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USB-C Power Delivery wattage explained
When you plug a laptop into a USB-C charger, the two have a short conversation before any real power flows. That conversation is defined by USB Power Delivery (USB PD), the standard that lets one charger safely charge an earbud case at 5 W and a laptop at 100 W. Understanding it explains most "why is this charging slowly?" puzzles.
The negotiation
- When first connected, a USB-C port supplies only a safe 5 V.
- The charger announces the voltage and current combinations it can provide, called power profiles.
- The device picks the best profile it supports and requests it.
- The charger switches to that voltage. If anything goes wrong, it drops back to 5 V.
The cable takes part too: if it lacks an e-marker chip, the charger assumes it can carry only 3 amps.
The standard voltage levels
USB PD uses fixed voltage steps:
| Voltage | Maximum power | Common devices |
|---|---|---|
| 5 V | 15 W | Earbuds, small accessories, older phones |
| 9 V | 27 W | Phones, tablets |
| 15 V | 45 W | Tablets, small laptops |
| 20 V | 60 W (3 A) or 100 W (5 A) | Laptops |
| 28 V | 140 W | Larger laptops (Extended Power Range) |
| 36 V | 180 W | High-performance laptops (EPR) |
| 48 V | 240 W | Workstation laptops, monitors (EPR) |
Voltages above 20 V belong to Extended Power Range (EPR), added in USB PD 3.1. EPR needs a charger, cable and device that all support it.
PPS: fine-tuned charging
Programmable Power Supply (PPS) is an optional part of USB PD that lets the device ask for voltage in small steps, for example 8.6 V instead of 9 V. Some phones, notably recent Samsung Galaxy models, use PPS for their fastest charging modes because it reduces heat inside the phone. If a charger lacks PPS, those phones still charge, just a little slower.
Why your device charges slower than the label says
- The cable is 3 A, capping power at 60 W.
- The charger is sharing power between several ports. See GaN chargers explained.
- The battery is nearly full. Devices charge fastest from low levels and slow down sharply above about 80 percent to protect the battery.
- Heat. Phones and laptops reduce charging speed when warm, for example when used for games while charging or left in the sun.
- The device's own limit. A phone that supports 25 W charges at 25 W even on a 100 W charger.
Can a smaller charger run a laptop?
Often, yes, just slower. A 65 W laptop on a 30 W charger will charge while idle or asleep, but may lose charge during heavy use. Some laptops refuse to charge below a minimum, often around 20 to 30 W, and show a warning.
Other fast-charging systems
Before USB PD became universal, phone makers used their own systems (such as Qualcomm Quick Charge on USB-A). Many current chargers support those too, but USB PD over USB-C is now the common standard, required for phones sold in the European Union under its common-charger rules.
Practical summary
- Match the charger's wattage to your most demanding device.
- Use a 5 A cable for anything above 60 W.
- Do not worry about "too big" a charger; the device takes only what it asks for.
For choosing the cable, read why USB-C cables are not all the same. For batteries on the go, see power banks: mAh versus Wh.