In short
- Homes have outlets of about 120 V (what many people call “110”) or about 230 V (“220”), depending on the country.
- Check the label on the appliance or its charger: if it says 100–240 V, it works everywhere and you only need an adapter for the plug shape.
- An appliance that says only 120 V plugged into 230 V can burn out or start a fire: heating appliances would try to deliver almost 4 times their power.
- A 230 V appliance plugged into 120 V won’t burn out, but heating appliances give only a quarter of their power.
What voltage your outlets have
There are two big families. Many countries have outlets of about 230 V, and others, especially in the Americas, about 120 V. The numbers people use, “110” and “220”, are old or rounded names; current rules use others:
- United Kingdom: 230 V and 50 Hz, under its electricity supply quality regulations.
- United States: the Federal Trade Commission’s own labeling rule measures light bulbs at 120 V.
- Spain: 230 V between phase and neutral, under its Low Voltage Electrical Regulation (ITC-BT-25).
- Mexico: its wiring standard (NOM-001-SEDE-2012) uses 120 and 127 V for circuits.
If you don’t know yours, ask your power company or check your bill or meter. What really matters isn’t the exact number but whether the appliance accepts it.
How to tell if your appliance will work: the label
Every appliance carries a label or marking (sometimes embossed, on the charger or underneath) with its voltage. It may say:
- A range, like “100–240 V ~ 50/60 Hz”: it works on any voltage and frequency in that range, meaning almost any country. That’s usual for phone and laptop chargers. You only need an adapter if the plug shape doesn’t fit.
- A single voltage, like “120 V ~ 60 Hz” or “230 V ~ 50 Hz”: it only works on that voltage. In another country you’d need a converter.
- A “110/220” or “115/230” switch: it works on both, but you must set the switch for your country before plugging it in.
Mexico’s standard sums it up in one line: an appliance’s rated voltage must not be lower than the actual voltage of the circuit it’s connected to.
What happens if you get it wrong: the sums
In appliances that heat with an element (kettle, iron, hair dryer, space heater, electric shower), power changes with the square of the voltage: power = voltage² ÷ resistance. So a voltage mistake isn’t a small difference, it’s a big one.
- A 120 V, 1,500 W kettle plugged into 230 V: its resistance is 120² ÷ 1,500 = 9.6 ohms. At 230 V it would try to deliver 230² ÷ 9.6 = about 5,500 W, almost 4 times as much, drawing 230 ÷ 9.6 = 24 amps. It isn’t built for that: it can burn out instantly, and take the cord or outlet with it.
- A 230 V, 2,000 W kettle plugged into 120 V: its resistance is 230² ÷ 2,000 = 26.45 ohms. At 120 V it gives 120² ÷ 26.45 = about 544 W, just over a quarter: it would take almost 4 times as long to boil.
Appliances with motors or electronics don’t follow this sum exactly, but the rule is the same: if the label doesn’t include your home’s voltage, don’t plug it in.
Adapter or converter: they’re not the same
- An adapter only changes the plug shape so it fits. It doesn’t change the voltage. It’s for appliances rated 100–240 V.
- A converter (or voltage transformer) changes the voltage, for example from 230 to 120 V. You need one for a single-voltage appliance that doesn’t match your home. It must handle at least the appliance’s watts: for the 1,500 W appliance in the picture, one rated 1,500 W or more, as stated on its label.
For the most power-hungry appliances (the ones that heat: hair dryers, irons, kettles, heaters), the converter has to be very large; it’s often more practical to buy the appliance made for your country’s voltage.
Hertz: 50 or 60 Hz
As well as voltage, mains power has a frequency: 50 or 60 hertz (Hz) depending on the country. The UK, for example, has 50 Hz by law. If the label says “50/60 Hz”, it doesn’t matter. If it says only one and your supply is the other, and the appliance has a motor or a clock, ask the manufacturer before using it.
When to call an electrician
If you don’t know what voltage an outlet in your home has, don’t guess: an electrician can measure it in a moment. And changing an outlet from one voltage to another, or adding a new one, is always a job for them.
To convert watts to amps at your home’s voltage, use the electricity usage calculator or read watts, kWh, volts and amps.
Sources and references
- United Kingdom. The Electricity Safety, Quality and Continuity Regulations 2002, regulation 27 (230 V and 50 Hz).
- Federal Trade Commission. 16 CFR Part 305 (Energy Labeling Rule): light bulbs are measured at 120 V.
- Royal Decree 842/2002, Low Voltage Electrical Regulation (Spain), consolidated text in the BOE, ITC-BT-25 (table 1: 230 V between phase and neutral), in Spanish.
- Diario Oficial de la Federación (Mexico). NOM-001-SEDE-2012, Electrical installations (use), 110-4 “Voltages”, in Spanish.
Frequently asked questions
Can I plug a 110 V charger into a 220 V outlet?
Check its label. Almost all phone and laptop chargers say “100–240 V”, and then yes: you only need an adapter if the plug shape doesn’t fit. If it says only 110 or 120 V, don’t plug it into 220 V: you’d need a converter.
What happens if I plug a 220 V appliance into 110 V?
It usually won’t burn out, but it won’t work properly. Heating appliances give only a quarter of their power: a 2,000 W kettle would give about 544 W. Appliances with motors or electronics don’t follow that sum: if the label doesn’t include your voltage, don’t use it.
What’s the difference between an adapter and a converter?
An adapter only changes the plug shape; it doesn’t change the voltage. A converter does change the voltage, and it must handle at least the watts of the appliance you connect.
