Watt

The watt (symbol: W) is a unit used to measure power or how fast energy is used. One watt means one joule of energy used in one second.

The watt is part of the International System of Units. It was named after James Watt, a Scottish inventor. In the 1700s, James Watt improved steam engines. These engines helped begin the Industrial Revolution, changing work and life for many people.

Overview

The watt is a unit that measures how much energy is used or moved in one second. It is named after James Watt, who improved steam engines a long time ago.

One watt happens when something moves at a steady speed of one meter per second against a steady force of one newton. For electricity, one watt is the power used when a current of one ampere flows across a voltage of one volt.

History

The watt is named after the Scottish inventor James Watt. In 1882, C. William Siemens suggested using the name "watt" for a unit of power. This was during a meeting of the British Association for the Advancement of Science.

In 1908, the watt was officially defined in London. In 1948, the definition was changed to match how scientists measure energy and time. In 1960, the watt became part of the International System of Units, used around the world to measure things.

Multiples

For additional examples of the size of these power units, see Orders of magnitude (power).

Attowatt

The sound intensity in water for a standard reference is about 0.65 aW/m².

Femtowatt

Very small powers, measured in femtowatts, are often used in radio and radar receivers. We measure the tiny energy devices like FM tuners receive, often in units called dBf, which means decibels compared to 1 femtowatt.

Picowatt

Picowatts are used in radio and radar receivers, acoustics, and radio astronomy. One picowatt is the standard reference for sound power when measured in decibels.

Nanowatt

Nanowatts are also used for radio and radar receivers.

Microwatt

Microwatts are used in medical devices like EEG and ECG machines, scientific tools, and small solar cells for calculators and watches.

Milliwatt

A typical laser pointer gives out about five milliwatts of light, and a hearing aid uses less than one milliwatt. Audio and other signals are often measured in dBm, which means decibels compared to one milliwatt.

Watt

Computer power supplies are measured in watts, and modern graphics cards can use a few hundred watts.

Kilowatt

The kilowatt is used for engines, electric motors, tools, machines, heaters, and the power of radio and TV transmitters. One kilowatt is about the same as 1.34 horsepower. A small electric heater can use one kilowatt.

Megawatt

Large machines and events, such as big electric motors, warships, data centers, and scientific equipment, use megawatts of power. Big buildings may use several megawatts. Modern electric trains can use 5 to 6 MW.

Gigawatt

A gigawatt is about the average yearly power use for a city of 1.8 million people and is used for large power plants, power grids, and big data centers. The largest unit at the Doel Nuclear Power Station can produce up to 1.04 GW.

Terawatt

Earth emits about 47 TW, much less than the energy it receives from the Sun. Very powerful lasers from the 1960s to 1990s could produce terawatts for very short times, and a typical lightning strike reaches about 1 TW but only lasts for very brief moments.

Petawatt

Today’s lasers can produce petawatts for very short times, measured in picoseconds. The total power of sunlight hitting Earth’s atmosphere is about 174 PW.

Yottawatt

The Sun’s power output is 382.8 YW, which is about 2 billion times the power that reaches Earth’s atmosphere.

Conventions in the electric power industry

In the electric power industry, there are special ways to talk about different kinds of power. Megawatt electrical (MWe or MW_e) is the electricity made by a generator. Megawatt thermal or thermal megawatt (MWt, MW_t, or MWth, MW_th) is the heat power made by a plant. And megawatt mechanical (MW_m) is the mechanical power. For example, the Embalse nuclear power plant in Argentina uses a fission reactor to make heat. This heat creates steam to turn a turbine and produce electricity.

When talking about alternating current electricity, there is a difference between the watt and the volt-ampere. These are the same for simple resistive circuits, but they can be different when circuits have electrical reactance.

Radio transmission

Main article: Effective radiated power

Radio stations often talk about their power in watts. This tells us how strong their signal is. It shows how much power a special antenna would need to send the same signal strength.

Distinction between watts and watt-hours

Power and energy are related but different ideas. Power tells us how fast energy is used or made, and we measure it in watts. For example, a light bulb that uses 100 watts for one hour uses 100 watt-hours of energy.

Power stations, like the Three Gorges Dam, are measured in big units such as megawatts or gigawatts, showing how much power they can make at one moment. But when we talk about how much energy they make in a year, we use units like gigawatt hours or terawatt hours. One terawatt hour means a constant power of one terawatt for one hour.