Unit designation W (watt). What is measured in watts: definition Unit of measurement of electrical power in the SI system

When choosing a hair dryer, blender or vacuum cleaner in a store, you will notice that its front panel always contains numbers with the Latin letter W. Moreover, according to sellers, the higher its value, the better and faster this appliance will perform its direct functions. Is such a statement correct? Perhaps this is another publicity stunt? How does W stand for, and what is this value? Let's find out the answers to all these questions.

Definition

The above letter is a Latin abbreviation for the value well known to everyone from physics lessons - watt (watt). According to the standards of the international SI system, W (W) is a unit of power.

If we return to the issue with the characteristics of household electrical appliances, then the higher the number of watts in any of them, the more powerful it is.

For example, there are two blenders in the window with the same cost: one of them is from a popular company with 250 W (W), the other is from a less well-known manufacturer, but with a power of 350 W (W).

These figures mean that the second will chop or beat the products faster than the first for the same period of time. Therefore, if the buyer is primarily interested in the speed of the process, it is worth choosing the second option. If speed does not play a key role, you can purchase the first one, as it is more reliable and possibly durable.

Who came up with the idea of ​​using watts

Oddly enough, it sounds today, but before the advent of watts, horsepower (hp, in English - hb) was the unit of power almost all over the world, less often foot-pound-force per second was used.

Watts were named after the person who invented and implemented this unit - the Scottish engineer and inventor James Watt. Because of this, this term is abbreviated with a capital W (W). The same rule applies to any unit in the SI system named after a scientist.

The name, like the unit of measurement itself, was first officially considered in 1882 in Great Britain. After that, it took a little less than a hundred years for watts to be accepted all over the world and become one of the units of the International SI system (this happened in 1960).

Formulas for finding power

From the lessons of physics, many people remember a variety of tasks in which it was necessary to calculate the current power. Both then and today, the formula is used to find watts: N \u003d A / t.

It was deciphered as follows: A is the amount of work divided by the time (t) during which it was completed. And if we also remember that work is measured in Joules, and time is measured in seconds, it turns out that 1 W is 1J / 1s.

The above formula can be slightly modified. To do this, it is worth remembering the simplest scheme for finding work: A \u003d F x S. According to it, it turns out that the work (A) is equal to the derivative of the force making it (F) to the path traveled by the object under the influence of this force (S). Now, to find the power (watts), we combine the first formula with the second. It turns out: N \u003d F x S / t.

Sub-multiple watts

Having dealt with the question “Watts (W) - what is it?”, It is worth knowing which submultiple units can be formed based on the available data.

In the manufacture of measuring instruments for medical purposes, as well as important laboratory research, it is necessary that they have incredible accuracy and sensitivity. After all, not just the result, but sometimes the life of a person depends on it. Such "sensitive" devices, as a rule, need little power - ten times less than a watt. In order not to suffer with degrees and zeros, submultiple watt units are used to determine it: dW (deciwatts - 10 -1), cW (centiwatts - 10 -2), mW (milliwatts - 10 -3), μW (microwatts - 10 -6 ), nW (nanowatts -10 -9) and several smaller ones, up to 10 -24 - iW (ioktowatts).

With most of the above submultiple units, an ordinary person does not encounter in everyday life. As a rule, only scientists-researchers work with them. Also, these values ​​appear in various theoretical calculations.

Watts, kilowatts and megawatts

Having dealt with the submultiples, it is worth considering multiple units of watts. Every person encounters them quite often when heating water in an electric kettle, charging a mobile phone or performing other daily “rituals”.

In total, scientists have identified about a dozen such units to date, but only two of them are widely known - kilowatts (kW - kW) and megawatts (MW, MW - in this case, the capital letter “m” is put so as not to confuse this unit with milliwatts - mW).

One kilowatt is equal to a thousand watts (10 3 W), and one megawatt is equal to a million watts (10 6 W).

