Watt (W)

Watt definition

Watt is the unit of power (symbol: W).

The watt unit is named after James Watt, the inventor of the steam engine.

One watt is defined as the energy consumption rate of one joule per second.

1W = 1J / 1s

One watt is also defined as the current flow of one ampere with voltage of one volt.

1W = 1V × 1A

Watt to mW, kW, MW, GW, dBm, dBW conversion calculator

Convert watt to milliwatt, kilowatt, megawatt, gigawatt, dBm, dBW.

Enter the power in one of the text boxes and press the Convert button:

mW
W
kW
MW
GW
dBm
dBW

Table of unit prefixes of watt

name symbol conversion example
picowatt pW 1pW = 10-12 W P = 10 pW
nanowatt nW 1nW = 10-9 W P = 10 nW
microwatt μW 1μW = 10-6 W P = 10 μW
milliwatt mW 1mW = 10-3 W P = 10 mW
watt W - P = 10 W
kilowatt kW 1kW = 103 W P = 2 kW
megawatt MW 1MW = 106 W P = 5 MW
gigawatt GW 1GW = 109 W P = 5 GW

How to convert watts to kilowatts

The power P in kilowatts (kW) is equal to the power P in watts (W) divided by 1000:

P(kW) = P(W) / 1000

How to convert watts to milliwatts

The power P in milliwatts (mW) is equal to the power P in watts (W) times 1000:

P(mW) = P(W) ⋅ 1000

How to convert watts to dBm

The power P in decibel-milliwatts (dBm) is equal to 10 times base 10 logarithm of the power P in milliwatts (mW) divided by 1 milliwatt:

P(dBm) = 10 ⋅ log10( P(mW) / 1mW)

How to convert watts to amps

The current I in ampers (A) is equal to the power P in watts (W) divided by the voltage V in volts (V):

I(A) = P(W) / V(V)

How to convert watts to volts

The voltage V in volts (V) is equal to the power P in watts (W) divided by the current I in ampers (A):

V(V) = P(W) / I(A)

How to convert watts to ohms

R(Ω) = P(W) / I(A)2

R(Ω) = V(V)2/ P(W)

How to convert watts to btu/hr

P(BTU/hr) = 3.412142 ⋅ P(W)

How to convert watts to joules

E(J) = P(W)t(s)

How to convert watts to horsepower

P(HP) = P(W) / 746

How to convert watts to kVA

The real power P in watts (W) is equal to 1000 times the apparent power S in kilo-volt-amperes (kVA) times the power factor (PF) or cosine of the phase angle φ:

P(W) = 1000 ⋅ S(kVA)PF = 1000 ⋅ S(kVA) ⋅ cos φ

How to convert watts to VA

The real power P in watts (W) is equal to the apparent power S in volt-amperes (VA) times the power factor (PF) or cosine of the phase angle φ:

P(W) = S(VA)PF = S(VA) ⋅ cos φ

Power consumption of some electrical components

How many watts does a house use? How many watts does a TV use? How many watts does a refrigerator use?

Electric component Typical power consumption in watts
LCD TV 30..300 W
LCD monitor 30..45 W
PC desktop computer 300..400 W
Laptop computer 40..60 W
Refrigerator 150..300 W (when active)
Light bulb 25..100 W
Fluorescent light 15..60 W
Halogen light 30..80 W
Speaker 10..300 W
Microwave 100..1000 W
Air conditioner 1..2 kW

Distinction between watts and watt-hours

The terms power and energy are closely related but distinct physical quantities. Power is the rate at which energy is generated or consumed and hence is measured in units (e.g. watts) that represent energy per unit time.

For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt hours (W·h), 0.1 kilowatt hour, or 360 kJ. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours.

Power stations are rated using units of power, typically megawatts or gigawatts (for example, the Three Gorges Dam in China, is rated at approximately 22 gigawatts). This reflects the maximum power output it can achieve at any point in time. A power station's annual energy output, however, would be recorded using units of energy (not power), typically gigawatt hours. Major energy production or consumption is often expressed as terawatt hours for a given period; often a calendar year or financial year. One terawatt hour of energy is equal to a sustained power delivery of one terawatt for one hour, or approximately 114 megawatts for a period of one year:

Power output = energy / time

1 terawatt hour per year = 1×1012 Wh / (365 days × 24 hours per day) ≈ 114 million watts,

equivalent to approximately 114 megawatts of constant power output.

The watt second is a unit of energy, equal to the joule. One kilowatt hour is 3,600,000 watt seconds.

While a watt per hour exists in principle (as a unit of rate of change of power with time[iii]), it is not correct to refer to a watt (or watt hour) as a "watt per hour".

James Watt (1736 - 1819)

Watt was a Scottish inventor and mechanical engineer, renowned for his improvements in steam engine technology.

James Watt was born in Greenock on 18 January 1736. His father was a prosperous shipwright. Watt initially worked as a maker of mathematical instruments, but soon became interested in steam engines.

The first working steam engine had been patented in 1698 and by the time of Watt's birth, Newcomen engines were pumping water from mines all over the country. In around 1764, Watt was given a model Newcomen engine to repair. He realised that it was hopelessly inefficient and began to work to improve the design. He designed a separate condensing chamber for the steam engine that prevented enormous losses of steam. His first patent in 1769 covered this device and other improvements on Newcomen's engine.

Watt's partner and backer was the inventor John Roebuck. In 1775, Roebuck's interest was taken over by Matthew Boulton who owned an engineering works in Birmingham. Together he and Watt began to manufacture steam engines. Boulton & Watt became the most important engineering firm in the country, meeting considerable demand. Initially this came from Cornish mine owners, but extended to paper, flour, cotton and iron mills, as well as distilleries, canals and waterworks. In 1785, Watt and Boulton were elected fellows of the Royal Society.

By 1790, Watt was a wealthy man and in 1800 he retired and devoted himself entirely to research work. He patented several other important inventions including the rotary engine, the double-action engine and the steam indicator, which records the steam pressure inside the engine.

Watt died on 19 August 1819. A unit of measurement of electrical and mechanical power - the watt - is named in his honour.


Kilowatt (kW) »


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