How to read a charger label: output lines and the watts you get

Here is the OUTPUT block of a 65 W single-port charger, copied from the maker's spec page (Xiaomi). The ⎓ sign means direct current. Each line pairs a voltage with the most current the charger will give at that voltage.

Multiply volts by amps on each line and you get 15 W, 27 W, 50 W, 36 W, 45 W, 65 W. The top line, 20 V × 3.25 A = 65 W, is where the “65W MAX” printed under the block comes from. The charger never gives all of these at once. It switches to the one line the device asks for.

USB Power Delivery has four fixed voltages: 5, 9, 15 and 20 V (USB-IF), and a charger may offer others on top (USB-IF). On this label, 10 V and 12 V sit outside the four. The label does not say whether they are optional PD voltages or part of another fast-charge scheme, so treat them as “not guaranteed” for a device that only speaks USB PD.

The USB-IF describes a charger's power rating (its PDP) as a figure the maker puts on the packaging, and that figure sets the voltages on offer and the current at each (USB-IF). The current is the PDP divided by the voltage (USB-IF): for 65 W at 20 V that is 65 ÷ 20 = 3.25 A, exactly the printed line. The current also depends on the cable (USB-IF), which matters for any line above 3 A (next section).

A higher-rated charger also looks like a lower-rated one to the device (USB-IF). That is why this 65 W charger still lists 5 V and 9 V lines: a phone that wants 9 V gets 9 V.

The 20 V line asks for 3.25 A. A standard USB-C cable is rated for 3 A, and only an electronically marked cable may carry up to 5 A (USB-IF). On a standard cable the top line drops to 20 V × 3 A = 60 W. The maker puts an e-marked cable rated for 5 A in the box (Xiaomi), which is the one that lets the full 65 W through.

The 10 V ⎓ 5 A line has the same catch: 50 W on a 5 A cable, 30 W on a standard one. Any line above 3 A needs a cable that is electronically marked (USB-IF).

A 67 W two-port charger from the same maker lists these lines for one USB-C port on its own (Xiaomi): 5V⎓3A, 9V⎓3A, 11V⎓6.1A, 12V⎓3A, 15V⎓3A, 20V⎓3.25A. The biggest product is 11 V × 6.1 A = 67.1 W, which is where “67W” comes from. The top line at a USB PD fixed voltage is 20 V × 3.25 A = 65 W.

6.1 A is more than the 5 A a marked USB-C cable is rated for (USB-IF), and the maker lists a “6A” cable in the box (Xiaomi). The spec page does not name the protocol behind the 11 V line. For a laptop that charges over plain USB PD, the useful figure on this label is 65 W.

The same charger prints separate lists for when both USB-C ports are in use (Xiaomi). They top out at 20 V ⎓ 2.25 A (45 W) and 9 V ⎓ 2 A (18 W), 63 W together, and the maker says the charger splits power by what each device needs and the order you plug them in.

For chargers in the USB-IF certification programme, the rules for shared ports are written down. Each port must be labelled with its own highest power level (USB-IF), but a shared port does not have to give that power all the time (USB-IF). The packaging must say that the other ports drop when one port takes the most (USB-IF), and the combined total must be printed on the charger (USB-IF). Each USB-C port still has to supply at least 7.5 W (USB-IF), and the wattage on the certified logo is the strongest single port, not the total (USB-IF).

• For a laptop, find the 20 V line and multiply. That is the most USB PD gives it on this charger. If the line shows more than 3 A, use an e-marked 5 A cable.
• The headline wattage can come from a line that is not USB PD. Take the top line at 5, 9, 15 or 20 V as the figure a plain USB PD phone or laptop can use.
• Charging two things at once? Read the "two ports in use" lists, not the single-port one. That is the power each device really gets.
• Both chargers on this page come with a cable rated above 3 A for their top line. Keep that cable with the charger; a spare 3 A cable caps the 20 V line at 60 W.
• Shopping online: zoom into the listing photo of the label or open the maker’s spec page. The OUTPUT block is there even when the title only shows one number.