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Kilowatt-hour (kWh)

What Is Kilowatt-hour (kWh)?

A kilowatt hour, written as kWh, measures how much electrical energy is used or produced over time. It tells you how much electricity something used when it ran for one hour.

Power companies use kilowatt hours to count how much electricity you used in your home or building, then they use that number to calculate your bill.

Definition

A kilowatt hour is the amount of energy used when something with a power of one kilowatt works for one full hour.

To break it down:

  • Watt W is a unit of power. It shows how fast energy is used.
  • Kilowatt kW means 1000 watts.
  • Hour h is a unit of time, equal to 60 minutes.

So:

  • 1 kilowatt hour 1 kWh is the energy used by a 1000 watt device running for 1 hour.
  • It is also equal to 1000 watt hours Wh.

In science, energy is often measured in joules J. One kilowatt hour is the same as 3,600,000 joules.

History / Origin

The kilowatt hour became common in the late 1800s and early 1900s, when electric power systems and power plants started to spread in cities.

Engineers already used the watt to measure power. To measure how much total energy a customer used over time, they needed a unit that mixed power with time. They chose watt hour, and for bigger amounts they used kilowatt hour.

Electricity meters were built to count how many kilowatt hours flowed into homes and factories. This made it easy for power companies to charge people based on how much electricity they actually used.

Symbol & Abbreviation

The standard way to write kilowatt hour is:

  • kWh most common spelling worldwide

Sometimes you may also see:

  • kW h with a space, less common
  • kW-hr in older or informal texts

The three letters each mean something:

  • k means kilo, or 1000.
  • W means watt, the unit of power.
  • h means hour, the unit of time.

Current Use Around the World

Kilowatt hour is used almost everywhere to measure electrical energy. Some common uses are:

  • Electricity bills Homes, stores, and factories are charged by how many kWh they use.
  • Appliance labels Fridges, air conditioners, TVs, and other devices often show how many kWh they use in a year.
  • Power plants Power companies talk about how many kWh or larger units they generate and sell.
  • Solar panels and wind turbines Output and savings are measured in kWh.
  • Electric vehicles EVs Battery size is often shown in kWh, and energy use is in kWh per 100 km or per mile.

In most countries, when people talk about using or saving electricity, they usually talk in kilowatt hours.

Example Conversions

Here are some simple conversions for 1 kilowatt hour:

  • To watt hours Wh
    1 kWh = 1000 Wh
  • To joules J
    1 kWh = 3,600,000 J
  • To megawatt hours MWh
    1 kWh = 0.001 MWh
    1000 kWh = 1 MWh
  • To gigajoules GJ
    1 kWh ≈ 0.0036 GJ

Everyday style examples:

  • A 100 watt light bulb running for 10 hours uses: 100 W × 10 h = 1000 Wh = 1 kWh.
  • A 2000 watt electric heater running for 30 minutes 0.5 h uses: 2000 W × 0.5 h = 1000 Wh = 1 kWh.
  • A phone charger that uses 5 watts and is on for 4 hours uses: 5 W × 4 h = 20 Wh = 0.02 kWh.

Units related to the kilowatt hour include:

  • Watt W Unit of power, shows how fast energy is used or produced.
  • Kilowatt kW 1000 watts, often used for the power of engines, heaters, and large devices.
  • Watt hour Wh Smaller energy unit, 1 kWh = 1000 Wh.
  • Megawatt hour MWh Larger energy unit, 1 MWh = 1000 kWh.
  • Joule J SI base unit of energy, 1 kWh = 3,600,000 J.
  • Gigawatt hour GWh Very large energy unit, 1 GWh = 1,000,000 kWh.
  • Calorie cal or kilocalorie kcal Energy units often used for food, can be changed to and from kWh.
  • British thermal unit BTU Energy unit used in heating and air conditioning, also linked to kWh by conversion.

FAQs

What is the difference between kW and kWh?

kW kilowatt measures power, or how fast energy is used at one moment. kWh kilowatt hour measures energy, or the total amount used over time. Power is like speed, energy is like distance traveled.

Why do electricity bills use kWh?

Electricity bills use kWh because they show the total energy you used over the whole month. This makes it easy to count up all the power used by all your devices over time.

How can I lower the kWh on my bill?

You can lower your kWh use by turning off lights and devices you do not need, using energy saving bulbs and appliances, and running heaters and air conditioners less or at efficient settings.

How many kWh does a typical home use?

The answer depends on the country, climate, home size, and how many devices you use. A small apartment may use a few hundred kWh per month, while a large home can use over 1000 kWh per month.

An electric car battery size is often written in kWh, for example 50 kWh or 75 kWh. A bigger number means the battery can store more energy, so the car can usually drive farther.

Is kWh an SI unit?

The official SI unit of energy is the joule J. The kilowatt hour is not an SI base unit, but it is accepted and widely used for practical work, especially for electricity and energy bills.

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