USB PD Charging: The Wattage on the Box Is a Ceiling
Quick answer / TL;DR: Short answer: USB PD negotiates voltage and current between charger, cable and device, so the delivered wattage is the lowest common capability. A cable rated above 3 A must contain an e-marker declaring its rating, and a cable without one caps the link at 3 A. USB PD 3.1 adds extended power range up to 240 W. To get the rated wattage, confirm the charger, the cable and the device all support it, and check the wattage the device actually reports.
Five shells, one family: the plug shape does not set the speed.USB Power Delivery is a negotiation, not a fixed supply. A charger advertises a set of voltage and current combinations, the device picks one it can accept, and the cable in between has to support what is chosen.
That is why a 100 W charger can deliver 45 W to a laptop, and why the same charger and laptop can charge faster with a different cable. Nothing is faulty; the negotiation simply settled lower.
How the Negotiation Works
The charger publishes power data objects describing what it can supply. The device requests one, and the cable's e-marker may further constrain the result.
| Element | What it contributes | If it is the limit |
|---|
| Charger | available voltage and current profiles | output caps at its own maximum | Cable e-marker | declared current rating | above 3 A is refused without one | Device | what it can accept | charges at its own maximum | Port | USB-C required for PD | USB-A cannot negotiate PD |
The cable row is the one buyers miss. A 100 W charger and a laptop that accepts 100 W will still charge at 60 W through a cable with no e-marker, because the system cannot confirm the cable's capability.
Voltage Profiles
PD works by raising voltage rather than pushing more current through the same 5 V. Higher voltage means lower current for the same power, which reduces loss in the cable.
- 5 V: baseline, always available
- 9 V and 12 V: common for tablets and small laptops
- 15 V and 20 V: laptops, up to 100 W on PD 3.0
- 28 V and above: PD 3.1 extended range, up to 240 W
- PPS: fine-grained voltage adjustment for fast charging
Real-World Applications
Laptop that charges slowly from a dock
The dock shares its supply between the laptop and its own peripherals. A 100 W dock often delivers 60 to 80 W to the host, and the laptop reports the lower figure.
Phone that will not fast charge
The phone may use a vendor protocol rather than standard PD, or the cable may be charge-only without the necessary data lines. Check both before replacing the charger.
Charging a laptop from a power bank
The bank must support PD at the voltage the laptop needs, and the cable must be rated for the current. A bank rated for 45 W will charge a 100 W laptop slowly, and that is expected.
Where PD does not apply
USB-A ports. They can supply more than the standard 500 mA through vendor protocols, but they cannot negotiate USB PD, so the rate is whatever the port and device agree outside the standard.
Related reading: Cable limits above 3 A — blogs/news/usb-c-extension-cable
Related reading: USB generations and power — blogs/news/usb-2-vs-usb-3
Standards and Verification
USB Power Delivery is specified by USB-IF, with PD 3.0 covering up to 100 W and PD 3.1 adding extended power range to 240 W. The e-marker requirement for cables above 3 A is part of the USB Type-C specification.
Verify by reading the wattage the device reports rather than the figure on the charger. Operating systems and phones both expose the negotiated rate, and that is the only figure that reflects the actual negotiation.
If the rate is lower than expected, substitute one element at a time: charger, cable, device. Substitution is faster than reasoning about which element is limiting, because the negotiation hides the reason.
Cheap cables sometimes contain a counterfeit e-marker that declares a rating the conductors cannot support. A cable that becomes warm at high current is a safety concern rather than a performance one, and it should be replaced.
Frequently Asked Questions
Why is my 100 W charger only delivering 45 W?
The device, the cable or the charger's profile is limiting the negotiation. A cable without an e-marker caps the link at 3 A, which is about 60 W at 20 V.
Do I need a special cable for fast charging?
Yes above 3 A. The cable must contain an e-marker declaring its rating, otherwise the system treats it as a 3 A cable.
What is USB PD 3.1?
An extension of USB Power Delivery that raises the maximum to 240 W using higher voltages, requiring matching chargers, cables and devices.
Can USB-A deliver 100 W?
Not through USB PD, which requires USB-C. Some vendor protocols exceed the basic 500 mA, but they are not standardised PD.
Does a USB-C cable always support PD?
No. Many cables carry power but not the data lines needed for negotiation, and charge-only cables behave differently.
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