Basics7 min read

How to read a device label: volts, hertz, amps and watts

Before choosing an adapter or converter, read the device label. The INPUT line tells you which voltage and frequency the device accepts; the OUTPUT line describes what the charger sends to the device.

In brief

✓INPUT describes what the device accepts from the mains.
✓OUTPUT is the supply delivered to the connected device.
✓A range such as 100–240 V, 50/60 Hz is widely compatible.

INPUT: the line that matters for travel

INPUT states the permitted mains voltage and frequency. For example, 100–240 V ~ 50/60 Hz covers the common 120 V and 230 V systems and both standard frequencies.

A single value or narrow range must not be extended by assumption. If the destination falls outside it, the device is not confirmed for direct use.

OUTPUT: the device side

OUTPUT values describe what a power supply provides to a phone, laptop or other device. USB-C chargers may list several voltage and current combinations because the charger and device negotiate a power profile.

Amps and watts

Power is commonly expressed in watts. For an adapter, the current rating and the destination voltage determine the practical maximum load. High-power appliances require a larger safety margin than small chargers.

Label	Meaning
V	Voltage
Hz	Frequency
A	Current
W	Power

When the label is unclear

Look for the manual or manufacturer specification. Do not infer worldwide compatibility from a USB connector or from the country where the product was purchased.

Common questions

FAQ

Is 100–240 V the same as dual voltage?+

It is a wide input range and normally covers both 120 V and 230 V supplies.

Which line should I check: INPUT or OUTPUT?+

Check INPUT first. OUTPUT describes the low-voltage side after the power supply.

What does the tilde symbol mean?+

It indicates alternating current, the type supplied by normal mains sockets.

Sources

Sources used

Reviewed: 2 August 2026

IEC 60417 Graphical Symbols

International Electrotechnical Commission (IEC) · Primary source

Official IEC database of graphical symbols used on electrical equipment.

World Plugs

International Electrotechnical Commission (IEC) · Primary source

IEC country overview of commonly used plug types, nominal voltages and frequencies.

USB Power Delivery

USB Implementers Forum (USB-IF) · Primary source

Official information on USB Power Delivery, power levels and USB-C charging.

The sources support the technical statements in this guide. For a particular device, its label, manual and manufacturer approval remain decisive.

All sources and methodology →

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