Beatrice Lang
Understanding units of electricity is essential for calculating your electricity bill and solving physics problems. Units of electricity measure the amount of energy consumed at a property or by an appliance. The standard unit of electricity is the kilowatt-hour (kWh), which represents the energy consumed by a 1-kilowatt appliance running for 1 hour. Each appliance uses a certain number of energy units, and you are charged based on the number of units used and the rate of your electricity plan.
| Characteristics | Values |
|---|---|
| Unit of Electricity | A foundational concept in Physics |
| Electric Parameter | Current, charge, voltage, resistance, power, or energy |
| Base SI Units | Ampere (A) for current, Coulomb (C) for electric charge, Volt (V) for electric potential, Ohm (Ω) for resistance, Watt (W) for power, Kilowatt-hour (kWh) for energy |
| Current (Ampere, A) | Measures the rate of flow of electric charge. Defined as 1 Coulomb per second |
| Charge (Coulomb, C) | The total quantity of electricity. 1 Coulomb is the charge carried by 1 Ampere in 1 second |
| Voltage (Volt, V) | The potential difference required to move 1 Coulomb of charge by 1 Joule of energy |
| Resistance (Ohm, Ω) | The opposition to current flow in a circuit |
| Power (Watt, W) | The rate at which electrical energy is consumed or converted |
| Energy (Kilowatt-hour, kWh) | The commercial unit shown on electricity bills. It represents the energy consumed by a 1-kilowatt appliance running for 1 hour |
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What You'll Learn
Kilowatt-hour (kWh)
The formula to calculate the energy in kilowatt-hours is: Energy (kWh) = Power (kW) x Time (hours).
For example, a 12-watt LED lamp lit constantly uses about 0.3 kWh per 24 hours and about 9 kWh per month. A 100-watt television running for 10 hours will use 1 kWh of energy. A 40-watt appliance operating continuously for 25 hours will also use 1 kWh of energy.
Kilowatt-hour is different from kilowatt (kW), which measures power or the rate at which something uses electricity. A kilowatt is equal to 1000 watts. A kilowatt-hour, on the other hand, measures the total amount of energy used over a period of time. For instance, if you use a 1000-watt drill for one hour, you will consume 1 kWh of energy.
Understanding the difference between kW and kWh is important for monitoring and managing electricity usage, especially for businesses and electric vehicles.
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Watt (W)
A watt (W) is the standard unit of power or radiant flux in the International System of Units (SI). It is used to measure the rate of energy transfer over a unit of time, with one watt equal to one joule (J) per second. In other words, watts are energy per second.
The watt is named after James Watt, the 18th-century Scottish inventor and engineer who improved steam engine technology, which was fundamental to the Industrial Revolution. The unit name was proposed by C. William Siemens in 1882, noting that units in the practical system were named after leading physicists.
In an electrical circuit, power is the product of current and voltage. Current, measured in amperes (A), is the rate at which energy flows. Voltage, measured in volts (V), is the difference in charge between two points. According to the SI standard, one volt is equal to one watt divided by one amp. Therefore, one watt is equal to one amp under the pressure of one volt.
Watts are used to measure the rate at which energy is generated or consumed. For example, a 60-watt lightbulb consumes electricity at a rate of 60 watts per hour. Radio stations use watts to report the power of their transmitters, referring to the effective radiated power. Power stations are rated using larger units of power, typically megawatts or gigawatts.
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$15.99
Ampere (A)
The ampere is named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism alongside Danish physicist Hans Christian Ørsted. Ampère studied electromagnetism and laid the foundation of electrodynamics. In recognition of Ampère's contributions to the creation of modern electrical science, an international convention, signed at the 1881 International Exposition of Electricity, established the ampere as a standard unit of electrical measurement for electric current.
The ampere definition exploits the fact that electric current is made up of a flow of billions of identical charged particles called electrons. We can create a standard ampere by using special nano-scale electric circuits that control the flow of electrons. The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634 x 10^-19 when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of ∆ν.
The ampere can alternatively be expressed in terms of other units using the relationship I = P/V, and thus 1 A = 1 W/V. Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance.
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Coulomb (C)
The coulomb (symbolised by C) is the unit of electric charge in the International System of Units (SI). It is named after the 18th–19th-century French physicist Charles-Augustin de Coulomb.
