Electric current

What Is a Watt?

The  watt  (symbol:  W)  is the power unit of the International System of Units. Its value indicates the amount of energy in joules that is converted, used or dissipated in one second. 

What is a watt?

One watt is equivalent to 1 joule per second (1 J / s).

It is a derived unit that takes its name from the Scottish engineer, inventor and builder of instruments James Watt, for his contribution to the development of the steam engine, which was one of the triggers for the start of the Industrial Revolution.

The watt was adopted by the Second Congress of the British Association for the Advancement of Science in 1889. This adoption led to its international recognition as a unit of power, incorporating it as an additional unit to the practical units defined in the CGS system. It was defined in the International System of Units 1948 through 9 to  General Conference on Weights and Measures, the same conference that joined AMP among others.

In electricity, the watt is the unit of power of a system of an intensity of current of one ampere under a voltage of one volt. It is the product of the voltage by the current intensity:

P(t) = I(t)·U(t)


P (t) is the power as a function of time t, It is expressed in watt.
U (i) is the electrical voltage as a function of time t, It is expressed in volt.
T (i) is the electric current as a function of time t, It is expressed in ampere

James Watt

We can speak of average, effective or instantaneous power depending on the nature of the current and the voltage (direct current or alternating current) and its variation over time.

If we integrate this power with respect to a time of one hour, we will obtain an amount of energy that is usually expressed in a unit of power called kilowatt-hours, equivalent to 3,600,000 joules. The megawatt (MW), one million watts, is a unit commonly used in the field of electrical production.

In an alternating current system, power is expressed in a special way due to the periodic nature of the functions involved, different magnitudes can be established: real power, reactive power, complex power or apparent power.

What Are an Electric Watt and a Thermal Watt?

Sometimes a difference is made between electrical watt and thermal watt in order to clarify whether we are talking about electrical power or thermal power generated.

The term electric watt (symbolized We) corresponds to the production of electrical power, the term stresses that only power generation is spoken of electricity regardless of the heat that can be generated. The usual multiple power units are the electric megawatt (MWe) and the electric gigawatt (GWe).

The term thermal watt (symbolized Wt) is the thermal power unit, with this concept we want to emphasize that only the heat generated is discussed. Its most common multiples are the thermal megawatt (MWt) and the thermal gigawatt (GWt).

This type of differentiation is often used in the field of energy production, for example to differentiate between electricity production and the thermal dissipation of a power plant or nuclear reactor.

The power of a plant is usually expressed in electrical power (that is, in Mwe). The thermal power of a nuclear power plant is usually approximately three times its electrical power. The difference corresponds to the thermodynamic performance, the limitations related to the Carnot cycle, directly related to the operating temperature and the conversion losses.

It must always be taken into account that the transformation of thermal energy into electrical energy can only be done with losses, the efficiency varies between 30 and 40%. The difference is explained due to the need for cooling the reactors, due to the inefficiency of the generators based on steam turbines, etc.

The use of these concepts and symbols is not recommended by the International Bureau of Weights and Measures, which considers that additional symbols cannot be added to symbolize other types of additional information, and the use of these specific symbols that are not defined in the SI is incorrect. . There is only one watt, what changes is what is measured, the measured physical magnitude, not the unit used for the measurement.

What Is the Difference Between Watt and Watt-hour?

When talking it is common to confuse power and energy (work). Actually, power is the rate at which energy (work) is used. One watt is one July per second. If for example a 10-watt light bulb is on for one hour, the energy it will consume will be 10 watt-hours (W · h) or 3,600 joules (J).

The capacity or power of a power plant is measured in watts, but the energy generated annually will be measured in watt-hours (W · h).

What Other Power Units Are There?

In addition to the power unit of the international measurement system, the watt (W). There are other units to measure power.

  • English system power units: horsepower or horsepower, English horsepower (hp). The horsepower conversions are as follows:
    1 HP = 550 ft · lbf / s
    1 HP = 745.7 W
  • In the technical system of units, the power can be expressed in kilometer per second, (kgm / s). The conversion to watts is as follows:
    1 kgm / s = 9.806215 W
  • The power unit can also be expressed in kilocalorie per hour (kcal / h). In this case, the kilocalories can be converted to watts as follows:
    1 kcal / h = 1000 cal / h = 1.1630556 W (watt).
  • In the cegesimal system we have the erg per second, (erg / s). In this case, 1 erg / s = 1x10-7 W.
  • Finally, the horsepower (CV), another unit of power. The horsepower to watts conversion is as follows:
    1 CV = 735.35375 W

Sometimes it happens that the vaito power unit is used in other measurement units that causes confusion. This would be the case of the unit vaito-hora commented previously, but there are other considerations to take into account.

Watt and Watt-hour

The kilowatt-hour (kWh) is not a unit of power, it is a unit of energy. It is common to confuse them due to the tendency to shorten the energy term using kilowatt instead of kilowatt-hour.

Power and energy are easy to confuse. Power can be said to be the rate of energy consumption (or generation). One watt is one joule per second. For example, if a 100W light bulb remains lit for one hour, the power consumed is 100 watt-hours (W · h) or 0.1 kilowatt-hour (kWh) or (60 × 60 × 100) 360,000 joules (J). This would be the same amount of energy that would make a 40W bulb glow for 2.5 hours.

The capacity of a photovoltaic solar power plant is measured in watts, but the energy generated annually is measured in watt-hours. This last unit is not usually used, it is usually passed directly to the kilowatt-hour, or megawatt-hour. The megawatt-hour is used to indicate the power of large nuclear power plants.

Watt-hour and Watt per Hour

Another source of confusion can be caused by the use of incorrect terminology when mentioning energy in kWh, which can be confused if it is read as kilowatt hours or kW / h. This type of unit is related to power generation and can be interesting to express the characteristics of a power plant.

Units of the above type, such as watt per hour (W / h) reflect the changeability of power per hour. Watts per hour (W / h) is useful to characterize the rate of power increase in generation plants. For example, a power plant that reaches a power of 1 MW from zero to 15 minutes has a rate or rate of increase of its power of 4 MW / h. Hydroelectric plants, related to hydropower, have a very high rate of increase in power, which makes them very suitable to face maximum loads and emergency situations.

Annual Consumption and Capacity Data

Most of the energy production or consumption in a period is expressed in terawatt hours consumed or produced. The period used is normally a calendar year or a financial year. A terawatt · hour equals a continuous power consumption (or production) of approximately 114 megawatts over a period of one year.

Sometimes a balance is made of the energy consumed in a year that would represent the installed power making transformations more visible to the receivers of the reports, for example, the continued consumption of 1 kW for a year would result in a need in terms of energy of about 8760 kWh / year. Sometimes watts · year is discussed at conferences on global warming and energy consumption.

Difference Between W and W '

In many physics books the notation W is included to denote work (from the English word work) that must be distinguished from units in watts (work / time). Normally, in books, the work is written with the letter W in italics or with a tracing similar to the freehand.


    Published: May 13, 2015
    Last review: March 25, 2020