48-455: A kilowatt-hour ( unit symbol : kW⋅h or kW h ; commonly written as kWh ) is a non-SI unit of energy equal to 3.6 megajoules (MJ) in SI units, which is the energy delivered by one kilowatt of power for one hour . Kilowatt-hours are a common billing unit for electrical energy supplied by electric utilities . Metric prefixes are used for multiples and submultiples of the basic unit,
96-483: A barleycorn . A system of measurement is a collection of units of measurement and rules relating them to each other. As science progressed, a need arose to relate the measurement systems of different quantities, like length and weight and volume. The effort of attempting to relate different traditional systems between each other exposed many inconsistencies, and brought about the development of new units and systems. Systems of units vary from country to country. Some of
144-428: A battery is usually expressed indirectly by its capacity in ampere-hours ; to convert ampere-hour (Ah) to watt-hours (Wh), the ampere-hour value must be multiplied by the voltage of the power source. This value is approximate, since the battery voltage is not constant during its discharge, and because higher discharge rates reduce the total amount of energy that the battery can provide. In the case of devices that output
192-401: A change over time. For example: miles per hour, kilometres per hour, dollars per hour. Power units, such as kW, already measure the rate of energy per unit time (kW= kJ / s ). Kilowatt-hours are a product of power and time, not a rate of change of power with time. Watts per hour (W/h) is a unit of a change of power per hour, i.e. an acceleration in the delivery of energy. It is used to measure
240-441: A different voltage than the battery, it is the battery voltage (typically 3.7 V for Li-ion ) that must be used to calculate rather than the device output (for example, usually 5.0 V for USB portable chargers). This results in a 500 mA USB device running for about 3.7 hours on a 2,500 mAh battery, not five hours. The Board of Trade unit (B.T.U.) is an obsolete UK synonym for kilowatt-hour. The term derives from
288-460: A factor of three. While smaller customer loads are usually billed only for energy, transmission services, and the rated capacity, larger consumers also pay for peak power consumption, the greatest power recorded in a fairly short time, such as 15 minutes. This compensates the power company for maintaining the infrastructure needed to provide peak power. These charges are billed as demand changes. Industrial users may also have extra charges according to
336-451: A kilowatt-hour over an eight-hour day. To convert a quantity measured in a unit in the left column to the units in the top row, multiply by the factor in the cell where the row and column intersect. All the SI prefixes are commonly applied to the watt-hour: a kilowatt-hour is 1,000 Wh (kWh); a megawatt-hour is 1 million Wh (MWh); a milliwatt-hour is 1/1,000 Wh (mWh) and so on. The kilowatt-hour
384-503: A period of one year. In 2020, the average household in the United States consumed 893 kWh per month. Raising the temperature of 1 litre of water from room temperature to the boiling point with an electric kettle takes about 0.1 kWh. A 12 watt LED lamp lit constantly uses about 0.3 kW⋅h per 24 hours and about 9 kWh per month. In terms of human power , a healthy adult male manual laborer performs work equal to about half
432-424: A quantity may be described as multiples of that of a familiar entity, which can be easier to contextualize than a value in a formal unit system. For instance, a publication may describe an area in a foreign country as a number of multiples of the area of a region local to the readership. The propensity for certain concepts to be used frequently can give rise to loosely defined "systems" of units. For most quantities
480-500: A result, units of measure could vary not only from location to location but from person to person. Units not based on the human body could be based on agriculture, as is the case with the furlong and the acre , both based on the amount of land able to be worked by a team of oxen . Metric systems of units have evolved since the adoption of the original metric system in France in 1791. The current international standard metric system
528-400: A small set of units is required. These units are taken as the base units and the other units are derived units . Thus base units are the units of the quantities which are independent of other quantities and they are the units of length, mass, time, electric current, temperature, luminous intensity and the amount of substance. Derived units are the units of the quantities which are derived from
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#1732780259667576-611: A subject of governmental regulation, to ensure fairness and transparency. The International Bureau of Weights and Measures (BIPM) is tasked with ensuring worldwide uniformity of measurements and their traceability to the International System of Units (SI). Metrology is the science of developing nationally and internationally accepted units of measurement. In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results