The coulomb is defined as the quantity of electricity transported in one second by a current of one ampere. In other words, if a current in a circuit is 1 ampere, 1 coulomb of charge passes through a point in the circuit every second. This relationship can also be expressed as one ampere being equal to one coulomb per second.
The SI defines the coulomb as "the quantity of electricity carried in 1 second by a current of 1 ampere" by fixing the value of the elementary charge, e = 1.602176634×10−19 C. One coulomb is the charge of approximately 6.24 x 10^18 electrons or protons, or about 6.24 quintillion particles. This is a very large number of charges—in everyday life, we rarely encounter charges as large as a coulomb. Charges produced by rubbing ordinary objects, such as a comb or plastic ruler, are typically around a microcoulomb (10^-6 C) or less.
The coulomb is considered a derived unit in the SI standard, meaning it is constructed from one or more of the seven base units. In this case, the coulomb is derived from the ampere and second.
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Volt (V)
The volt (symbolized as V) is the Standard International (SI) unit of electromotive force or the potential difference required to carry one ampere of current through a resistance of one Ohm. It is named after the 18th–19th-century Italian physicist Alessandro Volta.
The volt is the unit of measurement of electric potential, electric potential difference (voltage), and electromotive force. One volt is defined as the electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. This can also be expressed as the potential difference between two points in an electric circuit that imparts one joule (J) of energy per coulomb (C) of charge that passes through the circuit.
In terms of SI base units, a volt can be expressed as:
> {\displaystyle {\text{V}}={\frac {\text{potential energy}}{\text{charge}}}={\frac {\text{J}}{\text{C}}}={\frac {{\text{kg}}{\cdot }{\text{m}}^{2}{\cdot }{\text{s}}^{-2}}{{\text{A}}{\cdot }{\text{s}}}}={\text{kg}}{\cdot }{\text{m}}^{2}{\cdot }{\text{s}}^{-3}{\cdot }{{\text{A}}^{-1}}.}
Or, in terms of SI base units (m, kg, s, and A):
> {\displaystyle {\text{V}}={\frac {\text{power}}{\text{electric current}}}={\frac {\text{W}}{\text{A}}}={\frac {{\text{kg}}{\cdot }{\text{m}}^{2}{\cdot }{\text{s}}^{-3}}{\text{A}}}={\text{kg}}{\cdot }{\text{m}}^{2}{\cdot }{\text{s}}^{-3}{\cdot }{{\text{A}}^{-1}}.}
The volt can also be defined as electric potential along a wire when an electric current of one ampere dissipates one watt of power. It can be expressed in several other ways, including amperes times ohms (current times resistance, or Ohm's law), webers per second (magnetic flux per time), watts per ampere (power per current), or joules per coulomb (energy per charge).
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Frequently asked questions
A unit of electricity is a foundational concept in physics that helps us understand how we measure various electrical quantities. The unit of electricity is the kilowatt-hour (kWh), which is equal to 1 kilowatt (or 1000 watts) of power being consumed continuously over a period of 1 hour.
Every electric parameter—whether it is current, charge, voltage, resistance, power, or energy—has its specific unit of measurement, standardized under the International System of Units (SI). The base SI units include the Ampere (A) for current, the Coulomb (C) for electric charge, the Volt (V) for electric potential, the Ohm (Ω) for resistance, the Watt (W) for power, and the kilowatt-hour (kWh) for energy.
Your electricity bill is calculated based on the number of energy units you've used, charged at the rate offered by your electricity supplier. The formula for calculating electricity consumption for billing is: Energy (kWh) = Power (kW) x Time (hours).
Here are some examples of how much electricity common appliances use, measured in kilowatt-hours (kWh):
- A 100 W bulb running for 10 hours uses 1 kWh of electricity.
- A 40-Watt (0.04 kW) light bulb used for five hours uses 0.2 kWh of electricity.
- A 2000 W fan heater used for 4 hours uses 8 kWh of electricity.
- An electric shower with a power rating of 8-10 kW used for 1 hour uses 8 kWh of electricity.
Your electricity usage is measured using an electricity meter, which is usually located on the outside of your property where the power line enters. In the past, all electricity meters were mechanical devices that had to be read manually, but now many utilities use electronic smart meters, which provide wireless access to the meter's power usage data in real-time.
