computer off at night, for maximum energy savings. "We are all about energy savings, and when you shut off your computer at night, you save the most energy," says Craig Hershberg, program manager for office equipment and consumer electronics. "Every little bit helps. It all adds up."

But Energy Star supports the practice of putting computers in "hibernate" or "sleep" mode — most new Dell desktop PCs contain "sleep state" power-management programs that work similarly as "hibernate." Energy Star applauds companies such as Cisco Systems and Pitney Bowes that have made enabling computers to "hibernate/sleep" at night a company policy. (The organization recently issued press releases touting the dollar volumes in energy savings at each company.)

Hershberg estimates that as many as 50% of U.S. users are enabling their PCs to "hibernate/sleep" at night, a percentage Energy Star hopes will continue to climb — even if the users are doing it for the wrong reasons. Many users simply don't like the 3-8 minutes it takes to re-boot a shut-off computer; they're more concerned about the re-boot time than saving energy. For that reason, and because the power-management features in Windows continue to be improved, "the trend is for fewer people to be shutting off their computers at night," he says.

However, here are some consumer "myths" that are worth addressing:

Turning your PC off uses more energy than leaving it on. Not true. The small surge of power you use when turning it on — which varies per PC make and model — is still much smaller than the amount you use in keeping it on for lengthy periods.

Turning your PC on and off wears it out. Five or more years ago, there was something to this, but not today, say Hershberg and others. It used to be that PC hard disks did not automatically park their heads when shut off, and that frequent on/off cycling could damage the hard disks. Today's PCs are designed to handle 40,000 on/off cycles before a failure, and that's a number you likely won't reach during the computer's five-to-seven-year life span.

Screen savers save energy. Not true. Screen savers, at a minimum, can use 42 watts; those with 3D graphics can use as much as 114.5 watts, according to Don McCall, a Dell product marketing manager who does power measurement studies for the PC manufacturer. "It's absolutely wrong thinking that a screen saver will save energy," he says.

Your computer uses zero energy when "off." That's true only if it is unplugged. Otherwise, the PC utilizes "flea power," or about 2.3 watts, to maintain local-area network connectivity, among other things, McCall says. In "hibernate" mode, your PC uses the same 2.3 watts; in "sleep" mode, your PC uses about 3.1 watts. Monitors do use zero energy when turned off.

Lab tests done by Dell show that a PC running Microsoft Office uses 42.7 watts, McCall says. If it runs continuously at that rate for 365 days, at 7 cents per kilowatt-hour, the power consumption costs would be $26.18 for the PC and $45.99 for a regular monitor, for a total of $72.17 for the workstation.

Flat-panel monitors use less energy (22 watts when left on, 3.3 watts in "sleep" mode) than regular monitors (75 watts when left on, 5 watts in "sleep" mode), McCall says. So the same workstation with Microsoft Office running for a year would use $39.67 in power with a flat panel.

Meanwhile, if a PC was kept in "sleep" mode for 20 hours, for every four hours "on," as Dell recommends, the annual energy costs per PC would total $16.17 with a regular monitor and $9.88 with a flat panel. Using "hibernate," the costs would be slightly cheaper.

Enabling "hibernate"

As I said, "sleep" mode is available on most new Dell PCs, while "hibernate" is available to any user with Windows XP and previous versions (from Windows 98 Second Edition on). To enable "hibernate," simply go to your Control Panel, click on "power options," and set your PC to "hibernate" after a specified time (most recommend 30 minutes).

If you are away from your PC a lot during the work day, you may want to set it to "hibernate" after 45 minutes to an hour, and set it to "standby" to 15 minutes. Under "standby," you'll be conserving power but you won't be saving your computer memory onto your hard disk, as you will with "hibernate." "Standby" is meant for shorter absences.