624-433: A unit is necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given. But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only
672-485: A yearly basis, in units such as megawatt-hours per year (MWh/yr) gigawatt-hours/year (GWh/yr) or terawatt-hours per year (TWh/yr). These units have dimensions of energy divided by time and thus are units of power. They can be converted to SI power units by dividing by the number of hours in a year, about 8760 h/yr . Thus, 1 GWh/yr = 1 GWh/8760 h ≈ 114.12 kW . Many compound units for various kinds of rates explicitly mention units of time to indicate
720-470: Is a definite magnitude of a quantity , defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity . Any other quantity of that kind can be expressed as a multiple of the unit of measurement. For example, a length is a physical quantity . The metre (symbol m) is a unit of length that represents a definite predetermined length. For instance, when referencing "10 metres" (or 10 m), what
768-519: Is actually meant is 10 times the definite predetermined length called "metre". The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to the present. A multitude of systems of units used to be very common. Now there is a global standard, the International System of Units (SI), the modern form of the metric system . In trade, weights and measures are often
816-485: Is central to the scientific method . A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures historically developed for commercial purposes. Science , medicine , and engineering often use larger and smaller units of measurement than those used in everyday life. The judicious selection of the units of measurement can aid researchers in problem solving (see, for example, dimensional analysis ). In
864-482: Is commonly used by electrical energy providers for purposes of billing, since the monthly energy consumption of a typical residential customer ranges from a few hundred to a few thousand kilowatt-hours. Megawatt-hours (MWh), gigawatt-hours (GWh), and terawatt-hours (TWh) are often used for metering larger amounts of electrical energy to industrial customers and in power generation. The terawatt-hour and petawatt-hour (PWh) units are large enough to conveniently express
912-409: Is expressed as the product of a numerical value { Z } (a pure number) and a unit [ Z ]: For example, let Z {\displaystyle Z} be "2 metres"; then, { Z } = 2 {\displaystyle \{Z\}=2} is the numerical value and [ Z ] = m e t r e {\displaystyle [Z]=\mathrm {metre} } is the unit. Conversely,
960-450: Is measured in watts , or joules per second . For example, a battery stores energy. When the battery delivers its energy, it does so at a certain power, that is, the rate of delivery of the energy. The higher the power, the quicker the battery's stored energy is delivered. A higher power output will cause the battery's stored energy to be depleted in a shorter time period. Electric energy production and consumption are sometimes reported on
1008-467: Is now defined as exactly 0.0254 m , and the US and imperial avoirdupois pound is now defined as exactly 0.453 592 37 kg . While the above systems of units are based on arbitrary unit values, formalised as standards, natural units in physics are based on physical principle or are selected to make physical equations easier to work with. For example, atomic units (au) were designed to simplify
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#17327802596671056-489: Is the International System of Units (abbreviated to SI). An important feature of modern systems is standardization . Each unit has a universally recognized size. Both the imperial units and US customary units derive from earlier English units . Imperial units were mostly used in the British Commonwealth and the former British Empire . US customary units are still the main system of measurement used in
1104-514: Is the conversion of the unit of measurement in which a quantity is expressed, typically through a multiplicative conversion factor that changes the unit without changing the quantity. This is also often loosely taken to include replacement of a quantity with a corresponding quantity that describes the same physical property. One example of the importance of agreed units is the failure of the NASA Mars Climate Orbiter , which
1152-450: Is typically sold to consumers in kilowatt-hours. The cost of running an electrical device is calculated by multiplying the device's power consumption in kilowatts by the operating time in hours, and by the price per kilowatt-hour. The unit price of electricity charged by utility companies may depend on the customer's consumption profile over time. Prices vary considerably by locality. In the United States prices in different states can vary by
1200-467: Is used with loads or output that vary during the year but whose annual totals are similar from one year to the next. For example, it is useful to compare the energy efficiency of household appliances whose power consumption varies with time or the season of the year. Another use is to measure the energy produced by a distributed power source. One kilowatt-hour per year equals about 114.08 milliwatts applied constantly during one year. The energy content of
