How do I know how much battery life my laptop has left?
In Windows 8, you'll need to access the Desktop Environment by pressing the Windows button on your tablet or the Windows key on your notebook. When you get to the desktop, click on the battery icon in the Windows Notification Area in the bottom right-hand corner of your screen.
Windows 7, Vista, and XP
Mac OS X
In Mac OS X, click the battery icon in the menu bar at the top of your screen.
The icon displays the condition of the battery, tells you if the battery is being charged or drained, lets you know how much time is left to fully charge or drain the battery, and provides a list of any programs that are consuming a lot of power.
To configure the power consumption of your laptop, select Open Energy Saver Preferences... from this menu to bring up the Energy Saver options (which are also accessible from your System Preferences).
These options give you fine-grained control over how your laptop uses power. You can use the Battery / Power Adapter buttons to create separate configurations for when the laptop is plugged in and using battery power.
For instance, the GUI of the Ubuntu operating system (pictured) has an icon very similar to the icon in Mac OS X. Clicking on the icon will provide you with specific information about the battery's status, and access to various Energy Saver options.
If your OS has a different GUI and you cannot locate the battery meter or power management options, consult your documentation, or the official website of your OS.
Linux Command Line
From the Linux command line, you can use the acpi utility to display information about your battery provided by the ACPI (Advanced Configuration and Power Interface) daemon. The acpi utility is a quick and easy way to check how much charge your battery has left.
It is usually not installed by default, but it can be installed using most Linux package managers. For instance, on Ubuntu or Debian systems, the acpi utility can be installed with APT, using the command:
sudo apt-get install acpi
Once installed, you can run the command without any options to display how much power your battery has left:
Battery 0: Unknown, 96%
For more options, you can view the acpi manual using the command:
Viewing battery life on the battery itself
Some laptops have a power indicator on the physical battery itself. In the example shown above, pressing the button marked with the finger symbol would illuminate the lights to indicate the amount of battery charge remaining.
Note: To access this type of battery function, you may have to first remove the battery from the computer.