OS X Mountain Lion's Energy Saving Features
All Macs are Energy Star–compliant (and OS X Mountain Lion is no exception), allowing you to preset your machine to turn itself off at a specific time or after a specified idle period. To manage your Mac’s energy-saving features, open the Energy Saver System Preferences pane by choosing Apple→System Preferences and clicking the Energy Saver icon.
If you have a notebook computer, you have two mostly identical tabs — Battery and Power Adapter — in your Energy Saver System Preferences pane. The battery tab controls your MacBook, MacBook Air, or MacBook Pro’s behavior when it’s running on battery power (not plugged in); the Power Adapter tab controls its behavior when it is plugged in.
If you have a desktop Mac, you won’t have tabs, but you do have most of the same controls, including a pair of sliders that control sleep times for your computer and display. To enable Computer or Display sleep, move the appropriate slider to the desired amount of time.
You can choose any number between 1 minute and 3 hours, or turn off either type of sleep entirely by moving its slider all the way to the right, to Never.
Setting the display to sleep is handy if you want your Mac to keep doing what it’s doing, but you don’t need to use the monitor. And if you’re a notebook user, display sleep will save you battery power.
To wake up your Mac from its sleep, merely move your mouse or press any key.
Below the Sleep sliders are some check boxes for other useful energy settings, such as
Put the Hard Disk(s) to Sleep When Possible: Enabling this option forces your hard drive to sleep after a few minutes of inactivity. It’s not a particularly useful feature on a desktop Mac, but if you have a laptop, letting your hard drive sleep when it’s idle will save you some battery power.
Wake for Network Access: Enable this option if you want your Mac to wake up automatically for Ethernet network access (handy in a corporate setting where an IT person maintains system configurations).
Restart Automatically if the Computer Freezes: Enabling this option tells your Mac you want it to restart automatically if it becomes totally hosed.
Allow Power Button to Put the Computer to Sleep: Does just what it says: Enable it, and tapping your Mac’s power (on/off) button puts your computer to sleep immediately.
To turn your Mac off, use the Apple menu’s Shut Down command or its keyboard shortcut (Control+power key).
Automatically Reduce Brightness Before Display Goes to Sleep: Another one that does what it says.
If you have a laptop, you’ll have some additional options, including
Slightly Dim the Display When Using This Power Source (Battery tab only): The display dims slightly and uses less power when running on the battery.
Show Battery Status in the Menu Bar: This option adds a little battery-status indicator icon and menu.
Finally, to start up, shut down, or put your Mac to sleep at a predetermined time, click the Schedule button, and then select the appropriate check box and choose the appropriate options from the pop-up menus.