## Power vs. Energy – What Is a Watt?

Before defining what a watt is, we should first determine power and energy.

**Energy** is the ability of a system to produce work. **Power** is actually the rate at which the energy is generated or consumed and can be calculated with the following formula:

**P = E/t**

**P** is power, **E** is energy and** t** is time.

In the International System of Units (SI), **joule (J)** is used as a derived unit for the measurement of energy and **sec (s)** is the base unit for the measurement of time. The derived unit defined as one joule per one second (1J /1s), is called **watt **(symbol [**W**]) and it was named after James Watt (1736-1819), a Scottish engineer. Watt is used for measuring power.

## Conversion of Watts to Kilowatts, Megawatts and Other Multiples

Kilowatts and megawatts are two of the most popular multiples of watt. But what is a kilowatt and what is a megawatt?

A **kilowatt (kW)** is equal to one thousand watts. It is mostly used to express the power consumption of engines, motors, heaters and the electromagnetic power of radio transmitters.

A **megawatt (MW)** is equal to one million watts or one thousand kilowatts. This scale is used for lightning, large commercial buildings, or large mechanical devices and constructions (carriers, submarines, etc).

Below there is a list with the conversion of watts to kilowatts, megawatts and other multiples:

- 1 femtowatt (fW) = 10
^{−15}W - 1 picowatt (pW) = 10
^{−12}W - 1 nanowatt (nW) = 10
^{−9}W - 1 microwatt (μW) = 10
^{−6}W - 1 milliwatt (mW) = 10
^{−3}W - 1 kilowatt (kW) = 10
^{3}W - 1 megawatt (MW) = 10
^{6}W - 1 gigawatt (GW) = 10
^{9}W - 1 terawatt (TW) = 10
^{12}W

Or:

- 1 W = 10
^{15}fW - 1 W = 10
^{12}pW - 1 W = 10
^{9}nW - 1 W = 10
^{6}μW - 1 W = 10
^{3}mW - 1 W = 10
^{-3}kW - 1 W = 10
^{-6}MW - 1 W = 10
^{-9}GW - 1 W = 10
^{-12}TW

## How to Calculate Watt-hours, Kilowatt-hours and Watts per Hour

**Watt-hour** is a unit measuring energy. When an electric motor consumes 1 watt in one hour, the energy consumed is 1 watt-hours. If the same motor consumes 1kW in one hour, the energy will be 1 **kilowatt-hours (kW•h). **A power plant may produce hundreds of **megawatt-hours (MW•h)** of energy.

**Watts per hour (W/h)** is different than watt-hours. The unit does not express energy but rather the rate of power (change of power) consumed or generated per hour. The multiple units of watts per hour (**megawatt per hours [MW/h]**) is particularly used to measure the rate of power generated by power plants (for example* *a power output of 3 MW from 0 MW in 30 minutes has a rate of 6 MW/h*).** *

## Other Units of Power

Regarding electrical systems, power can be calculated by multiplying the voltage V by the current I. When current is measured in **amps** and voltage in volts, the power is 1* ***W**.

**P = VI **=>** 1 watt = 1volt x 1amp **

**BTU **(Btu) is a British unit used to measure energy (mostly for heat) and not power. The conversion is: 1 Btu = 1 055.05585 J. On the other hand, the unit that measures power is **BTU hour (BTU.h). **

1 W is approximately 3.41214 BTU.h and 1kW is 3412.14 BTU.h

## Sources:

- "The Physics Factbook" 17 February 2009.
- A Practical Guide to the International System of Units,
- www.physicsclassroom.com

Image Credits: