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

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 a user presses the button and powers off when they press it again.

A hard power button is mechanical—you can feel a click when it's 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 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 it 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 I/O or the "I" and "O" characters on top of each other as a single character, as in this photo.

Power Buttons on Computers

Power buttons are on all kinds of computers, like desktops, tablets, netbooks, laptops, and more. On mobile devices, they're 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 computer case. The power switch on the back of the case is actually for the power supply.

When to Use the Power Button on a Computer

The ideal time to shut down a computer is after all the software programs are closed and you've saved your work. However, 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 shut down 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 cause some files to become corrupt. Depending on the damaged files, 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 we discussed in the introduction, is that users can configure them to do different things since they're electrical and communicate directly with the computer.

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

If you need to force your computer to shut down, 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 to turn it back on. This type of restart is called a hard reboot or hard reset (tip: reset and reboot mean different things).

How to Turn Off a Device Without Using the Power Button

If possible, avoid just killing the power to your computer or 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 seen.

Another reason you might need to turn off or restart a computer without using the power button is if the button is broken and won't work like it's supposed to. It can happen on phones and computers alike.

See How to Properly Reboot (Restart) a Windows Computer for instructions on properly turning off your Windows computer. Turning off other devices like tablets and smartphones usually involves holding down the power button and following the on-screen prompts.

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 is triggered by the power button, but is 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 and go into the Hardware and Sound section. It's called Printers and Other Hardware in Windows XP.

   Don't see it? If you're viewing Control Panel where you see all the icons and not categories, you can skip down to Step 2.

2. Choose Power Options.

   In Windows XP, this option is off to the left side of the screen in the See Also section. Skip down to Step 4.

3. From the left, pick Choose what the power buttons do or Choose what the power button does, depending on the Windows version.

4. Choose an option from the menu next to When I press the power button. It can be Do nothing, Sleep, or Shut down. In some setups, you might also see Hibernate and Turn off the display.

   Windows XP Only: Go into 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.

   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 a battery and the other for when the computer is plugged in. You can have the power button do something different for either scenario. 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 start over at Step 1.

5. Select Save changes or OK when you're done making changes to the power button's function.

You can now close down any Control Panel or Power Options windows. When you press the power button from now on, it will do whatever you chose it to do in Step 4.

Other operating systems might also support changing what happens when the power button is used, but they probably only support non-shutdown options like opening apps and adjusting the volume.

Buttons Remapper is one example of a tool for Android devices that should be able to remap the power button to make it do something other than power down the device. It can open the last app you were in, adjust the volume, open the flashlight, start the camera, begin a web search, and lots more. ButtonRemapper is very similar.