1248-884: The 4th and 3rd millennia BC among the ancient peoples of Mesopotamia , Egypt and the Indus Valley , and perhaps also Elam in Persia as well. Weights and measures are mentioned in the Bible (Leviticus 19:35–36). It is a commandment to be honest and have fair measures. In the Magna Carta of 1215 (The Great Charter) with the seal of King John , put before him by the Barons of England, King John agreed in Clause 35 "There shall be one measure of wine throughout our whole realm, and one measure of ale and one measure of corn—namely,
1296-629: The International System of Units (SI). Other representations of the unit may be encountered: The hour is a unit of time listed among the non-SI units accepted by the International Bureau of Weights and Measures for use with the SI. An electric heater consuming 1,000 watts (1 kilowatt) operating for one hour uses one kilowatt-hour of energy. A television consuming 100 watts operating continuously for 10 hours uses one kilowatt-hour. A 40-watt electric appliance operating continuously for 25 hours uses one kilowatt-hour. Electrical energy
1344-484: The United States outside of science, medicine, many sectors of industry, and some of government and military, and despite Congress having legally authorised metric measure on 28 July 1866. Some steps towards US metrication have been made, particularly the redefinition of basic US and imperial units to derive exactly from SI units. Since the international yard and pound agreement of 1959 the US and imperial inch
1392-433: The annual electricity generation for whole countries and the world energy consumption . A kilowatt is a unit of power (rate of flow of energy per unit of time). A kilowatt-hour is a unit of energy. Kilowatt per hour would be a rate of change of power flow with time. Work is the amount of energy transferred to a system; power is the rate of delivery of energy. Energy is measured in joules , or watt-seconds . Power
1440-401: The metric system , the imperial system , and United States customary units . Historically many of the systems of measurement which had been in use were to some extent based on the dimensions of the human body. Such units, which may be called anthropic units , include the cubit , based on the length of the forearm; the pace , based on the length of a stride; and the foot and hand . As
1488-405: The power factor of their load. Major energy production or consumption is often expressed as terawatt-hours (TWh) for a given period that is often a calendar year or financial year . A 365-day year equals 8,760 hours, so over a period of one year, power of one gigawatt equates to 8.76 terawatt-hours of energy. Conversely, one terawatt-hour is equal to a sustained power of about 114 megawatts for
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1536-553: The social sciences , there are no standard units of measurement. A unit of measurement is a standardized quantity of a physical property, used as a factor to express occurring quantities of that property. Units of measurement were among the earliest tools invented by humans. Primitive societies needed rudimentary measures for many tasks: constructing dwellings of an appropriate size and shape, fashioning clothing, or bartering food or raw materials. The earliest known uniform systems of measurement seem to have all been created sometime in
1584-626: The London quart;—and one width of dyed and russet and hauberk cloths—namely, two ells below the selvage..." As of the 21st century, the International System is predominantly used in the world. There exist other unit systems which are used in many places such as the United States Customary System and the Imperial System. The United States is the only industrialized country that has not yet at least mostly converted to
1632-414: The base quantities and some of the derived units are the units of speed, work, acceleration, energy, pressure etc. Different systems of units are based on different choices of a set of related units including fundamental and derived units. Following ISO 80000-1 , any value or magnitude of a physical quantity is expressed as a comparison to a unit of that quantity. The value of a physical quantity Z
1680-506: The crew confusing tower instructions (in metres) and altimeter readings (in feet). Three crew and five people on the ground were killed. Thirty-seven were injured. In 1983, a Boeing 767 (which thanks to its pilot's gliding skills landed safely and became known as the Gimli Glider ) ran out of fuel in mid-flight because of two mistakes in figuring the fuel supply of Air Canada 's first aircraft to use metric measurements. This accident
1728-556: The daily variation of demand (e.g. the slope of the duck curve ), or ramp-up behavior of power plants . For example, a power plant that reaches a power output of 1 MW from 0 MW in 15 minutes has a ramp-up rate of 4 MW/h . Other uses of terms such as watts per hour are likely to be errors. Several other units related to kilowatt-hour are commonly used to indicate power or energy capacity or use in specific application areas. Average annual energy production or consumption can be expressed in kilowatt-hours per year. This
