What Is Watt-hour (Wh)?
A watt hour is a unit that tells you how much electrical energy is used or stored when a device runs for a certain time. It combines power in watts and time in hours. If a 1 watt device works for 1 hour it uses 1 watt hour of energy.
Definition
A watt hour is a unit of energy. It is equal to using power of 1 watt for 1 hour.
In science energy is often measured in joules. One watt is one joule of energy used every second. There are 3,600 seconds in one hour. So
- 1 watt hour Wh equals 3,600 joules J
- 1 joule J equals about 0.000278 watt hours Wh
So when you see Wh on a battery label or power bill it tells you the total energy not just how strong the device is at one moment.
History / Origin
The watt hour is named after James Watt a Scottish engineer who helped improve the steam engine in the 1700s. His work was very important in the Industrial Revolution so the unit of power watt was named after him.
Later when people started using electricity engineers needed a simple way to show how much electrical energy was used over time. They joined the power unit watt with the time unit hour to create the energy unit watt hour Wh. This made it easier for power companies to measure and charge for electricity and to compare different electrical devices.
Symbol & Abbreviation
The standard symbol for watt hour is
- Wh for watt hour
Related common multiples are
- mWh milliwatt hour one thousandth of a Wh
- kWh kilowatt hour 1,000 Wh
- MWh megawatt hour 1,000,000 Wh
Note that the W is uppercase for watt and the h is lowercase for hour. There is no dot between them.
Current Use Around the World
The watt hour is used worldwide as a practical way to measure electrical energy especially in homes and businesses. The most common form on power bills is the kilowatt hour kWh.
Everywhere in the world you will see Wh or kWh
- On home electricity meters and energy bills to show how much energy you used
- On batteries for phones laptops cameras and power banks to show how much energy they can store
- For solar panels wind turbines and other power systems to show how much energy they produce or store over time
- In electric vehicles to show battery capacity usually in kilowatt hours
Scientists often use the joule in research but for everyday life and products Wh and kWh are easier to understand and compare.
Example Conversions
Here are some simple examples to show how watt hours work.
Basic idea
- A 10 W light bulb on for 3 hours uses 10 W multiplied by 3 h equals 30 Wh.
- A 100 W device on for 0.5 hours 30 minutes uses 100 W multiplied by 0.5 h equals 50 Wh.
Wh to kWh
To change watt hours to kilowatt hours divide by 1,000.
- 1,000 Wh equals 1 kWh
- 500 Wh equals 0.5 kWh
- 2,500 Wh equals 2.5 kWh
kWh to Wh
To change kilowatt hours to watt hours multiply by 1,000.
- 1 kWh equals 1,000 Wh
- 3 kWh equals 3,000 Wh
- 0.2 kWh equals 200 Wh
Wh to joules
To change watt hours to joules multiply by 3,600.
- 1 Wh equals 3,600 J
- 10 Wh equals 36,000 J
- 100 Wh equals 360,000 J
Related Units
These units are closely related to the watt hour.
- Watt W a unit of power which is how fast energy is used or produced energy per second
- Kilowatt hour kWh 1,000 watt hours the main unit on home electricity bills
- Milliwatt hour mWh one thousandth of a watt hour used for very small batteries
- Joule J the main scientific unit for energy in the International System of Units SI
- Calorie cal an older energy unit often used for food energy not usually used for electricity
- British thermal unit BTU an energy unit used in heating and air conditioning systems
FAQs
What is the difference between watt W and watt hour Wh
A watt W measures power which is how fast energy is used at one moment. A watt hour Wh measures total energy used over time. Power is like speed. Energy is like distance traveled.
Why do power bills use kilowatt hours kWh instead of joules
Joules are very small for everyday use. Using kWh gives smaller easier numbers. For example saying you used 300 kWh this month is simpler than saying you used over one billion joules.
How can I reduce the watt hours I use at home
You can reduce Wh by using devices that need less power watts and by using them for less time. For example use LED bulbs instead of old bulbs and turn off devices when you do not need them.
Why are batteries often rated in Wh instead of only in mAh
Milliamp hours mAh speak about electric charge but do not include the battery voltage. Watt hours Wh show the real energy stored because they include both current and voltage. This makes comparing different batteries easier.
Is watt hour an SI unit
No the official SI unit of energy is the joule J. The watt hour is not an SI unit but it is accepted for use with SI because it is very practical for electricity and batteries.
How many Wh are in a phone battery
Many phone batteries store around 10 to 20 Wh of energy. For example a 4,000 mAh battery at 3.7 V has about 4 Ah multiplied by 3.7 V equals 14.8 Wh of energy.