USB Power Delivery works by negotiation: the charger publishes a list of voltage and current combinations, and the device picks one. A headline wattage is only the highest entry on that list. Two chargers rated at the same wattage can offer different combinations, so one will charge a given device quickly and the other will fall back to a lower rate without showing an error. The list, not the wattage, is what determines compatibility.
The model, in one paragraph
A USB-C charger does not simply supply power. When a device is connected, the two sides have a conversation. The charger presents a list of the voltage and current combinations it can provide. The device reads that list, chooses the entry that suits it, and the two confirm the choice before any power flows.
That is USB Power Delivery. It replaced an older world in which a charger offered one fixed voltage and a device took whatever it could get.
What the list contains
Each entry on the list is a power data object, usually shortened to PDO. There are two kinds, and the difference between them is the source of most confusion in this product category.
Fixed PDOs name a specific voltage. In the standard power range these are 5 V, 9 V, 15 V and 20 V. In the extended power range, fixed voltages at 28 V, 36 V and 48 V are added.
Adjustable PDOs describe a voltage range the charger can produce in small steps, rather than a set of discrete values.
The adjustable kind is what enables modern fast charging. A battery does not want a constant voltage; it wants a controlled current while empty and a controlled voltage as it fills. A charger restricted to four discrete voltages can only approximate that curve in coarse jumps, and each jump wastes energy as heat inside the device. A charger that can step the voltage finely can follow the curve closely, which means less heat and a higher charging rate sustained for longer.
The two ranges, and why the split exists
The specification divides power delivery at the 100 W line.
Standard Power Range (SPR) covers everything up to 100 W, using the fixed voltages above with a maximum current of 5 A. This is where phones, tablets, power banks and most small chargers live.
Extended Power Range (EPR) covers 100 W to 240 W. It adds the higher fixed voltages and its own adjustable mode.
The reason for the split is arithmetic. Power is voltage multiplied by current. To raise power without raising voltage, you have to raise current, and current is what heats cables and connectors. Pushing past 5 A through a USB-C connector is the option that runs out first, so the specification raised the voltage instead — from a 20 V ceiling in the standard range to 48 V in the extended range.
PPS and AVS are not the same feature
Both are adjustable modes. They operate in different parts of the specification and they are not interchangeable.
PPS — Programmable Power Supply — is the adjustable mode in the standard power range. It is what phones and small devices use, and it is the feature people mean when they say a charger "supports PPS".
AVS — Adjustable Voltage Supply — is the adjustable mode introduced with the extended power range. It exists because a notebook wants a smooth voltage curve just as a phone does, and the fixed high voltages of the extended range are a poor fit for that on their own.
A charger supporting one does not support the other. A device that requests AVS will not be served by a charger offering only PPS.
The cable is part of the circuit
Above 100 W, the cable stops being a passive accessory and becomes part of the electrical design.
A cable rated for the extended power range contains a small chip — an e-marker — that declares its current and voltage capability to the connected devices. A charger will not offer the higher voltages to a cable that has not declared them. This is a safety behaviour: a cable not built for the current would heat up, and the negotiation is the mechanism that prevents the system from using one.
The consequence is the most common failure mode in this category. A device charges slowly from a charger that is nominally far more capable, no error is shown, and the cause is the cable. Substituting a known-good cable is the fastest diagnostic step available, and it isolates the cable from the charger and the device in a single action.
How to read a specification sheet
Look for the capability list, not the wattage. A charger that publishes its PDO list is telling you what it can actually do. A charger that publishes only a wattage figure is telling you the maximum across entries you cannot see.
Check whether an adjustable range is listed. If the sheet shows only discrete voltages, the charger does not support adjustable output, whatever the marketing copy says about fast charging. An adjustable entry looks like a voltage range with a step size, not a single number.
Check the current at each voltage. This is where two chargers with the same headline wattage diverge. A 65 W charger offering 20 V at 3.25 A will not charge a notebook that requires 20 V at 5 A, even though the notebook's requirement is 100 W and the charger is nominally in the same territory.
Check the range in the extended tier if the application needs it. Above 100 W, confirm both the fixed voltages and which adjustable mode is implemented.
Conformance and certification are separate
The USB Implementers Forum publishes the specification and operates its own certification programme. A product can be designed to conform to the specification without going through that programme, and a manufacturer claiming conformance is making a statement about the design that no external body has verified.
This is not a technicality. Where a supplier says a product is "USB PD compliant", the useful follow-up question is whether it has been certified under the programme or simply designed to the specification — and, if certified, under which revision.
The practical summary
The wattage on the box is a ceiling. The list of capabilities is what the product actually does. The cable determines whether the upper part of that list is reachable. And the device decides which entry gets used.
Four variables, one number on the packaging. That gap is where most charger complaints originate, and it is entirely a specification-reading problem rather than a fault.