What Is a Power Button and What Are the On/Off Symbols?

Definition of a Power Button or Power Switch and When to Use a Power Button

A picture of the power button on a laptop computer
© Martin Hospach / Getty Images

The power button is a round or square button that powers an electronic device on and off. Nearly all electronic devices have power buttons or power switches.

Typically, the device powers on when the button is pressed and powers off when the button is pressed again.

A hard power button is mechanical - you can feel a click when pressed and usually see a difference in depth when the switch is on versus when it's not.

A soft power button, which is much more common, is electrical and appears the same when the device is on and off.

Some older devices instead have a power switch that accomplishes the same thing as a hard power button. A flip of the switch in one direction turns the device on, and a flip in the other turns the device off.

On/Off Power Button Symbols (I & O)

Power buttons and switches are usually labeled with "I" and "O" symbols.

The "I" represents power on and the "O" represents power off. This designation will sometimes be seen as I/O or as the "I" and "O" characters on top of each other as a single character, as in the photo on this page.

Power Buttons on Computers

Power buttons are found on all kinds of computers, like desktops, tablets, netbooks, laptops, and more. On mobile devices, these are usually on the side or top of the device or sometimes next to the keyboard, if there is one.

In a typical desktop computer setup, power buttons and switches appear on the front and sometimes back of the monitor and on the front and back of the case.

The power switch on the back of the case is actually the power switch for the power supply installed in the computer.

When to Use the Power Button on a Computer

The ideal time to shut down a computer is only after all the programs are closed and your work is saved, and even then using the shutdown process in the operating system is a better idea.

A common reason you'd want to use the power button to turn off a computer is if it's no longer responding to your mouse or keyboard commands. In this case, forcing the computer to power off using the physical power button is probably your best option.

Please know, however, that forcing your computer to shutdown means all the open software and files will also be ended without any notice. Not only will you lose what you're working on, but you can actually cause some files to become corrupt. Depending on the files that are damaged, your computer may fail to start back up.

Pressing the Power Button Once

It might seem logical to press the power once to force a computer to shut down, but that often doesn't work, especially on computers made in this century (i.e. most of them!).

One of the advantages of soft power buttons, which was talked about in the introduction above, is that, since they're electrical and communicate directly with the computer, they can be configured to do different things.

Believe it or not, most computers are set up to sleep or hibernate when the power button is pressed, at least if the computer is working properly.

If you really need to force your computer to shutdown, and a single press isn't doing it (pretty likely), then you'll have to try something else.

How to Force a Computer to Turn Off

If you have no choice but to force the computer off, you can usually hold down the power button until the computer no longer shows signs of power - the screen will go black, all the lights should go off, and the computer will no longer make any noises.

Once the computer is off, you can press the same power button once to turn it back on. This type of restart is called a hard reboot or hard reset.

Important: If the reason you're powering off a computer is because of a problem with Windows Update, be sure to see What to Do When Windows Update Gets Stuck or is Frozen for some other ideas.

Sometimes a hard power-down is the best way to go, but not always.

How to Turn Off a Device Without Using the Power Button

If at all possible, avoid just killing the power to your computer, or to any device! Ending running processes on your PC, smartphone, or another device without a "heads up" to the operating system is never a good idea, for reasons you've already read about.

See How Do I Restart My Computer? for instructions on properly turning off your Windows computer. See How to Restart Anything for more information on turning off computers, tablets, smartphones, and other devices.

More Information on Powering Off Devices

A strictly software-based method to turn off a device is usually available, but not always. The shutdown of some devices are triggered by the power button but even then are finished up by the operating system it's running.

The most notable example is the smartphone. Most require that you hold down the power button until the software prompts you to confirm that you wish to turn it off. Of course, some devices don't run an operating system in the typical sense and can be safely shut down by simply pressing the power button once - like a computer monitor.

How to Change What the Power Button Does

Windows includes a built-in option to change what happens when the power button is pressed.

  1. Open Control Panel.
  2. Go into the Hardware and Sound section.

    It's called Printers and Other Hardware in Windows XP.
  3. Choose Power Options.

    In Windows XP, Power Options is off to the left side of the screen in the See Also section. Skip down to Step 5.
  4. From the left, click or tap Choose what the power buttons do or Choose what the power button does, depending on the Windows version.
  5. Choose an option from the menu next to When I press the power button:. It can be Do nothing, Sleep, Hibernate, or Shut down.

    Windows XP Only: Go into the the Advanced tab of the Power Options Properties window and select an option from the When I press the power button on my computer: menu. In addition to Do nothing and Shut down, you have the options Ask me what to do and Stand by.

    Note: Depending on whether your computer is running on a battery, like if you're using a laptop, there will be two options here; one for when you're using battery and the other for when the computer is plugged in. You can have the power button do something different for either scenario.

    Note: If you can't change these settings, you might first have to select the link called Change settings that are currently unavailable. If the hibernate option isn't available, run the powercfg /hibernate on command from an elevated Command Prompt, close down every open Control Panel window, and then start over at Step 1.
  6. Be sure to hit the Save changes or OK button when you're done making changes to the power button's function.
  7. You can now close down any Control Panel or Power Options windows.