As in the case of submultiple units, there are special ones among multiples that are used only in narrow-profile enterprises. So, power plants sometimes use GW (gigawatts - 10 9) and TW (terawatts - 10 12).

In addition to those mentioned above, there are petawatts (PVt - 10 15), exawatts (EWt - 10 18), zettawatts (ZWt - 10 21) and iottawatts (IVt - 10 24). Like extra small submultiples, large multiples are mainly used in theoretical calculations.

Watt vs Watt Hour: What's the Difference?

If power is displayed on electrical appliances with the letter W (W), then when looking at a conventional household electricity meter, you can see a slightly different abbreviation: kW⋅h (kWh). It stands for "kilowatt hour".

In addition to them, watt-hours (W⋅h - W⋅h) are also distinguished. It is worth noting that, according to international and domestic standards, such units in abbreviated form are always written only with a dot, and in the full version - through a dash.

Watt hours and kilowatt hours are distinct units from watts and kW. The difference lies in the fact that with their help it is not the power of the transmitted electricity that is measured, but it is directly measured. That is, kilowatt-hours show how much of it was produced (transferred or used) per unit of time (in this case, one hour).

A table with power units is given in OK 015-94 (MK 002-9) OKEI. Power units are included in

OKEI is the All-Russian Classification of Units of Measurement (OKEI), which is a document in the field of the national standardization system.

OKEI is developed on the basis of:

• UNECE International Classification of Units of Measurement "Codes for units of measurement used in international trade"
• of the Commodity Nomenclature of Foreign Economic Activity (TN VED) in terms of the units of measurement used and taking into account the requirements of international standards ISO 31 / 0-92 “Values ​​and units of measurement. Part 0. General principles” and ISO 1000-92 "SI units and recommendations on the use of multiple units and some other units.

SI — international system of units physical quantities, a modern version of the metric system. (the metric system is the common name for the international decimal system of units based on the use of the meter and kilogram)

Let's single out from tables with only tables with power measurement values.

According to section 1 OK 015-94 (MK 002-9):

International power units (SI) included in OKEI

According to section 2 OK 015-94 (MK 002-9):

National Power Unitsincluded in OKEI

According to Appendix A OK 015-94 (MK 002-9):

International power units (SI) not included in OKEI

Watt (symbol: Tue, W) - in the SI system, a unit of power. The unit is named after the Scotch-Irish mechanical inventor James Watt (Watt), creator of the universal steam engine.

The watt as a unit of power was first adopted at the Second Congress of the British Scientific Association in 1889. Prior to this, most calculations used the horsepower introduced by James Watt, as well as foot-pounds per minute. At the XIX General Conference on Weights and Measures in 1960, the watt was included in the International System.

One of the main characteristics of all electrical appliances is the power they consume, so on any electrical appliance (or in its instructions) you can find information about the number of watts required for its operation.

What is Watt. Definition

1 watt is defined as the power at which 1 joule of work is done in 1 second of time.

Thus, the watt is a derived unit of measurement and is related to other SI units by the following relationships:

W = J / s = kg m² / s³

W = H m/s

W = VA

In addition to mechanical (the definition of which is given above), there are also thermal and electrical power:

1 watt of heat flow power is equivalent to 1 watt of mechanical power.

1 watt of active electrical power is also equivalent to 1 watt of mechanical power and is defined as the power of direct electric current of 1 ampere, doing work at a voltage of 1 volt.

Conversion to other power units

Watt is related to other units of power as follows:

1 W = 107 erg/s

1 W ≈ 0.102 kgf m/s

1 W ≈ 1.36×10−3 l. With.

1 cal/h = 1.163×10−3 W

How is a kilowatt different from a kilowatt hour?

The prefix "kilo" before any measurement value (watts, amps, volts, grams, etc.) means "thousand".

1 kilowatt (kW) = 1000 watts (W).

Watt- unit power. Power is the rate at which energy is expended. One watt is equal to the power at which work (energy costs) of one joule is performed in one second.

