It consumes more energy to leave your PC running than it does to simply turn it off and reboot later. If you're loath to do this because of the lengthy startup time, putting your computer in sleep mode (not to be confused with your screensaver) is the next best bet for energy savings. Surprisingly, however, only about 10 percent of computers today have their power-management features enabled, according to the EPA. It's pretty simple to set up the power-saving options in Windows.
Power consumption in Windows Vista is more adjustable than in Windows XP. In fact, Vista can adjust its performance to fit whatever task you're working on, boosting processing power when you're watching or editing a video, then lowering it when you're just tapping out a document in Word.
To access your PC's power-savings options, click the Start menu, choose Control Panel, then click Power Options (nested under "System and Maintenance" in Vista, if you're not using Classic View). In Vista, you can extend your options by clicking on "Create a Power Plan," then selecting one of three options: "Balanced" (equal parts energy savings and performance), "Power saver" (greater energy savings, but reduced performance), and "High performance" (reduced energy savings, but maximum performance). For further customization, click on "Change plan settings" under the plan you've selected. This takes you to the Edit Plan Settings page, where you can specify how long your PC and monitor remain idle before they enter hibernation. Hit "Save changes" to implement your tweaks.
In Mac OS X, you can access power-management options by choosing System Preferences from the Apple menu, then selecting Energy Saver from the View menu. In the Energy Saver pane, you can choose to turn on and off features such as "Reduce processor performance," as well as determine how long your PC should be allowed to remain inactive before going to sleep. For even greater energy savings, select the checkbox next to "Put the hard disk(s) to sleep when possible."