Watt
SI derived unit · quantity: power, flow of energy
01 · Definition
The power at which one joule of energy is spent per second. Not an amount but a rate: the watt answers “how fast”, not “how much”.
The difference between the watt and the watt-hour is the commonest confusion in household energy. A two-thousand-watt kettle does not “contain” energy; it spends energy at a certain rate, and how much has been spent is what the meter shows in kilowatt-hours. The same job can be done quickly at high power or slowly at low: a litre of water is boiled both by a kettle in three minutes and by a weak immersion heater in half an hour, and the energy used is the same. Hence the rule: power decides the time, energy decides the bill.
An appliance’s power says how fast energy flows; the meter counts how much has flowed in. That is why an eight-watt LED lamp and a sixty-watt incandescent give the same light yet produce different figures on the monthly bill. For the same reason a battery’s capacity is quoted in watt-hours rather than watts: the battery stores a quantity and releases it at whatever rate the load asks for.
02 · Conversion
Watts, horsepower, calories per hour
There are two horsepowers: the metric one of seven hundred and thirty-five and a half watts and the imperial one of seven hundred and forty-six. The difference is about one and a half per cent, and in car specifications it shows.
James Watt derived the horsepower from watching horses at a brewery: he took it that a horse lifts thirty-three thousand pounds by one foot in a minute, and sold steam engines by comparison with that figure. The measure proved generous — a real horse cannot sustain it for long. The metric horsepower came later as seventy-five kilogram-metres per second and does not agree with the imperial one.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| W | watt, the SI unit | 1 | domestic and scientific use |
| kW | kilowatt | 0.001 | engines, heating |
| MW | megawatt | 1·10⁻⁶ | industry, power plants |
| GW | gigawatt | 1·10⁻⁹ | generating units, power grids |
| mW | milliwatt | 1000 | electronics, lasers |
| J/s | joule per second | 1 | the same as the watt |
| hp | metric horsepower | 0.0014 | European vehicle papers |
| hp | imperial horsepower | 0.0013 | English-language engineering |
| kcal/h | kilocalorie per hour | 0.8598 | boiler heat output |
| BTU/h | British thermal unit per hour | 3.41 | air conditioners and heaters |
| kgf·m/s | kilogram-force metre per second | 0.102 | older mechanical calculations |
| erg/s | erg per second, the CGS system | 1·10⁷ | astrophysics, luminosities |
The joule per second and the watt are one and the same. The current is computed for a resistive load at unity power factor; for motors and switching power supplies it will be higher
03 · Orders of magnitude
from a wristwatch to the luminosity of the SunOne watt — a charger left idle
04 · Measuring instruments
What power is measured with
The electrodynamic instrument multiplies current by voltage mechanically: one coil sits in the break of the circuit, the other across the load. The needle shows active power directly and takes the phase shift into account with no correction at all.
The engine is loaded with a brake, the shaft torque and the speed of rotation are measured and multiplied — and out comes the power. This is exactly how the figures in a car’s papers are obtained; on a rolling road the transmission losses are added in and the result falls below the factory value.
A thermopile with a blackened face absorbs the beam entirely and puts out a voltage proportional to the heat flow. The range runs from the milliwatts of a bench laser to the kilowatts of industrial cutting.
Under a glass dome a thermopile compares the warming of a black and a white patch and gives the flux density in watts per square metre. At a clear noon the instrument reads about a thousand — and that is the ceiling for any solar panel beneath it.
05 · Writing rules
Capital W, no full stop
The symbol comes from a surname and is therefore capital, while the name in running text is lowercase: watt, but W. The prefix kilo takes a small letter, mega and giga capitals. The Russian symbol is Вт.
The third example confuses power with energy: kilowatt-hours can be consumed, whereas a kilowatt is a rate of consumption. The watt-hour is written with a multiplication dot and a space after the number. Horsepower is not recommended in technical documents but survives in vehicle papers: in many countries the tax is reckoned by it, and both figures appear on the registration.
06 · Neighbouring units
The joule divided by the second gives the watt; the watt multiplied by an hour gives energy back. The volt-ampere has the same dimension but is a different quantity.
In the neighbouring Simetrium passports: joule as energy, and kilowatt-hour as its everyday unit, volt-ampere and var for alternating-current power, horsepower for engineering, and lumen — luminous flux, which is often confused with the electrical power of a lamp.
07 · Historical section
How a machine came to be compared with a horse
James Watt had to explain to buyers how many horses his engine would replace. He measured the work of horses at a gin and set the horsepower at thirty-three thousand foot-pounds per minute, with a margin in the buyer’s favour.
Continental Europe rendered the same idea in its own units: seventy-five kilogram-metres per second. The number came out different, the name stayed the same, and ever since two horsepowers have coexisted in engine specifications.
The International Congress of Electricians fixed the watt for electrical power and named it after Watt. So the inventor of the steam engine gave his name to the unit by which lamps and transformers are measured.
For decades buyers chose a lamp by its watts, used to the idea that sixty meant a familiar brightness. With LEDs the link broke: the same light comes from eight watts, and lumens had to be printed on the box instead.
The watt became the unit through which the kilogram is defined
Power was long thought a purely practical quantity, until it turned up at the foundation of the metrology of mass. The watt balance, devised by Bryan Kibble in seventy-five, balances the weight of a mass against the force on a coil in a magnetic field, and in two modes of measurement compares mechanical power with electrical. The electrical side is expressed through the Josephson and Hall quantum effects, that is, through the Planck constant. After the system was revised in twenty nineteen the constant was fixed as a number, and the balance began to work the other way round: it no longer measures the kilogram but realises it. The instrument was renamed the Kibble balance, but the principle stayed the same — the equality of two powers, mechanical and electrical, expressed in the very same watts.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with names of their own, and the non-SI units that neither engineering nor daily life can do without. Each gets its own sheet: definition, conversion, instruments, writing rules. In 14 languages.
SI derived units with names of their own
the watt passport is openSI base units
in terracotta — the kilogram, metre and second, out of which the watt is assembledNon-SI units
in terracotta — units of power and of energyPassport language
14 languages. The symbol W is international, but the usual measure of engine power differs: metric horsepower in Europe, imperial in the United States, kilowatts in the paperwork.