What the Numbers on Your Charger Actually Mean

Every USB-C port looks the same, which is exactly the problem. The number stamped on a charger is a ceiling, not a promise. What actually happens is a short negotiation between the charger and the device before any power moves.

The two ends agree

USB Power Delivery lets a device ask for what it needs rather than take whatever the charger has. A pair of headphones asks for very little, a laptop asks for a lot, and the same charger serves both. That is why an over-powered charger does no harm: the device only draws what it requested.

Direction is not fixed either. Whichever end has the power supplies it, so a monitor plugged into the wall can charge a laptop while still driving the display.

From 100 watts to 240

Before 2021 the ceiling was 100 watts, reached through 20 volts over cables rated at 5 amps. Revision 3.1 of the specification lifted that ceiling to 240 watts.

It brought new fixed voltages with it: 28 volts for 140 watts, 36 volts for 180 watts, and 48 volts for 240 watts. There is also an adjustable mode that lets a device request an intermediate voltage anywhere between 15 volts and the highest fixed voltage the charger offers.

The cable is not a detail

The higher ceiling did not happen in the charger alone. The USB Type-C specification was updated to Release 2.1 specifically to define what a 240-watt cable must do. A strong charger paired with an unrated cable will not deliver the number printed on its casing.

What to look at when buying

Check three things rather than one number: whether the charger supports Power Delivery at all, the highest figure it can actually supply, and whether the cable is rated for that figure. Then compare against what your device asks for. An over-specified charger will not hurt your phone; an under-specified cable will slow your laptop.

Photo: a USB-C plug above a USB-C receptacle on the edge of a device. By Wikideas1, CC0 via Wikimedia Commons.

Source: USB Implementers Forum (USB-IF), USB Power Delivery page.