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As the title says, what's the power output of a USB port?

Is it a standard value, or it may change depending on manufacturer/model, and so on?

If that value is not standard, how can one determine it?

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up vote 42 down vote accepted

As stated in Wikipedia

The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices.

A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of 5 unit loads (500 mA) from a port in USB 2.0; 6 (900 mA) in USB 3.0.

As power is equal to current times voltage, all you have to do is multiply 5V with the current the device is drawing from the port.

Note there also exists a convention for charging devices. These kinds of ports allow for currents up to 1.5 A (also using 5V). However, the USB port is rated to withstand current up to 5 A--so some manufacturers may go out of spec and offer a higher maximum current.

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Some motherboards have ultra high ampage USB ports to support charging devices off them also. – Lawrence Dec 18 '13 at 12:29
Are these values standard, or they can be different? How can I determine if my laptop has such high amperage ports, and physically identify it between the other standard ports? – Sekhemty Dec 18 '13 at 12:37
The values need to be standard (of course, there exists a certain level of tolerance in the current and voltage levels). I usually see a lightning-bolt signal in the outside of the port, that should mean the high-discharge port. As for certainty, check the laptop's documentation. – Doktoro Reichard Dec 18 '13 at 12:43
@Lawrence, that's charging downstream port, which allows the device to draw more than 1 unit without negotiation. – Free Consulting May 19 '15 at 14:57
As far as I understood, this is not the complete answer as of now. Going by USB Power Delivery section, there might already be devices that output 20 V. I wonder what happens if the controller of such a device has a bug and puts +20 V on a legacy device... Or do I get it all wrong? – Pavel Gatilov Jul 28 '15 at 15:09

There are USB power adapters on the market explicitly stating "10W adapter". As USB is 5V the 10W result in 2A = 2000 mA. The net effect is that devices connected to this adapter charges its battery 4 times faster than with a "normal" 500 mA USB port.

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Just be careful that the device (and its control circuitry) can handle the extra current! – Andrew Aug 9 '15 at 10:03

I used the 'Battery Doctor' free app to determine how much amperage the usb charging port is offering. I use the word offering intentionally, since each device has a maximum amperage amount that it will take in regardless of what is offered.

I found that my 3.0 port on my hp envy laptop, which has a lightning bolt next to it offers 1.5 amps (1500mA), while the 2.0 usb offers only 0.5 amps (500mA).

Although some forums have stated that it is not possible for an app to determine the amount of amps offered to a device, the Battery Doctor app clearly states amps offered accurately and immediately on my ipad (although it may only display up to the maximum allowed by the device - I have not tried this). I have tested the app with a 1.8 amp out wall charger, and a 2.1 amp out power bank, and both are marked as such on the charger. The amperage readings displayed accurately and immediately on the app.

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1500mA is more than the USB 3.0 specification even allows. What is likely was that this particular model offered a "USB Charging" port. – Ramhound Nov 7 '14 at 18:46
The USB Power Delivery spec appears to combine with, rather than replace, the USB3 standard. So you can still be in-standard while delivering far, far more power than this - up to 100 watts.… – Chris Moschini Jun 22 '15 at 12:24
@Ramhound This is wrong. The spec says the usb device is not allowed to draw more than 900 mA, but this is the minimum and not the maximum for the host. Confusing? Makes sense for charging a device. Although its interesting that a mobile usb host (notebook) is allowed to share the power. That is the reason why some older ultrabooks are not able to power some external drives even with two usb cables (as the 500 mA are shared). And except of the external drive (as it needs more than 500 mA) all devices respect the spec. – mgutt Aug 3 '15 at 22:42
My Lenovo Ideapad gives 2.4Ah for charging devices through its USB3 ports. – Umair Ahmed Sep 3 '15 at 6:28

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protected by JakeGould Oct 22 '15 at 14:07

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