Electric current

Watt - Power unit

Watt - Power unit

The watt (symbol: W) is the power unit of the International System, it is the amount of energy in joules that is converted, used or dissipated in one second. It is a derived unit that takes its name from the engineer, inventor and constructor of Scottish instruments James Watt, for his contribution to the development of the steam engine, which was one of the triggers of the beginning of the Industrial Revolution.

The watt was adopted by the Second Congress of the British Association for the Advancement of Science in 1889, which meant its international recognition as a power unit, incorporating it as an additional unit to the practical units defined in the CGS system. It was defined in the International System of Units in 1948 to the 9th General Conference of Weights and Measures, the same conference in which the ampere was incorporated among others.

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

Watt in electricity

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

P (t) = I (t) · U (t)


P (t) is the power as a function of time t, is expressed in watt.U (i) is the voltage as a function of time t, 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-hour, equivalent to 3,600,000 joules. The megawatt (MW), one million watts, is a unit usually used in the field of electrical production.

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

Electric and thermal watt

Sometimes there is a difference between electric watt and thermal watt in order to clarify whether we talk about electric power or thermal power generated.

The term electric watt (symbolized by We) corresponds to the production of electrical power, the term emphasizes that only power generation power is spoken without the heat that can be generated. The usual multiple units of potnecy are the electric megawatt (MWe) and the electric gigawatt (GWe).

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

This type of differentiation is usually used in the field of energy production, for example to differentiate between the production of electricity and the thermal dissipation of a power plant or a nuclear reactor. The power of a plant is usually expressed in electrical power (ie in Mwe). The thermal power of a nuclear power plant is usually about 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 consideration that the transformation of thermal energy into electrical energy can only be done with losses, the yield varies between 30 and 40%. The difference is explained due to the need for cooling of the reactors, due to the inefficiency of 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 can not be added to symbolize other types of additional information, and the use of these specific symbols that are not defined by the SI is wrong There is only one watt, what changes is what is measured, the measured physical magnitude, not the unit used for the measurement.

Difference between watt and watt-hour

When speaking, 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 bulb is lit for one hour, the energy it consumes will be 10 watt-hours (W · h) or 3,600 joules (J).

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

Other power units

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

  • English system power units: horsepower or horsepower, horsepower in English, (hp). Horse power 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 kilogram 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, you can convert kilocalories to watts in the following way:
    1 kcal / h = 1000 cal / h = 1.1630556 W (watt).
  • In the cegesimal system we have the ergium per second, (erg / s). In this case, 1 erg / s = 1x10-7 W.
  • Finally, the horse of steam (CV), another power unit. The conversion of the horsepower to watts is as follows:
    1 CV = 735,35375 W

Common mistakes related to watt

Occasionally, the vaito power unit is used in other measurement units that cause confusion. This would be the case of the vaito-hora unit discussed above, 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 term energy by using kilowatt instead of kilowatt-hour.

Power and energy are easy to confuse. It can be said that power is the rhythm of consumption (or generation) of energy. One watt is one joule per second. For example, if a 100 W bulb stays lit for one hour, the energy consumed is 100 watt-hours (W · h) or 0.1 kilowatt-hour (kW · h) or (60 × 60 × 100) 360,000 joules (J). This would be the same amount of energy that would make a 40 W bulb shine 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, normally it is 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 the energy in kWh, which can be confused if it is read as if it were kilowatt hour or kW / h. This type of unit is related to the generation of energy and can be interesting to express the characteristics of a power plant.

Units of the previous type, such as watt per hour (W / h) reflect the capacity to change the power per hour. The watts per hour (W / h) are useful to characterize the speed of power increase in the generation plants. For example, a power plant that reaches a power of 1 MW from zero to 15 minutes has a rate or speed of increase of its power of 4 MW / h. Hydroelectric plants, related to hydraulic energy, 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

The majority of energy production or consumption in a period expressed in terawatt hours · hour consumed or produced. The period that is used is normally a calendar year or a financial year. A terawatt-hour equals a continuous consumption (or production) of energy 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 more visible transformations for the receivers of the reports, for example, the continuous consumption of 1 kW during a year would result in a need in terms of energy of about 8760 kW · h / year. Sometimes we talk about watts · year at conferences in the field of global warming and energy consumption.

Difference between W and W '

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

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Last review: March 13, 2017