1776-468: The different systems include the centimetre–gram–second , foot–pound–second , metre–kilogram–second systems, and the International System of Units , SI. Among the different systems of units used in the world, the most widely used and internationally accepted one is SI. The base SI units are the second, metre, kilogram, ampere, kelvin, mole and candela; all other SI units are derived from these base units. Systems of measurement in modern use include
1824-410: The kilowatt-hour is kWh , derived from its component units, kilowatt and hour. It is commonly used in billing for delivered energy to consumers by electric utility companies, and in commercial, educational, and scientific publications, and in the media. It is also the usual unit representation in electrical power engineering. This common representation, however, does not comply with the style guide of
1872-629: The metric system. The systematic effort to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up such a unit system. This system was the precursor to the metric system which was quickly developed in France but did not take on universal acceptance until 1875 when The Metric Convention Treaty was signed by 17 nations. After this treaty
1920-555: The name of the Board of Trade which regulated the electricity industry until 1942 when the Ministry of Power took over. This should not be confused with a British Thermal Unit (BTU) which is 1055 J. In India, the kilowatt-hour is often simply called a unit of energy. A million units, designated MU , is a gigawatt-hour and a BU (billion units) is a terawatt-hour. Unit symbol A unit of measurement , or unit of measure ,
1968-425: The numerical value expressed in an arbitrary unit can be obtained as: Units can only be added or subtracted if they are the same type; however units can always be multiplied or divided, as George Gamow used to explain. Let Z {\displaystyle Z} be "2 metres" and W {\displaystyle W} "3 seconds", then There are certain rules that apply to units: Conversion of units
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2016-434: The same unit for the distance between two cities and the length of a needle. Thus, historically they would develop independently. One way to make large numbers or small fractions easier to read, is to use unit prefixes . At some point in time though, the need to relate the two units might arise, and consequently the need to choose one unit as defining the other or vice versa. For example, an inch could be defined in terms of
2064-454: The size of the degree and for the circumference of the Earth was therefore about 25% too small. Historical Legal Metric information List of countries by electricity production This is a list of countries and dependencies by annual electricity production . China is the world's largest electricity producing country, followed by the United States and India. Data are for
2112-432: The watt-hour (3.6 kJ). The kilowatt-hour is a composite unit of energy equal to one kilowatt (kW) sustained for (multiplied by) one hour. The International System of Units (SI) unit of energy meanwhile is the joule (symbol J). Because a watt is by definition one joule per second , and because there are 3,600 seconds in an hour, one kWh equals 3,600 kilojoules or 3.6 MJ. A widely used representation of
2160-515: The wave equation in atomic physics . Some unusual and non-standard units may be encountered in sciences. These may include the solar mass ( 2 × 10 kg ), the megaton (the energy released by detonating one million tons of trinitrotoluene , TNT) and the electronvolt . To reduce the incidence of retail fraud, many national statutes have standard definitions of weights and measures that may be used (hence " statute measure "), and these are verified by legal officers. In informal settings,
2208-458: Was accidentally destroyed on a mission to Mars in September 1999 (instead of entering orbit) due to miscommunications about the value of forces: different computer programs used different units of measurement ( newton versus pound force ). Considerable amounts of effort, time, and money were wasted. On 15 April 1999, Korean Air cargo flight 6316 from Shanghai to Seoul was lost due to
2256-480: Was signed, a General Conference of Weights and Measures (CGPM) was established. The CGPM produced the current SI, which was adopted in 1954 at the 10th Conference of Weights and Measures. Currently, the United States is a dual-system society which uses both the SI and the US Customary system. The use of a single unit of measurement for some quantity has obvious drawbacks. For example, it is impractical to use
2304-560: Was the result of both confusion due to the simultaneous use of metric and Imperial measures and confusion of mass and volume measures. When planning his journey across the Atlantic Ocean in the 1480s, Columbus mistakenly assumed that the mile referred to in the Arabic estimate of 56 + 2 / 3 miles for the size of a degree was the same as the actually much shorter Italian mile of 1,480 metres. His estimate for
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