Common Power Conversions and Symbols

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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.

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) = 10³ W
• 1 megawatt (MW) = 10⁶ W
• 1 gigawatt (GW) = 10⁹ W
• 1 terawatt (TW) = 10¹² W

Or:

• 1 W = 10¹⁵ fW
• 1 W = 10¹² pW
• 1 W = 10⁹ nW
• 1 W = 10⁶ μW
• 1 W = 10³ mW
• 1 W = 10^-3 kW
• 1 W = 10^-6 MW
• 1 W = 10^-9 GW
• 1 W = 10^-12 TW

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).

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

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

Image Credits:

James Watt 1736-1819