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?


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
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    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
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    @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
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    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
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    @PavelGatilov The answer is still accurate regarding the older current ratings, which still apply, although it does need some revision due to recent developments. About the +20V output, I read the article you linked (albeit diagonally): it seems only Type C devices are covered by it, so this does not affect legacy (i.e. USB 2.0 or older) devices. Power Delivery devices are also supposed to implement some sort of management capability, such that they can revert to older protocols. – Doktoro Reichard Jul 28 '15 at 17:21

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
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    @Andrew, I believe that devices will only draw the current they need / can use. As long as the device is getting the correct voltage ("electrical pressure") of 5 volts, the available amps can be any amount. Consider how you can take a home's internal wiring (which is capable of many, many amps, enough to power a whole household), wire a 60W lightbulb straight into it, and it'll only draw the power it needs, because it's rated for that voltage (120V AC). – JamesTheAwesomeDude Oct 10 '16 at 20:31
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    @Andrew absolutely wrong. A supply of X amps can supply anything up to X amps. The device only draws what it needs. Where there may be an issue is when it's the other way around, if a device wants more amps than the supply can put out. – barlop Oct 5 '17 at 10:24
  • The intensty (=amperes) of the current depend on the voltage and the resistance / impedance of the circuits of the device. You can't change that latter nor the voltage, the time of charge will be the same as long as enough current (=amperes) are provided. If your adapter has 4x more amperes than required it may be able to charge 4 devices at the same time but there won't be any change in the time of charge of a single device. Not because "it's rated" but because it's a physical law (Ohm's Law) – runlevel0 May 3 '18 at 8:36

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
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    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. venturebeat.com/2012/07/24/… – 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
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    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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    Can you provide some link to the Battery Doctor app? I found some Android power management apps with this name, is that what you mean? – Suma Jan 24 '17 at 8:33

Power that must be delivered by a USB port is defined in Section 7.2.1 of USB 2.0 Specifications.

To start, the power delivery is defined in "units of load". For USB 2.0 one unit is 100 mA, and for USB 3.x one unit is 150 mA.

USB standard defines two classes of USB ports, "high-power ports", and "low-power ports"

The specs says, page 171:

"Systems that obtain operating power externally, either AC or DC, must supply at least five unit loads to each port. Such ports are called high-power ports."

So, if you have a desktop PC or laptop connected to AC outlet, each USB port MUST supply 500 or 900 mA of current. Note the language, "at least". So it could be more, unless an OPTIONAL overcurrent functionality is supported in hardware. For example, a common desktop PC in sleep mode derives the VBUS power from +5VSB rail of its PSU, which at least is capable to deliver 2 A of current. Or more, which is specified in particular PSU.

For example, if a Raspberry Pi3 gadget gets its power from AC-DC adapter from a wall AC power, it must supply at least 500 mA per each (of 4) ports. Unfortunately, it fails to do so, and therefore is not USB-compliant.

However, if a USB host is a skinny battery-powered device (such as MP3 player or smartphone), this can be declared by manufacturer as "low-power host", and the USB port can be limited by design to deliver 100/150 mA only. This limit is very inconvenient to customers, and is rarely enforced.

If a USB system (host or hub) is declared as normal host, the ports are tested to USB-IF test specifications using specialized USB port testers. The tester either applies a load equal to 5 units and checks if the voltage drop doesn't exceed specifications (5% or 10% margin), or applies a step-wise increasing load and determines at which point the (optional) overcurrent circuit trips over.

Under household conditions the port capability can be checked by applying a big 10 Ohm (or 5.5 Ohm if USB 3.x) resistor to a stripped-off cable. Or using a dedicated variable load found on e-Bay.

The requirements for power delivery from a normal USB port should not be confused with requirements for USB DEVICES: USB devices should NOT take more than one unit of load until host completes the device enumeration. USB hosts must keep track of consumed power declared by attached devices. During enumeration a host reads mandatory power requirements of the device within its descriptor, and if the host believes that its power capabilities are maxed out, it can refuse the connection.


USB 2.0 = 5V x 0.5a(500ma) = 2.5W

USB 3.0 = 5V x 0.9a(900ma) = 4.5W

500ma and 900ma is the max current a normal spec'd port will take (see other answers for exceptions).

This Wikipedia article has a nice usb power chart.

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