Watt

The watt (symbol: W) is a unit used to measure power or the rate at which energy is used. One watt equals one joule of energy used in one second. This unit helps us understand how quickly energy moves from one place to another.

The watt is part of the International System of Units, which includes many measurements we use every day. It was named to honor James Watt, a Scottish inventor and engineer. In the 1700s, James Watt made big improvements to steam engines. These engines helped start the Industrial Revolution, changing how people worked and lived.

Overview

The watt is a unit that measures how much energy is being used or transferred in one second. It is named after James Watt, who made big improvements to 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 during a meeting of the British Association for the Advancement of Science.

Later, in 1908, the watt was officially defined at a meeting in London. In 1948, the definition was changed to match the way scientists measure energy and time. Then, in 1960, the watt became part of the International System of Units, which is used all over 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. For example, to know how well devices like FM tuners work, we measure the tiny energy they 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, and the average home in the United States uses about one kilowatt of electricity.

Earth’s surface gets about one kilowatt of sunlight on a clear day at midday near the equator.

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, and some use even more. U.S. nuclear power plants can have capacities from about 500 MW to 1300 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. By 2010, power shortages in China’s Shanxi province were expected to reach 5–6 GW, and Germany had 25.8 GW of wind power. The largest unit at the Doel Nuclear Power Station can produce up to 1.04 GW. HVDC converters can be built for up to 2 GW.

Terawatt

In 2019, the world's total energy use was about 160,000 terawatt-hours, meaning an average continuous power of 18 TW. 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. For example, Lawrence Livermore’s Nova laser reached 1.25 PW, and Japan’s LFEX laser reached 2 PW. The total power of sunlight hitting Earth’s atmosphere is about 174 PW, and Earth’s warming was about 0.5 PW in 2019.

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, which 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 the power of their transmitters using watts. This measures how strong the signal is. It shows how much power a special type of antenna would need to send out the same strength of signal as the station does.

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.