Kilowatt hour- unit of measure used for electricity measurement at home. Means the amount of energy that a device with a power of 1 kilowatt produces / consumes in one hour.

Watt/kilowatt and kilowatt-hour are different concepts.

In order to characterize the speed with which work is done ($A$), the concept of power (P) is used, which is defined as:

expression (1) is the instantaneous power.

Instantaneous power can be defined as:

where $\overline(F)$ is the vector of the force that does the work; $\overline(v)$ - velocity vector of the point, to which the force $\overline(F)$ is applied.

Watt is a unit of power in the SI system

From the definition of power, it can be seen that the unit of power can be taken as:

\[\left=\frac(J)(s).\]

However, the unit of power has its own name: watt - a unit of power. Watt is denoted as W. The power is 1 watt if one joule of work is done in one second. It should be noted that the watt is a unit of power in the International System of Units (SI). The watt is not a basic unit of measure in the SI system. The watt received its name in honor of the inventor J. Watt.

Watt as a unit of power began to be used after 1882. Up to this point, power has been calculated in horsepower or foot-pounds per minute. In the SI system, watt is a unit of power since 1960 (since the adoption of the system itself).

Using the definition of instantaneous power (2), it is easy to get the combination of basic units from which the watt is derived.

\[\left=H\cdot \frac(m)(s)=kg\cdot \frac(m)(s^2)\cdot \frac(m)(s)=kg\cdot \frac(m^2 )(c^3).\]

Definitions (1) and (2) are mechanical definitions of power. Let's single out the electric instantaneous power:

where $I$ is the current strength in a certain section of the circuit; $U$ - voltage in the area under consideration. Watt is a unit of measurement of electrical power, while from definition (3), it follows that:

\[\left=A\cdot B,\]

where $\left=A$ (amps); $\left=B$ (volts).

Power units in other systems of units

In the CGS system (the system in which the main units are: centimeter, gram and second), the unit of power does not have a special name. In this system:

\[\left=\frac(erg)(c),\]

where $erg$ is the CGS unit of energy (work) measurement.

Horsepower (hp) is a non-systemic unit of power. In the world, several different units are distinguished, calling them "horsepower". In our country, we mean "metric horsepower", they consider:

\ \

This unit has practically been taken out of use in calculations. However, it is still used, for example, when calculating vehicle taxes.

Examples of problems with a solution

Example 1

Exercise. Show that the unit of electrical power is the watt.

Solution. We will take the definition of instantaneous electric power as the basis for solving the problem:

The unit of current (ampere) is the main one in the International System of Units:

\[\left=A\ (1.2).\]

The unit of voltage is auxiliary, let's figure out how to express it through the basic units of the SI system. We use the definition of voltage ($U$) in the form:

where $A"$ is the work of the electric field on the transfer of a test charge from one point of the field to another; $q$ is the magnitude of the charge.

\[\left=H\cdot m=kg\cdot \frac(m^2)(c^2)(1.4).\] \[\left=Cl=A\cdot c(1.5).\]

From the two previous equalities we have:

\[\left=kg\cdot \frac(m^2)(s^2):A\cdot c=kg\frac(m^2)(A\cdot c^3)\left(1.6\right). \]

To obtain the power dimension, we use (1.1), (1.2) and (1.6):

\[\left=kg\frac(m^2)(A\cdot c^3)\cdot A=kg\frac(m^2)(c^3)\ \left(1.7\right).\]

In expression (1.7) we have received the unit of measurement of mechanical power, namely the watt, expressed in terms of the basic units of the SI system.

Example 2

Exercise. A body with mass $m,$ falls from a height $h$. What is the instantaneous power of gravity at the height $\frac(h)(2)$? Air resistance is ignored. Check the units of the resulting value.

Solution. Let's make a drawing.

Knowing that the body moves under the action of gravity, we write the kinematic equation of body motion:

where from the choice of reference system (Fig. 1) it can be seen that $y_0=0.\ $The initial velocity of the body is equal to zero ($v_0=0$).

