We have to pay for the electrical energy we utilize in our houses, shops, mills and factories. There is an electricity meter in the houses those use electricity, which maintains the accounts of spent electrical energy. Throughout the world, the electricity supply authority measures the amount of electrical energy consumed in units of a kilowatt-hour (kWh). We call this kilowatt-hour unit as ‘board of trade’ unit or in brief ‘unit’. From the difference of the readings of two times in the electricity meter, we get the amount of consumed electric power during this period.

Since power, **P = Work done / time = Convened energy / time = W / t**

So, **W = Pt**

If P = 1 kW and t = 1h, then W = 1 kW x 1h = 1 kWh.

Therefore, the amount of electrical energy converted or spent when an electric device of 1-kilowatt power works for 1 hour is called 1 kilowatt-hour or 1 unit.

1 kWh = 3.6 x 10^{6} J

When power is expressed in watt and time in the hour, the amount of electrical energy consumed can be expressed as:

**FV = Pt Wh**

Dividing this by 1000, the electricity consumed is found to be in kWh.