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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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6 Answers 6

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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, 2013 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. Dec 18, 2013 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. May 19, 2015 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? Jul 28, 2015 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. Jul 28, 2015 at 17:21
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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, 2015 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). Oct 10, 2016 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, 2017 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, 2018 at 8:36
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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.

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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, 2014 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/… Jun 22, 2015 at 12:24
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    @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, 2015 at 22:42
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    My Lenovo Ideapad gives 2.4Ah for charging devices through its USB3 ports. Sep 3, 2015 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, 2017 at 8:33
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Unfortunately all these answered missed out on the charging and or power delivery options. Also USB type C.

USB type C is capable of 20v at 5a or 100w enough to power many laptops.

In addition the PD or Power Delivery protocol and the QC, Quick Charge, protocol allow for a wide variety of power. Basically 0-20v at 0-5amps.

Also there are different versions of PD and QC to further muddy the waters.

enter image description here

In fact there are even more standards: PD, PPS, QC5, SCP, VOOC, QC3.0, AFC, FCP,DASH,DCP,FCP,MTK,PE2.0

So while a max of 500ma for USB 2 and 900ma for 3.0 port is normal the "normal" standards can be easily ignored if the USB port supports one of the previously mentioned charging protocols.

USB type C can support 100w, but apparently it doesn't have to, so you can't depend on it.

In fact you have to carefully study the technically specs for any USB device to determine what it actually supports. In fact you may have to contact the manufacturer in order to determine the actual output.

Typically you have to do hours of searching in order to find a good devices.

If all of this isn't bad enough the new specs that are either approved or going to be approved allow 48v at 5A for a total 240w of power.

Regular USB

This is Power delivery only. PD

This is Quick Charge Only Quick Charge

As of 2020, VOOC / SuperVOOC comes in five variations:

VOOC 2.0 (2015), same as the first version that was introduced in 2014, which operates at 5 V/4 A.
VOOC 3.0 (2019), a technology that claims to be 23.8% faster than VOOC and "based on a new technology". It appears to be a 5 V/5 A version of VOOC. which can charge the phone up to 55% in 30 minutes.[7]
SuperVOOC (2018), a successor of VOOC 2.0 with 10 V/5 A (50W). It charges a two-cell battery in series.[8]
VOOC 4.0 (2020), a successor of VOOC 3.0, which operates at 5 V/6 A (30W), which can charge the phone up to 67% in 30 minutes.
SuperVOOC 2.0 (2020), a successor of Super VOOC with 10 V/6.5 A (65W), its also the first charger to introduce GaN technology (Gallium Nitride).

Even this does not convey the confusion unleashed on us by the USB ports.

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USB 2.0 = 5 V x 0.5 A (500 mA) = 2.5 W

USB 3.0 = 5V x 0.9 A (900 mA) = 4.5 W

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

This Wikipedia article has a nice usb power chart.

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    " 500ma and 900ma is the max current a normal spec'd port will take (see other answers for exceptions). " No, that is not the maximum a USB port will output. That's the maximum allowed by the USB 2.0 and USB 3.0 protocol. Ports that support USB-BC, USB-PD, or some vendor specified protocol can provide more power. And most any port on a computer that supports USB 2.0 or USB 3.0 will support more power with one of these protocols. 1.5 amps from most any USB-A or USB-C port is common. 2.4 amps on USB-A and 3.0 amps on USB-C isn't rare. That's from computer ports, not just chargers.
    – MacGuffin
    Jun 11, 2021 at 9:33
  • Based on your comment, the max allowed of 500ma and 900ma is not the max allowed at all. Its a measly suggestion. With other protocols in place or if the port feels like it, the max allowed gets thrown out the window. Either way, thanks for your comment, I've changed the word 'take' to 'allow' to be more correct.
    – Jeff Luyet
    Jun 14, 2021 at 14:38
  • Your interpretation of "at least" as "at most" puzzles me. Apr 14 at 17:28

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