Find the moment of time ($t"$) at which the body reaches the height $\frac(h)(2)$. To do this, set $y=\frac(h)(2)$:

\[\frac(h)(2)=\frac(g(t")^2)(2)\to t"=\sqrt(\frac(h)(g))\left(2.2\right). \]

Equation for body speed:

\[\overline(v)=\overline(g)t\ \to v=gt\ \left(2.3\right).\]

The speed of the body at the time equal to $t"$:

We find the instantaneous speed as:

in our case $(\cos \alpha =1,\ )\ $since the force doing the work (gravity) is co-directed with the velocity vector of the body. For the moment of time we are considering ($t"$), we get the instantaneous power equal to:

Let's check the units of measurement of the value, which is obtained on the right side of the final formula:

\[\left=kg\ \sqrt(m\cdot \frac(m^3)(s^6))=kg\frac(m^2)(s^3)=W\]

Answer.$P\left(t"\right)=m\sqrt(hg^3)$

Kilowatt is a multiple unit derived from "Watt"

Watt

Watt(W, W) - system unit of power measurement.
Watt- a universal derived unit in the SI system, which has a special name and designation. As a unit of power, "watt" was recognized in 1889. Then this unit was named after James Watt (Watt).

James Watt - the man who invented and made the universal steam engine

As a derived unit of the SI system, "watt" was included in it in 1960.
Since then, the power of everything is measured in Watts.

In the SI system, in Watts, it is allowed to measure any power - mechanical, thermal, electrical, etc. The formation of multiples and submultiples from the original unit (Watt) is also allowed. To do this, it is recommended to use a set of standard SI system prefixes, such as kilo, mega, giga, etc.

Power units, multiples of watts:

• 1 watt
• 1000 watts = 1 kilowatt
• 1000,000 watts = 1000 kilowatts = 1 megawatt
• 1000,000,000 watts = 1000 megawatts = 1000,000 kilowatts = 1 gigawatt
• etc.

Kilowatt hour

There is no such unit of measure in the SI system.
Kilowatt hour(kW⋅h, kW⋅h) is a non-systemic unit that was developed solely to account for the electricity used or produced. In kilowatt-hours, the amount of electricity consumed or produced is taken into account.

The use of "kilowatt-hour" as a unit of measurement in Russia is regulated by GOST 8.417-2002, which clearly indicates the name, designation and scope for "kilowatt-hour".

Download GOST 8.417-2002 (downloads: 3230)

Extract from GOST 8.417-2002 “State system for ensuring the uniformity of measurements. Units of quantities”, clause 6 Units not included in the SI (fragment of table 5).

Non-systemic units acceptable for use on a par with SI units

What is a kilowatt hour for?

GOST 8.417-2002 recommends using "kilowatt-hour" as the basic unit of measure for accounting for the amount of electricity used. Because "kilowatt-hour" is the most convenient and practical form that allows you to get the most acceptable results.

At the same time, GOST 8.417-2002 absolutely does not object to the use of multiple units formed from "kilowatt-hour" in cases where this is appropriate and necessary. For example, during laboratory work or when accounting for the generated electricity at power plants.

Educated multiples of "kilowatt-hour" look, respectively:

• 1 kilowatt hour = 1000 watt hour
• 1 megawatt hour = 1000 kilowatt hour
• etc.

How to write kilowatt-hour?

Spelling of the term "kilowatt-hour" according to GOST 8.417-2002:

• the full name must be written with a hyphen:
  watt hour, kilowatt hour
• short designation must be written with a dot:
  Wh, kWh, kWh

Note. Some browsers misinterpret the HTML code of the page and display a question mark (?) or some other shorthand instead of a dot (⋅).

Analogs GOST 8.417-2002

Most of the national technical standards of the current post-Soviet countries are linked to the standards of the former Soviet Union, therefore, in the metrology of any country in the post-Soviet space, you can find an analogue of the Russian GOST 8.417-2002, or a link to it, or its revised version.

