The higher units are kilowatt(kW), megawatt(MW), and Gigawatt(GW).

Unit of power

Power is a scalar quantity. Its dimension is [ML2T-3]. The SI unit of power is watt (W), named after the inventor of the steam engine James Watt. *One watt is defined as* the power *when one joule of work is done in one second*,* *(1 W = 1 J s-1).

The higher units are kilowatt(kW), megawatt(MW), and Gigawatt(GW).

1kW = 1000 W = 103 watt

1MW = 106 watt

1GW = 109 watt

For motors, engines and some automobiles an old unit of power still commercially in use which is called as the horse-power (hp). We have a conversion for horse-power (hp) into watt (W) which is,

1 hp = 746 W

All electrical goods come with a definite power rating in watt printed on them. A 100 watt bulb consumes 100 joule of electrical energy in one second. The energy measured in joule in terms of power in watt and time in second is written as, 1 J =1 W s. When electrical appliances are put in use for long hours, they consume a large amount of energy. Measuring the electrical energy in a small unit watt. second (W s) leads to handling large numerical values. Hence, electrical energy is measured in the unit called kilowatt hour (kWh).

1 electrical unit = 1 kWh = 1 x (103W) x (3600 s)

1 electrical unit = 3600×103 Ws

1 electrical unit = 3.6×106 J

1 kWh = 3.6×106 J

Electricity bills are generated in units of kWh for electrical energy consumption. 1 unit of electrical energy is 1 kWh. (Note: kWh is unit of energy and not of power.)

Calculate the energy consumed in electrical units when a 75 W fan is used for 8 hours daily for one month (30 days).

Power, P = 75 W

Time of usage, t = 8 hour × 30 days = 240 hours

Electrical energy consumed is the product of power and time of usage.

Electrical energy = power × time of usage = P × t

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11th Physics : Work, Energy and Power : Unit of power |

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