In the International System of Units (SI) it is define as a derive unit of 1 joule per second, and is use to quantify the rate of energy transfer. Moreover, The wattage (symbol: **W**) is a unit of power. In SI base units, the wattage is describe as kg⋅m^{2}⋅s^{−3}. In 18th-century the name watt is honor of Scottish inventor James watt. who invent the watt.

## Wattage Example

When an object’s velocity is held constant at one meter per second against a constant opposing force of one newton, the rate at which work is done is one watt.

In terms of electromagnetism, one watt is the rate at which electrical work is performed when a current of one ampere (A) flows across an electrical potential difference of one volt (V), meaning the watt is equivalent to the volt-ampere (the latter unit, however, is used for a different quantity from the real power of an electrical circuit).

Two additional unit conversions for watt can be found using the above equation and Ohm’s law.

Where ohm is the SI derive unit of electrical resistance.

### Multiples

- Attowatt

The attowatt (aW) is equal to one Quintilian (10^{−18}) of a watt.

- Femtowatt

The femtowatt (fW) is equal to one quadrillion (10^{−15}) of a watt.

- Picowatt

The picowatt (pW), not to be confuse with the much larger petawatt (PW), is equal to one trillionth (10^{−12}) of a watt.

- Kilowatt

The **kilowatt (kW)** is equal to one thousand (10^{3}) watts.

- Megawatt

The megawatt (MW) is equal to one million (10^{6}) watts.

#### Distinction between watts and watt-hours

The terms power and energy are frequently confuse. Power is the rate at which energy is generate or consume and hence is measure in units (e.g. watts) that represent energy *per unit time*.

For example, when a light bulb with a power rating of 100W is turn on for one hour, the energy use is 100 watt hours (W·h), 0.1 kilowatt hour, or 360 kJ. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours.

Power stations are rated using units of power, typically megawatts or gigawatts (for example, the Three Gorges Dam in China, is rated at approximately 22 gigawatts). This reflects the maximum power output it can achieve at any point in time.

A power station’s annual energy output, however, would be record using units of energy (not power), typically gigawatt hours. Major energy production or consumption is often express as terawatt hours for a given period; often a calendar year or financial year.

One terawatt hour of energy is equal to a sustained power delivery of one terawatt for one hour, or approximately 114 megawatts for a period of one year:

Power output = energy / time1 terawatt hour per year = 1×10^{12} Wh / (365 days × 24 hours per day) ≈ 114 million watts,

equivalent to approximately 114 megawatts of constant power output.

The watt second is a unit of energy, equal to the joule. One kilowatt hour is 3,600,000 watt seconds.

While a watt per hour exists in principle (as a unit of rate of change of power with time), it is not correct to refer to a watt (or watt hour) as a “watt per hour”

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