Designation of power of electrical appliances

A common practice is to mark the power of electrical appliances on their case.
The following designation of the power of electrical equipment is possible:

• in watts and kilowatts (W, kW, W, kW)
  (designation of the mechanical or thermal power of an electrical appliance)
• in watt-hours and kilowatt-hours (W⋅h, kW⋅h, W⋅h, kW⋅h)
  (designation of the consumed electrical power of the electrical appliance)
• in volt-amperes and kilovolt-amperes (VA, kVA)
  (designation of the total electrical power of the electrical appliance)

Units of measure for indicating the power of electrical appliances

watt hour and kilowatt hour (W⋅h, kW⋅h, W⋅h, kW⋅h)- off-system units of measurement of consumed electrical energy (power consumption). Power consumption is the amount of electricity consumed by electrical equipment per unit of time of its operation. Most often, "watt-hours" and "kilowatt-hours" are used to refer to the power consumption of household electrical equipment, according to which it is actually chosen.

volt-ampere and kilovolt-ampere (VA, kVA, VA, kVA)— Units of measurement of electrical power in the SI system, equivalent to watts (W) and kilowatts (kW). Used as units of measure for apparent AC power. Volt-amperes and kilovolt-amperes are used in electrical calculations in cases where it is important to know and operate with electrical concepts. In these units of measurement, you can designate the electrical power of any AC electrical appliance. Such a designation will best meet the requirements of electrical engineering, from the point of view of which all AC electrical appliances have active and reactive components, so the total electrical power of such a device should be determined by the sum of its parts. As a rule, in "volt-amperes" and multiples of them, they measure and designate the power of transformers, chokes and other purely electrical converters.

The choice of units of measurement in each case occurs individually, at the discretion of the manufacturer. Therefore, you can find household microwaves from different manufacturers, the power of which is indicated in kilowatts (kW, kW), in kilowatt-hours (kWh, kWh) or in volt-amperes (VA, VA). And the first, and the second, and the third - will not be a mistake. In the first case, the manufacturer indicated the thermal power (as a heating unit), in the second - the consumed electrical power (as an electrical consumer), in the third - the total electrical power (as an electrical appliance).

Since household electrical equipment is low enough to take into account the laws of scientific electrical engineering, then at the household level, all three numbers are practically the same

Given the above, we can answer the main question of the article

Kilowatt and kilowatt hour | Who cares?

• The biggest difference is that kilowatt is a unit of power while kilowatt hour is a unit of electricity. Confusion and confusion arises at the household level, where the concepts of kilowatts and kilowatt-hours are identified with the measurement of the produced and consumed power of a household electrical appliance.
• At the level of a household electrical converter, the difference is only in the separation of the concepts of energy produced and consumed. In kilowatts, the output of thermal or mechanical power of the generating set is measured. In kilowatt-hours, the consumed electrical power of the generating set is measured. For a household appliance, the figures for generated (mechanical or thermal) and consumed (electrical) energy are almost the same. Therefore, in everyday life there is no difference in what concepts to express and in what units to measure the power of electrical appliances.
• Linking units of measurement of kilowatts and kilowatt-hours is applicable only for cases of direct and reverse conversion of electrical energy into mechanical, thermal, etc.
• It is absolutely unacceptable to use the unit of measurement "kilowatt-hour" in the absence of an electricity conversion process. For example, in "kilowatt-hour" you cannot measure the power consumption of a wood-fired heating boiler, but you can measure the power consumption of an electric heating boiler. Or, for example, in "kilowatt-hour" you cannot measure the power consumption of a gasoline engine, but you can measure the power consumption of an electric motor
• In the case of direct or reverse conversion of electrical energy into mechanical or thermal energy, you can link a kilowatt-hour with other units of energy measurement using the online calculator of the site tehnopost.kiev.ua: