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I have 2 monitors plugged into my MacBook Pro 2016, one via USB-C and another through a 5-port hub with HDMI. When the MacBook is not plugged in to power (through the hub) it seems to drain power very quickly. Would the monitors cause this? Thx.

Learn about the power available via USB-A on Apple computers and displays. Also find out how to use peripheral devices that might require more power than the USB port provides.

About the power provided through USB-A ports

Apple computers equipped with USB 3 provide up to 900 mA (milliamps) at 5 V (Volts) to most Apple USB peripherals as well as all third-party USB peripherals that comply with USB specifications.

Apple computers and displays equipped with USB 1.1 or USB 2 provide up to 500 mA (Milliamps) at 5 V (Volts) to most Apple USB peripherals as well as all third-party USB peripherals that comply with USB specifications.

Some USB peripheral devices not manufactured by Apple might have unique power requirements that exceed the power allotment on an individual USB port. These devices might require you to connect them to multiple USB ports or might have external power sources that must be connected for them to function.

Apple devices can request extra power

Apple peripheral devices might request more than 500 mA (Milliamps) at 5 V (Volts) from a port to function or to allow for faster charging. These devices include:

  • Apple MacBook Air SuperDrive (when connected to supported computers)
  • Aluminum Wired Keyboard*
  • iPod
  • iPhone
  • iPad

Some Apple computers and displays** can provide up to 1100 mA at 5 V through the port the Apple device is connected to. This power is available under certain conditions:

  1. An Apple device must be plugged directly into an Apple computer or display. Apple devices connected to hubs won't have access to extra power above the standard USB specification of the port the device is connected to.
  2. Your Apple computer or display must be powered on and be awake. If it's asleep, all ports provide their normal maximum output.
  3. The port providing extra power is determined by the first Apple device to connect to the Apple computer or display that requires extra power. The remaining ports continue to offer their normal maximum output. Some Apple computer and displays offer the ability to operate more than one USB port at 1100 mA at 5 V. On those computers, the second or third port is enabled when an appropriate device is connected.
  4. An Apple computer started up to Windows via BootCamp doesn't provide extra power.

*When connected to a computer that supports a connection of 1100 mA at 5 V, the first port on the keyboard to have a device or peripheral connected to it that requests standard 500mA power receives that power. At that point, 100 mA at 5 V is available through the remaining port on the keyboard. The keyboard doesn't support extra power out of its two ports simultaneously; it requests extra power from the host computer to provide power out of either one of its two ports, then the second keyboard port receives the standard 500mA.

**Apple computers and displays that were introduced before 2007 support only 500 mA at 5 V from their ports and don't offer additional power.

Connecting multiple devices

Multiple devices can receive high power simultaneously until the extra power is consumed. If more devices are connected requesting additional power, they default to the maximum the port allows, or the device won't function until the original device using the extra power is removed.

If you have multiple supported devices that require additional power, an Apple LED Cinema display can supply additional power to a device connected directly to the display.

Confirming power usage in System Information

You can use System Information to find more information about peripheral power requirements, or you can contact the manufacturer of your peripheral. For the most accurate information about power usage, make sure your device is connected directly to your Apple computer or display before you open System Information.

To open System Information, hold down the Option key and choose Apple menu () > System Information. You can also use Spotlight to find System Information, or open it from the Utilities folder of your Applications folder. 

In the example above, the USB port offers the default 500 mA, as shown in the Current Available (mA) entry. The Current Required (mA) entry indicates the current the device needs to operate. Extra Operating Current (mA) indicates that this USB port is providing an additional 1600 mA for use with the device.

Learn more

If you have a USB peripheral that's acting unexpectedly—such as not turning on or not being recognized by the computer—the peripheral might require more power than is available. To fix the issue, try one of these options:

  • Connect the device directly to your Mac rather than through a hub.
  • Use a USB hub that comes with a power adapter.
  • Check to see if the peripheral offers the ability to connect to more than one USB port at once.
  • Find out if the peripheral comes with a power adapter.
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Every device you plug into a USB port has a power requirement.

Some are quite small in the milliamp range and some can be quite large, up to 5A of current. The more devices, the more draw.

When you have monitors hooked up via USB, it's your CPU that is actually driving the display as there is no separate video adapter. Higher CPU cycles, more power draw. As your CPU works harder, fans have to kick on - another device, more draw.

So, in short, yes, your monitors are causing the more rapid drain on your battery.

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