The capacity to do work is called energy.The most important types of energy are electrical energy,mechanical energy and thermal energy.Various units are assigned to each energy and since all these energies are interchangeable we could assign same unit of energy through out

Electrical Energy

The unit of electrical energy is watt sec or joule and is defined as

One watt sec or joule of energy is transfered between two points if a potential difference of one volt exists between them and one ampere current passes between them for one sec. ie

Electrical energy in watts or joules = voltage in volts ×current in amperes × time in seconds

joule or wattsecond is practically a very small unit of energy and hence we use larger units like watt-hour and kilowatt hour.

1 watt hour = 1 watt × 1hour

= 1 watt × 3600sec=3600 wattsec

1 kilowatt hour(kWh) =1kW × 1 hr = 1000watt ×3600 sec=36×10ˆ5 watt-sec

Efficiency of an electrical system is defined as the ratio of output power to input power

ie η = output power /input power

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